Objects and Classes

Using Helper.factorial(...) computing $(\begin{matrix} n \\ k \end{matrix}) = \frac{n!}{k! (n - k)!}$

Objects and Classes
- ➟ Static methods

public class Add {

    static void main() {
        int i = 2, j = 5;
        int result = sum (i, j);
        IO.println("The sum of " + i +         
              " and " + j + " is " + result);
    }
    static int sum ( int a, int b) {
        return a + b;
    }
}

The sum of 2 and 5 is 7

\begin{matrix} sum : ℤ \times ℤ \to ℤ \\ (a, b) \mapsto a + b \end{matrix}

   Method's return type:    Method's arguments:
   Method returns           Method expects two
   int values               arguments of type int
          ↘               ↙      ↙
    static int sum ( int a, int b) {
        int result = a + b;
        return result; 
    }                ↖
                      Returning a value 
                      of type int

static void main() {
     ...
    // Compile error: Non-static method 'sum(int, int)' 
    // cannot be called from a static context
    int result = sum (i,j);
         ...
}

// No static modifier
int sum ( int a, int b) {
   return a + b;
}

/**
 * Calculates the sum of two integers.
 *
 * @param a the first integer to add
 * @param b the second integer to add
 * @return the sum of the two integers
 */
static int sum (int a, int b) {
    int result = a + b;
    return result;
}

Method definition

Method usage

public class Helper {

    static int sum(int a, int b) {              
        int result = a + b;
        return result;
    }
}

public class Add {

    static void main() {
        int i = 2, j = 5;

        int result = Helper.sum(i, j);
        IO.println("The sum of " + i +              
           " and " + j + " is " + result);
    }
}

Objects and Classes
- ➟ Working with objects
  
  The concept of classes: Blueprints for objects.
  
  UML diagram visualization.
  
  Classes and related instances.
  
  The «new» operator.
  
  Assignment of attribute values. Working with «null».

In object oriented languages classes:

are blueprints for objects.
contain attributes and methods.
allow for implementation hiding.
allow for tailored access to methods and attributes.

public class Rectangle {
  int width;
  int height;

  // solid or dashed:
  boolean hasSolidBorder;
}

Rectangle dashedRectangle = new Rectangle();

Rectangle solidRectangle = new Rectangle();

...

new class-name ([argument 1[, argument 2] ...] )

Wording examples:

“Create an instance of class Rectangle”.
“Create a Rectangle object.”
“Create a Rectangle”.

Rectangle dashedRectangle = new Rectangle();

dashedRectangle.width = 28;
dashedRectangle.height = 10;
dashedRectangle.hasSolidBorder = false;

Syntax accessing object attributes:

variable.attributeName = value;

Rectangle r = new Rectangle();// Creating an object

r.width = 28; // o.K.

r = null;// removing reference to object

r.width = 28; // Runtime error: NullPointerException (NPE)

Exception in thread "main" java.lang.NullPointerException
  at de.hdm_stuttgart.mi.classdemo.App.main(App.java:17)

Rectangle r;

... // possible object assignment to variable r.

if (null == r) {
  IO.println("No rectangle on offer");
} else {
  IO.println("Width:" + r.width);
}

Objects and Classes
- ➟ Packages
  
  Significance for larger projects.
  
  Fully qualified vs. «import».
  
  java.lang and other exceptions.

Grouping of related classes (e.g. subsystems).
Structuring big systems.
Provide access restrictions using:

public, private and protected modifier
Resolving class name clashes. Example:

java.lang.String vs. my.personal.String

Package names below java. are reserved.
Package names should not start with javax. either.
Package names must not contain operators:

mi.hdm-stuttgart.de → de.hdm_stuttgart.mi.
Packages should start with reversed DNS avoiding clashes.

Fully qualified class name:

java.util.Scanner ❶ scanner =          // Clumsy and
   new java.util.Scanner ❷(System.in); // redundant

Using import :

import java.util.Scanner; ❶

public class Q {

  static void main() {
     Scanner ❷ scanner = new Scanner ❷(System.in);
       ...
  }
}

Bad

Good

import java.util.*;

public class Q {
  static void
        main() {
    Scanner s = 
      new Scanner(System.in);
    Date today = new Date();
  }
}

import java.util.Scanner;
import java.util.Date;

public class Q {
  static void
         main() {
    Scanner s =
       new Scanner(System.in);
    Date today = new Date();
  }
}

import java.lang.String; ❶  // Optional
import java.util.Scanner;❷  // Required
public class Q {

  static void main() {
    String message = "Hello!";
    Scanner ❸ s = new Scanner(System.in);
  }
}

package my.first.javapackage;
        ❷ ❸     ❹
public class Print { 
              ❺
  ...


}

Objects and Classes
- ➟ Object methods
  
  Object behaviour, state transitions, derived values.
  
  Selected examples both of state-changing and state-conserving methods.
  
  Rectangle sample methods.

Get (dependent) values: Example: Calculate a rectangle's area.
Change an object's state.: Example: Scale a rectangle.
Combined: Scale a rectangle and calculate its new perimeter.

public class Rectangle {

    int width, height;

    int getArea() {
        return width * height;       
    }
}

static void main() {       
   Rectangle  r =  new Rectangle();
   r.width = 20;
   r.height = 30;

   int area = r.getArea();
        
  IO.println(
    "Area of rectangle is: " + area);
}

Area of rectangle is: 600

public class Rectangle {
    int width, height;
 
    int getArea() {...}

    public void scale (int factor) {    
        width *= factor;
        height *= factor;
    }
}

Rectangle  r =  new Rectangle();
r.width = 20;
r.height = 30;

IO.println("Area before scaling: "     
      + r.getArea());
r.scale(2);
IO.println("Area after scaling: "
      + r.getArea());

Area before scaling: 600
Area after scaling: 2400

void ❶ scale (int factor ❷) {
...}

❶	No value is being returned to caller.
❷	A single value of type `int` is being provided as method argument.

Rectangle r = new Rectangle();
r.width = 33;
r.height = 22;

r.scale(2);

IO.println("width=" + r.width);
IO.println("height=" + r.height);

width=66
height=44

public ❶ void ❷ scale❸ (int factor ❹) { ❺
   width *= factor; ❻
   height *= factor;
}

[access modifier] ❶ return_type ❷ methodName ❸ ([arguments] ❹) {❺
   [statement(s)] ❻
}

public class Rectangle {
    int width, height;
    boolean hasSolidBorder;

    public void scale (int factor) { ... }

    public int getPerimeter() {
        return 2 * (width + height);
    }
}

Rectangle r = new Rectangle();

r.width = 33;
r.height = 22;

IO.println("Perimeter=" + r.getPerimeter());

Perimeter=110

Objects and Classes
- ➟ Object methods
  - ➟ Encapsulation and access control
    
    Justifying access restrictions.
    
    public, protected «package local» and private.
    
    Best practices.

Fine-grained control on attributes and methods.
Support encapsulation / Information hiding.

Purpose: Hide implementation details within class or package

public class DayTime {
  private int ❶ minutesSinceMidnight;

  public int getMinute() { ❷
    return minutesSinceMidnight % 60;
  }
  public int getHour() {❷
    return minutesSinceMidnight / 60;
  }
}

public class DayTime {
  private int minute, hour;❶
         
  public int getMinute() { ❷
        return minute;
  }
  public int getHour() { ❷
        return hour;
  }
}

No access to private field of alien class Time:

public class Q {
  static void main() {

      DayTime t = new DayTime();

      // Error: 'minutesSinceMidnight' has private access in 'Time'
      t.minutesSinceMidnight = 371;
  }
}

No access modifier = package - local access

package a;

public class X{
  // No public  
  // modifier
  int i;
}

package a;

public class Y {

  void displayValues(X x) {
      // o.K.
      IO.println(x.i);  
  }
}

package b;

// X resides in different package
import a.X;

public class Z {
  void displayValues(X x) {
    // Error:
    // 'i' is not public in 'a.X'    
    // Cannot be accessed from
    // outside package
    IO.println(x.i);
  }
}

Access Level	Other package	Child class	Same package	Same class
`public`	yes	yes	yes	yes
`protected`	no	yes	yes	yes
[omitted]	no	no	yes	yes
`private`	no	no	no	yes

Use the most restrictive access level that makes sense for a particular member.
Use private unless you have a good reason not to.
Avoid public fields except for constants. Public fields tend linking to a particular implementation and limit your flexibility in changing your code.

Objects and Classes
- ➟ Object methods
  - ➟ Getter and setter methods

Direct access Setter access

public class Time {
    public int hour, minute;
}

Time time = new Time();
    
time.hour = 17;
time.minute = 45;

public class Time {
    private int hour, minute;
    public void setTime(int h, int m) {
        minute = m;
        hour = h;
    }
}

Time time = new Time();
time.setTime(17, 45);

Allow for change of implementation.
Allow for non business - logic concerns. Examples:
- Logging
- Adding persistence (Databases)

public class Time {
    private int hour, minute;
    public void setTime(int h, int m) {
        minute = m;
        hour = h;
        IO.println("Time has been set to " + hour + ":" + minute);
    }
}

Direct access Setter / getter access

public class Time {
  // Minutes since 00:00
  public int minute;
}

final Time time = new Time(); 
// To be changed at multiple
// project locations
time.minute = 17 * 60 + 45;

public class Time {
  private int minute; // Minutes since 00:00
  public void setTime(int h, int m) {
    minute = m + 60 * h;
  }
  public int getMinute(){/* coming soon */}
  public int getHour(){/* coming soon */}
}

final Time time = new Time()
time.setTime(17, 45);

Implementing getter methods

Objects and Classes
- ➟ Object methods
  - ➟ Signatures
    
    Type signatures.
    
    Method signatures: Prerequisite understanding method overloading.

boolean ❶ startsWith(String prefix, int toffset) ❷

❶	Return type boolean
❷	Arguments among with their respective types and order: Type `String` Type `int`

Method	Return type	Argument type list
`void print()`	void	(void)
`int add (int a, int b)`	int	(int, int)
`int max (int a, int b)`	int	(int, int)
`void print (int a, float b)`	void	(int, float)
`void display (float a, int b)`	void	(float, int)

boolean startsWith❶(String prefix, int toffset) ❷

❶	Method name `startsWith`.
❷	Number of arguments among with their respective types and order.

Method	Method name	Method signature
`void print()`	print	(void)
`int add (int a, int b)`	add	(int, int)
`int max (int a, int b)`	max	(int, int)
`void print (int a, float b)`	print	(int, float)
`void display(float a, int b)`	display	(float, int)

Method signature variants

Objects and Classes
- ➟ Object methods
  - ➟ Method overloading
    
    Same name, different argument types.
    
    «C» programming language comparison.
    
    Common examples.

public class Print {
  public void print() {             // (void)
    IO.println("No argument");
  }
  public void print(int i) {        // (int)
    IO.println("int value " + i);
  }
  public void print(double d) {     // (double)
    IO.println("double value " + d);
  }
  public void print(int i, int j) { // (int, int)
      IO.println("Two int values "+
         i + " and " + j);
  }
}

Print p = new Print();
p.print();
p.print(33);
p.print(4.333);
p.print(-1, 7);

No argument
int value 33
double value 4.333
Two int values -1 and 7

Static polymorphism.
Compile time binding.
Early binding.

#include <stdio.h>

void print() {
  printf("No argument\n");
}

void print(int i) {              /* Error: redefinition of ‘print’ */
  printf("int value %d\n", i);
}

void main(void) {
  print();
  print(33);
}

Code in file print.c Compile / execute

#include <stdio.h>

void print() {
  printf("No argument\n");
}

/* Different function name */
void printIntValue(int i) {   
  printf("int value %d\n", i);
}

void main(void) {
  print();
  printIntValue(33);
}

Compiling print.c to executable file print

> cc -o print print.c

Executing file print

> ./print
No argument
int value 33

public class Person {
  String getDetails() { return "dummy";}
  int getDetails() { return 1;} // Error: 'getDetails()' is already
}                                // defined in 'Person'

Return type	Method signature
Return type	Method name	Argument type list
String	getDetails	(void)
int	getDetails	(void)

Only method signature support in Java™ ignoring return type.

In Java™ method signatures allow for uniquely addressing a method within a given class e.g.:

The method named print having an int argument followed by a double:

print(int, double)

Will a match be found?

`print(boolean b)`	`print(char c)`
`print(char[] s)`	`print(double d)`
`print(float f)`	`print(int i)`
`print(long l)`	`print(Object obj)`
`print(String s)`

Objects and Classes
- ➟ Object methods
  - ➟ Constructors
    
    Combined instance creation and initialization.
    
    Constructors are methods, but ...
    
    Constructor overloading and a class' default constructor.

Combining statements desired:
`int a; a = 33;`	`Rectangle r = new Rectangle(); r.width = 28; r.height = 10; r.hasSolidBorder = false;`
`int a = 33; // works!`	// how to ??? Rectangle r = new Rectangle(28, 10, false);

public class Rectangle {
    int width, height;
    boolean hasSolidBorder;
         ...
    public ❶ Rectangle ❷ (int width, int height, boolean hasSolidBorder){
        this.width = width;
        this.height = height;
        this.hasSolidBorder = hasSolidBorder;
    }
}

[access modifier] constructorName (listOfArguments) {
    [constructor body]
}

Empty argument list

Default constructor e.g. obj = new MyClass().

Non-empty argument list

Non-default constructor e.g. :

obj = new String("xyz");

Can only be executed on object creation.
Are being called prior to any non-constructor method.
Only one of potentially multiple constructors will be executed exactly one time.

However nesting is possible.

public class Rectangle {
    int width, height;

    public Rectangle() {
        width = height = 1;
    }
    public Rectangle(int width, int height){
        this.width = width;
        this.height = height;
    }
    public Rectangle(int widthAndHeight) {
        width = height = widthAndHeight;
    }
}

public class Rectangle {
  int width, height;

  public Rectangle(int width,
                int height){
    this.width = width;
    this.height = height;
  }
  public Rectangle() {
    width = height = 1;
  }
  public Rectangle(
     int widthAndHeight) {
    width = height =
          widthAndHeight;
  }
}

   public class Rectangle {
     int width, height;

 ┏━▶ public Rectangle(int width,
 ┃                    int height){
 ┃     this.width = width;
 ┃     this.height = height;
 ┃   } 
 ┃   public Rectangle() { 
 ┣━◀━━ this(1, 1);           ❶
 ┃   }
 ┃   public Rectangle(
 ┃     int widthAndHeight) {
 ┗━◀━━ this(widthAndHeight,   ❷
            widthAndHeight); 
     }
   }

Rectangle standard = new Rectangle(); // 1 x 1
Rectangle square = new Rectangle(2); // 2 x 2
Rectangle individual = new Rectangle(2, 7); // 2 x 7

Equivalent:

Rectangle r =
                              new Rectangle();

public class Rectangle {
    int width, height;
    boolean hasSolidBorder;

    // Default constructor, empty body.
    public Rectangle ( ){}
}

public class Rectangle {
    int width, height;
    boolean hasSolidBorder;

  // No constructor at all
}

public class Rectangle {
  int width, height;
  boolean hasSolidBorder;

  // Non-default constructor
  public Rectangle(int width,
                  int height,
    boolean hasSolidBorder){
      this.width = width;
      this.height = height;
      this.hasSolidBorder =
                hasSolidBorder;
  }

 // No defined default constructor.
}

// o.K.: Using non-default
// constructor.

Rectangle r = 
   new Rectangle(3, 6, false);

// Wrong: Default constructor
// undefined, but non-default
// constructor present.

Rectangle r = new Rectangle();

Objects and Classes
- ➟ Object methods
  - ➟ Scopes
    
    Class, method and block scopes.
    
    Documentation, «good» variable names and the this keyword.

/** Representing circles.
 */
public class Circle {
  private double radius;

  /** Creating a circle.
   *  @param r representing the circle's radius
   */
  public Circle(double r) {
    radius = r;
  }
  public double getDiameter() {
    return 2 * radius;
  }
}

/** Representing circles.
 */
public class Circle {
  private double radius;

  /** Creating a circle.
   *  @param radius Circle's size
   */
  public Circle(double radius) {
    radius = radius; // Warning: Variable 'radius' is assigned to itself.
  }
  public double getDiameter() {
    return 2 * radius;
  }
}

Constructors variable names and “this”.

Objects and Classes
- ➟ Class members and methods
  
  Motivating class attributes and methods.
  
  Example: A club and its affiliated members.
  
  Syntax of static and employment considerations

public class Helper {
  // Find the larger of two values
  public int maximum(int a, int b) {
    if (a < b) {
      return b;
    } else {
      return a;
    }
  }
}

final Helper instance = new Helper();

// Why do we need an instance just for
// computing the maximum of two values?
IO.println("Maximum: " +
            instance.maximum(-3, 5));

Maximum: 5

Observation: The instance's state is irrelevant for finding the maximum of two values.

public class Helper { ❶

  static ❷ public int maximum(int a, int b)
   {
    if (a < b) {
      return b; 
    } else {
      return a;
    }
  }
}

// No instance required any longer
IO.println("Maximum: " +
     Helper.maximum(-3, 5)); ❶

Maximum: 5

Each club member has got a name.
Each member has got an ascending unique membership number.
The overall club's member count needs to be accounted for.

Solution:

Class level: Advance club's member count by each new member.
Instance level: New members receive name and current member count plus 1.

public class ClubMember {

  final private String name;

  public ClubMember(final String name) {
    this.name = name;
  }
  public String toString() {
    return "Member " + name;
  }
}

final ClubMember
   john = new ClubMember("John"),
  karen = new ClubMember("Karen");

IO.println(john.toString());
IO.println(karen.toString());

Member John
Member Karen

public class ClubMember {

  static ❶ private int memberCount = 0;

  final private int memberNumber; ❷
  final private String name;

  public ClubMember(final String name) {
    this.name = name;
    memberNumber = ++memberCount; ❸
  }
  public String toString() {
    return "Member " + name + ", member number " +  memberNumber ❹;
  }
}

final ClubMember
   john = new ClubMember("John"),
  karen = new ClubMember("Karen");

IO.println(john); ❶ // toString() is being
IO.println(karen);   // called implicitly

Member John, member number 1
Member Karen, member number 2

public class ClubMember {

  static private int memberCount = 0;
  ...
  public ClubMember(final String name) {
    this.name = name;
    memberNumber = ++memberCount;
  }
  static public int getMemberCount() ❶ {
    return memberCount; ❷
  }
  ...
}

final ClubMember
   john = new ClubMember("John"),
  karen = new ClubMember("Karen"),
  petra = new ClubMember("Petra");

IO.println(karen.toString());

IO.println("Club's member count:"
   + ClubMember.getMemberCount());
// Good: Prevent tampering memberCount
// variable.

Member Karen, member number 2
Club's member count:3

Class Variable: {class name}.{variableName}
Class Method: {class name}.{methodName}([parameter])

public class X {
         int a; ❶
  static int b; ❷

❶	Variable `a` defined once per instance of class `X`.
❷	Variable `b` defined once per class `X`.

   ❶   ❷    ❸
IO.print(...)

❶	Class `System` in package java.lang.
❷	Class variable (static) out of type `PrintStream` in class `System`
❸	One of 9 overloaded methods in class `PrintStream`.

Objects and Classes
- ➟ Maven project dependencies
  
  Cross project dependencies.
  
  Repositories and project artifacts.

Automated third-party class import and dependency management
Executing automated tests
Complete project lifecycle:

compile, test, execute, package, deploy
Extensible plugin architecture

Source: CreatePdf.java PDF output

import com.itextpdf.kernel.pdf.PdfDocument;
...
final PdfWriter writer = new PdfWriter("helloWorld.pdf");

final PdfDocument pdf = new PdfDocument(writer);

final Document document = 
   new Document(pdf, PageSize.A8.rotate());

document.add(new Paragraph("Hello World!").
     setFontColor(ColorConstants.MAGENTA).
     setFontSize(20));

document.close();

<project ...>
  ...
  <dependencies>
    <dependency>
      <groupId>com.itextpdf</groupId>
      <artifactId>itext-core</artifactId>
      <version>8.0.3</version>
      <type>pom</type>
    </dependency>
  </dependencies>
...

</project>

mvn dependency:tree 
...
 --- dependency:3.6.1:tree (default-cli) @ hellopdf ---
 de.hdm_stuttgart.mi:hellopdf:jar:0.9
 \- com.itextpdf:itext-core:pom:8.0.2:compile
    +- com.itextpdf:barcodes:jar:8.0.2:compile
    |  \- org.slf4j:slf4j-api:jar:1.7.36:compile
    +- com.itextpdf:font-asian:jar:8.0.2:compile
    +- com.itextpdf:forms:jar:8.0.2:compile
    +- com.itextpdf:hyph:jar:8.0.2:compile
    +- com.itextpdf:io:jar:8.0.2:compile
    |  \- com.itextpdf:commons:jar:8.0.2:compile
    +- com.itextpdf:kernel:jar:8.0.2:compile ...

CDN: Maven Central
Company local: Sonatype Nexus, e.g. MI Maven repository

Dealing with IBAN numbers

Blueprints for projects.
Based on the pom.xml file:

Project Object Model
Initial project creation.
CLI and IDE support.
Extensibility by archetype catalogs.

mvn archetype:generate
[INFO] Scanning for projects...
...
1: remote -> am.ik.archetype:elm-spring-boot-blank-archetype ...
2: remote -> am.ik.archetype:graalvm-blank-archetype ...
3: remote -> am.ik.archetype:graalvm-springmvc-blank-archetype ...
...
2083: remote -> org.apache.maven.archetypes:maven-archetype-quickstart...
...
3330: remote -> za.co.absa.hyperdrive:component-archetype_2.12 (-)
Choose a number or apply filter ...

Helper.java pom.xml

package de.hdm_stuttgart.common;
public class Helper {
  static public long
    factorial(int n) {
    long result = 1;
    for (int i = 2;
        i <= n; i++) {
      result *= i;
    }
    return result;
  }
}

<project xmlns="http://maven.apache.org...>
     ...
 <groupId>de.hdm-stuttgart</groupId>
 <artifactId>common</artifactId>
 <version>1.0</version>
     ...
</project>

goik@goiki Helper> mvn install
    ...
[INFO] Building common 1.0
    ...
[INFO] Installing .../target/common-1.0.jar to
  /home/goik/.m2/repository/de/hdm-stuttgart/common/1.0/common-1.0.jar

goik@goiki tmp> unzip  .../de/hdm-stuttgart/common/1.0/common-1.0.jar
Archive:  /home/goik/.m2/repository/de/hdm-stuttgart/common/1.0/common-1.0.jar
     ...
   creating: de/
   creating: de/hdm-stuttgart/
   creating: de/hdm-stuttgart/common/
  inflating: de/hdm-stuttgart/common/Helper.class  
   creating: META-INF/maven/
   creating: META-INF/maven/de.hdm-stuttgart/
   creating: META-INF/maven/de.hdm-stuttgart/common/
  inflating: META-INF/maven/de.hdm-stuttgart/common/pom.xml  
  inflating: META-INF/maven/de.hdm-stuttgart/common/pom.properties  
    ...
  inflating: org/apache/logging/log4j/core/AbstractLifeCycle.class  ...

<project ...>
  ...
  <groupId>de.hdm-stuttgart</groupId>
  <artifactId>lottery</artifactId>
  <version>0.9</version>

  <dependencies>
    <dependency>
      <groupId>de.hdm-stuttgart</groupId>
      <artifactId>common</artifactId>
      <version>1.0</version>
    </dependency>
...
</project>

static public long binomial(int n, int k) {
  return (Helper.factorial(n) / Helper.factorial(k)
                              / Helper.factorial(n - k));
}

static void main() {
  IO.println("There are " + binomial(5, 2) +
      " ways to draw 2 out of 5 numbers");

  IO.println("There are " + binomial(49, 6) +
      " ways to draw 6 out of 49 numbers");
}

Objects and Classes
- ➟ Maven project dependencies
  - ➟ Maven command line usage

Download file mavenTemplate.zip from here.
Extract mavenTemplate.zip to folder template.
Optional: Edit template/pom.xml reflecting your project needs i.e. <groupId> and related.
Optional: Import your project in Intellij IDEA.

> mvn --batch-mode -e archetype:generate \
-DgroupId=de.hdm_stuttgart.mi.sd1 -DartifactId=second -Dversion=0.9 \
-DarchetypeGroupId=org.apache.maven.archetypes -DarchetypeArtifactId=maven-archetype-quickstart -DarchetypeVersion=1.4

[INFO] Scanning for projects...
      ...
[INFO] BUILD SUCCESS ...

See artifact reference.

MI nexus repository server

Configuration:

mkdir -p ~/.m2 ❶
nano ~/.m2/settings.xml ❷

> mvn --batch-mode -e archetype:generate \
  -DgroupId=de.hdm_stuttgart.mi.sd1 -DartifactId=second -Dversion=0.9 \
  -DarchetypeGroupId=de.hdm_stuttgart.mi -DarchetypeArtifactId=mi-maven-archetype-quickstart -DarchetypeVersion=2.5
[INFO] Error stacktraces are turned on.
[INFO] Scanning for projects...
      ...
[INFO] BUILD SUCCESS ...

> mvn --batch-mode ❶ -e archetype:generate ❷ \
                                              \
 -DarchetypeGroupId=de.hdm_stuttgart.mi \ ❸
 -DarchetypeArtifactId=mi-maven-archetype-quickstart \
 -DarchetypeVersion=2.5 \
                        \
 -DgroupId=de.hdm_stuttgart.mi.sd1 ❹ \
 -DartifactId=second  \
 -Dversion=0.9

> tree -a
├── dependency-reduced-pom.xml
├── .gitignore
├── pom.xml ❶
├── Readme.md
└── src
    ├── main
    │   ├── java
    │   │   └── de
    │   │       └── hdm_stuttgart
    │   │           └── mi
    │   │               └── sd1
    │   │                   ├── App.java ❷
    │   │                   ├── HighlightSample.java
    │   │                   └── Statistics.java
    │   └── resources
    │       └── log4j2.xml
    └── test
        └── java
            └── de
                └── hdm_stuttgart
                    └── mi
                        └── sd1
                            └── AppTest.java

DNS inflicted groupId / package names clashes

> mvn compile
[INFO] Scanning for projects...
       ...
[INFO] Building second 0.9
   ...
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 1 source file to /ma/goik/second/target/classes
[INFO] --------------------------------------------------------
[INFO] BUILD SUCCESS

> tree target/classes
target/classes/
├── de
│   └── hdm_stuttgart
│       └── mi
│           └── sd1
│               ├── App.class
│               ├── HighlightSample.class
│               └── Statistics.class
└── log4j2.xml

> mvn package
[INFO] -------------------< de.hdm_stuttgart.mi.sd1:second >-------------------
[INFO] Building second 0.9
...
[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running de.hdm_stuttgart.mi.sd1.AppTest
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.253 s -- in de.hdm_stuttgart.mi.sd1.AppTest
...
[INFO] Building jar: /home/goik/Downloads/second/target/second-0.9.jar
...
[INFO] BUILD SUCCESS

> java -jar target/second-0.9.jar
Hi there, let's have
fun learning Java!

2026-01-11 07:40:33,903 INFO  [main] sd1.App (App.java:27) - The following two statements will log to file A1.log, ...
2026-01-11 07:40:33,905 DEBUG [main] sd1.App (App.java:28) - You may configure 'src/main/resources/log4j2.xml' 
2026-01-11 07:40:33,905 DEBUG [main] sd1.App (App.java:29) - for adapting both console and 'A1.log' file output

Remark: This will execute App.class being contained in second-0.9.jar.

Details on execution

> mvn javadoc:javadoc
[INFO] Scanning for projects...
...
Generating /ma/goik/second/target/site/apidocs/allclasses-noframe.html...
Generating /ma/goik/second/target/site/apidocs/index.html...
Generating /ma/goik/second/target/site/apidocs/overview-summary.html...
Generating /ma/goik/second/target/site/apidocs/help-doc.html...

See e.g. class String documentation.

> mvn clean
...
[INFO] 
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ second ---
[INFO] Deleting /ma/goik/second/target
[INFO] ------------------------------------------------------------------------
[

Objects and Classes
- ➟ Maven project dependencies
  - ➟ Intellij IDEA on top of Maven

Built in Maven support in Intellij IDEA.
MI supplementary archetypes require MI archetype configuration in ~/.m2/settings.xml.

Your Maven project depends on its pom.xml file.
Reloading changed pom.xml file.

Objects and Classes
- ➟ Unit testing
  
  Test categories.
  
  Why unit testing? Idea of test «driven development».
  
  Required steps.

Unit test: Test individual methods, classes and packages in isolation.
Integration Test: Test a group of associated components/classes.
Acceptance / Functional Test: Operate on a fully integrated system, testing against the user interface.
Regression Test: Ensure system integrity after (implementation) change.
Load test: Responsiveness vs. system load.

Informal problem specification:

A prime number is a whole number greater than 1 whose only factors are 1 and itself.

Sample values: 2, 3, 5, 7, 11, 13, 17, 23, ...

First write tests, then implement.

Specify but not yet implement classes / methods.
Write skeleton (dummy) implementations.
Write corresponding unit tests.
Implement skeleton.
Test your implementation.

Objects and Classes
- ➟ Unit testing
  - ➟ Tests and implementation
    
    Providing both an implementation and related tests possibly revealing coding flaws.
    
    Providing prime number related tests.

/**
 * Identifying prime numbers.
 */
public class Prime {
  /**
   * Check whether a given integer candidate is prime or not ❶
   * @param candidate A positive integer value
   * @return true if and only if candidate is a prime number.
   */
  public static boolean isPrime(int candidate) {
    return true ❷; //TODO: Dummy value to be implemented correctly 
  }
}

Code

for (int i = 1; i < 20;i++) {
  IO.println(i + " is " + 
      (Prime.isPrime(i) ? " a " : " not a ") + " prime number");
}

Result

1 is a prime number
2 is a prime number
3 is a prime number
4 is a prime number
5 is a prime number
...

Input	Expected output	Input	Expected output
`1`	`false`	`7`	`true`
`2`	`true`	`8`	`false`
`3`	`true`	`9`	`false`
`4`	`false`	`10`	`false`
`5`	`true`	`11`	`true`
`6`	`false`	`12`	`false`

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test; 

public class PrimeTest {

  @Test ❶ public void testIsNotPrime() {
   ❷ Assertions.assertFalse(Prime.isPrime(1));
  }
  @Test ❶ public void testIsPrime() {
    Assertions.assertTrue(Prime.isPrime(2));
  }
  void someOrdinaryMethod() ❸ {...}
...

❶

A @Test annotation triggers automatic method execution by the Junit framework.

❷

The Assertion class hosts multiple test methods e.g. assertFalse(boolean) and assertTrue(boolean).

See Assertions for details.

❸

Any methods not being annotated by @Test will not be called by the Junit framework. They may however be called indirectly by test methods e.g. testIsNotPrime() thus acting as helper methods.

goik@goiki Prime_v01> mvn test
...
[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running de.hdm_stuttgart.mi.sd1.PrimeTest
[ERROR] Tests run: 5, Failures: 3, Errors: 0, Skipped: 0, Time elapsed: 0.063 s <<< FAILURE! -- in de.hdm_stuttgart.mi.sd1.PrimeTest
[ERROR] de.hdm_stuttgart.mi.sd1.PrimeTest.testOddNonPrimes -- Time elapsed: 0.030 s <<< FAILURE!
org.opentest4j.AssertionFailedError: expected: <false> but was: <true>
        at org.junit.jupiter.api.AssertionFailureBuilder.build(AssertionFailureBuilder.java:158)
...

Code

@Test public void testIsNotPrime() {
    Assertions.assertFalse(Prime.isPrime(1), "1 is no prime number");
}

@Test public void testIsPrime() {
    Assertions.assertTrue(Prime.isPrime(2), "2 is a prime number");
}

Result

org.opentest4j.AssertionFailedError: 1 is no prime number ==> 
Expected :false
Actual   :true

@Test public void test_Primes() {
  Assertions.assertTrue(Prime.isPrime(3));
  Assertions.assertTrue(Prime.isPrime(5));
  Assertions.assertTrue(Prime.isPrime(7));
  Assertions.assertTrue(Prime.isPrime(11));
 ...  }
@Test public void testOddNonPrimes() {
  Assertions.assertFalse(Prime.isPrime(9));
  Assertions.assertFalse(Prime.isPrime(15));
  Assertions.assertFalse(Prime.isPrime(21));
...}

@Test public void testEvenNonPrimes() {
  for (int i = 2; i < 100; i++) {
    Assertions.assertFalse(Prime.isPrime(2 * i));
  }
}

public static boolean isPrime(int candidate) {
  for (int i = 2; i < candidate; i++) {
    if (0 == candidate % i) { // i divides value
      return false;
    }
  }
  return candidate != 1;
}

goik@goiki Prime_v01> mvn test
...
[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running de.hdm_stuttgart.mi.sd1.PrimeTest
[INFO] Tests run: 5, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.047 s -- in de.hdm_stuttgart.mi.sd1.PrimeTest
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 5, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------

Objects and Classes
- ➟ Unit testing
  - ➟ Improving the algorithm
    
    Try improving the algorithm.
    
    Introducing an error.

101 / 2  = 50 remainder 1
101 / 3  = 33 remainder 2
101 / 4  = 25 remainder 1
101 / 5  = 20 remainder 1
101 / 6  = 16 remainder 5
101 / 7  = 14 remainder 3
101 / 8  = 12 remainder 5
101 / 9  = 11 remainder 2
101 / 10  = 10 remainder 1
101 / 11  = 9 remainder 2
101 / 12  = 8 remainder 5
101 / 13  = 7 remainder 10
...

Big performance gain:

public static boolean isPrime(int candidate) {
  for (int i = 2; i * i < candidate; i++) {
    if (0 == candidate % i) {
      return false;
    }
  }
  return candidate != 1;
}

public static boolean isPrime(int candidate) {

//for (int i = 2; i * i < candidate; i++) {
  for (int i = 2; i * i <= candidate; i++) {
    if (0 == candidate % i) {
      return false;
    }
  }
  return candidate != 1;
}

Objects and Classes
- ➟ Unit testing
  - ➟ Parameterized testing

Separate both test parameters and expected values from code.
Provide parameter values either by variables or by separate CSV files.
Read Parameterized Classes and Tests for details.

Hard-coded

public void testPrimes() {
  Assertions.assertEquals(false, Prime.isPrime(1));
  Assertions.assertEquals(true,  Prime.isPrime(2));
  Assertions.assertEquals(true,  Prime.isPrime(3));
  Assertions.assertEquals(false, Prime.isPrime(4));}

Parameterized

@ParameterizedTest
@CsvSource( textBlock = """
   false, 1
   true,  2
   true,  3
   false, 4""")
public void testPrimes(boolean expected, int primeCandidate) {
  Assertions.assertEquals(expected, Prime.isPrime(primeCandidate));         
}

mvn test ─ de.hdm_stuttgart.mi.sd1.PrimeTest - 0.098 s
           ├─ ✔ testPrimes(int, boolean)[1] 1, false - 0.035 s
           ├─ ✔ testPrimes(int, boolean)[2] 2, true - 0.001 s
           ├─ ✔ testPrimes(int, boolean)[3] 3, true - 0.001 s
           └─ ✔ testPrimes(int, boolean)[4] 4, false - 0.001 s

@ParameterizedTest
@CsvFileSource(resources = "/primes.csv")
public void testPrimesByFile 
  (boolean expected, int primeCandidate, ) {
     Assertions.assertEquals(
        expected
       ,Prime.isPrime(primeCandidate)
     );
}

File test/resources/primes.csv:

false, 1
true,  2
true,  3
false, 4

mvn test ─ de.hdm_stuttgart.mi.sd1.PrimeTest - 0.098 s
           ├─ ✔ testPrimesByFile(int, boolean)[1] false, 1  - 0.031 s
           ├─ ✔ testPrimesByFile(int, boolean)[2] true,  2 - 0.001 s
           ├─ ✔ testPrimesByFile(int, boolean)[3] true,  3 - 0.001 s
           └─ ✔ testPrimesByFile(int, boolean)[4] false, 4 - 0.001 s

Objects and Classes
- ➟ Unit testing
  - ➟ Helpful Junit methods
    
    Selected assertion methods.
    
    Considerations when comparing float and double values.

org.opentest4j.AssertionFailedError: expected: <3.6> but was: <3.5999999999999996>
            ...
        at de.hdm_stuttgart.AppTest.test1_3(AppTest.java:35)

IO.println("3.6 - 3 * 1.2 == " + (3.6 - 3 * 1.2));

Result:

3.6 - 3 * 1.2 == 4.440892098500626E-16

public class doubleCompareTest {
  static final double delta = 1E-15;
  /**
   * Comparing 3.6 and 3 * 1.2 within delta's limit
   */
  @Test
  public void test1_3() {
    Assertions.assertEquals(3.6, 3 * 1.2 , delta);
  }
}

Objects and Classes
- ➟ Unit testing
  - ➟ Technical details
    
    Junit Building Blocks: Classes and interfaces
    
    Integration using Maven.

public
@interface @Test {
...
}

@interface defining an annotation.
Purpose: Adding meta information for automated detection of test methods.

public class Assertions {

  public static void assertTrue(boolean condition, String message) {         
      ...
  }

  public static void assertEquals(long expected, long actual) {         
      ...
  }
  ...
}

<project ...>
...
  <dependencies>
    <dependency>
      <groupId>org.junit.jupiter</groupId>
      <artifactId>junit-jupiter-api</artifactId>
      <version>6.0.2</version>
      <scope>test</scope>
    </dependency>
    ...
  </dependencies>
  ...
</project>

Local: /home/goik/.m2/repository/org/junit/jupiter/junit-jupiter-api/6.0.2/junit-jupiter-api-6.0.2.jar

Remote: https://mvnrepository.com/artifact/org.junit.jupiter/junit-jupiter-api

> jar -tf junit-jupiter-api-6.0.2.jar
META-INF/
META-INF/MANIFEST.MF
  ...
org/
org/junit/
org/junit/jupiter
org/junit/jupiter/api
  ...
org/junit/jupiter/api/Assertions.class
  ...

Objects and Classes
- ➟ Value types and reference types
  
  Primitive and value types revisited.

Value type: Holding data in associated memory location

byte
short

int
long

float
double

char
boolean

Reference type: Holding a reference to an object

Array or class instances i.e. String, java.util.Scanner etc.

Value type

int a = 1;
int b = a;
a = 5;
IO.println("a=" + a);
IO.println("b=" + b);

a=5
b=1

Reference type

StringBuffer r = new StringBuffer("Joe");
StringBuffer s = r;
r.append(" Simpson");
IO.println("r=" + r);
IO.println("s=" + s);

r=Joe Simpson
s=Joe Simpson

static void main() {
   int value = 300;
      sub(value);
      IO.println("value = " + value);
}

static void sub(int n) {
   n = 1000;
   IO.println("n =  " + n);
}

n =  1000
value = 300

static void main() {
  StringBuffer buffer = new StringBuffer("My");
  IO.println("Before duplicateString: "
    + buffer);
  duplicateString(buffer);
  IO.println("After duplicateString: "
    + buffer);
}
static void duplicateString(StringBuffer b) {
    b.append(b); // Append self
}

Before duplicateString: My
After duplicateString: MyMy

static void main() {
  StringBuffer buffer = new StringBuffer("My");
  IO.println("Before duplicateString: "
    + buffer);
  replaceString(buffer);
  IO.println("After duplicateString: "
    + buffer);
}
static void replaceString(StringBuffer b) {
  b = new StringBuffer("Replacement");
}

Before duplicateString: My
After duplicateString: My

int a = 1;

int &b = a;  

cout << a << " : " << b << endl;

a = 5;

cout << a << " : " << b << endl;

1 : 1
5 : 5

// Passing a reference
// to variable n
void printDuplicateValue(int& n) {
  n = 2 * n;
  cout << "duplicateValue: " << n 
       << endl;
}
int main() {
  int value = 3;
  cout << "Before call: "
    << value << endl;
  printDuplicateValue(value);
  cout << "After call: "
    << value << endl;
}

Before call: 3
duplicateValue: 6
After call: 6

Objects and Classes
- ➟ Arrays reconsidered

Combining array allocation and value assignment:

public class Rectangle {
    private int width, height;
    private boolean hasSolidBorder;
    public Rectangle(int width, int height,
                   boolean hasSolidBorder) {
        this.width = width;
        this.height = height;
        this.hasSolidBorder = hasSolidBorder;
    }
}

final Rectangle[] rectList = new Rectangle[] {
  new Rectangle(2, 5, true),
  new Rectangle(4, 1, false)
};

...println("        String: " + "".getClass().getTypeName());
...println("         int[]: " + new int[]{}.getClass().getTypeName());
...println("      double[]: " + new double[]{}.getClass().getTypeName());
...println("     boolean[]: " + new boolean[]{}.getClass().getTypeName());
...println("StringBuffer[]: " + new StringBuffer[]{}.getClass().getTypeName());

        String: java.lang.String
         int[]: int[]
      double[]: double[]
     boolean[]: boolean[]
      String[]: java.lang.String[]
StringBuffer[]: java.lang.StringBuffer[]

static void main() {
  final int [] lectures = new int[3]; // Three lectures
  fill(lectures, 25); // Default lecture having 25 participants
  IO.println("Second lecture has got " + lectures[1] +
          " participants");
}
/**
  * Initialize array with default value.
  *
  * @param values Array to be initialized.
  * @param common New value for all array elements.
  */
static void fill(final int[] values, final int common) {
  for (int i = 0; i < values.length; i++) {
    values[i] = common;
  }
}

Second lecture has got 25 participants

// Value type
final boolean values[] = {true, true, false, true};

// Reference type
final String shapes[] = {"Triangle", "Circle"};

Objects and Classes
- ➟ Arrays reconsidered
  - ➟ Varargs method parameter

Code	`public class Q { static void main() { printInfo("Winner", 34); } public static void printInfo(String msg, int value) { IO.println(msg + ": " + value); } }`
Result	Winner: 34

Code

printInfo("Winner", 34);         // One int value
printInfo("Winner", 31, 7);      // Two int values
printInfo("Winner", 2, 8, 5);    // Three int values
---------------------------------------------------------------------------------
public static void printInfo(String msg, int value) { ...}

public static void printInfo(String msg, int value1, int value2) {
    IO.println(msg + ": " + value1 + " " + value2 );
}
public static void printInfo(String msg, int value1, int value2, int value3) {
    IO.println(msg + ": " + value1 + " " + value2 + " " + value3);
}

Result

Winner: 34
Winner: 31 7
Winner: 2 8 5

Similar code in multiply overloaded methods.
Tedious: Each added argument requires adding another method.

Objective: Get rid of boilerplate code!

Code

printInfo("Winner", new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 });
---------------------------------------------------------------------------------
public static void printInfo(String msg, int[] values) {
    IO.print(msg + ": " );
    for (int v: values) {
        IO.print(v + " ");
    }
    IO.println();
}

Result

Winner: 1 2 3 4 5 6 7 8 9

Code

printInfo("Winner", new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 }); // Array of int

printInfo("Winner", 20, 19, 18, 17, 16, 15);                  // Separate values
---------------------------------------------------------------------------------
public static void printInfo(String msg, int... values) {
    IO.print(msg + ": " );
    for (int v: values) {
        IO.print(v + " ");
    }
    IO.println();
}

Result

Winner: 1 2 3 4 5 6 7 8 9 
Winner: 20 19 18 17 16 15

Only one varargs parameter per method.
Must be a method's last argument.

Video

Java Tutorial: Method VarArgs

Text

Uncommon Java Syntax: Ellipses…
Varargs in Java
The varargs section in chapter 4 of [Kurniawan].

Objects and Classes
- ➟ Arrays reconsidered
  - ➟ java.util.Arrays helpers
    
    Sorting and searching arrays.
    
    Fill in values.
    
    Compare array contents.

final String[] names = {"Eve", "Aaron", "Paul", "Mandy"};

IO.println("     toString: " + Arrays.toString(names));

Arrays.sort(names);

IO.println("sort|toString: " + Arrays.toString(names));

Result:

     toString: [Eve, Aaron, Paul, Mandy]
sort|toString: [Aaron, Eve, Mandy, Paul]

final String[] names = {"Aaron", "Eve", "Mandy",  "Paul"};

// Precondition: Array must be ordered!
...println("sort|find(Mand): " + Arrays.binarySearch(names, "Mand"));
...println("sort|find(Mandy): " + Arrays.binarySearch(names, "Mandy"));
...println("sort|find(Mandyer): " + Arrays.binarySearch(names, "Mandyer"));

Result:

sort|find(Mand): -3
sort|find(Mandy): 2
sort|find(Mandyer): -4

Understanding search results

final String[] names = 
  {"Eve", "Aaron", "Paul", "Mandy"};

IO.println("toString: " +
   Arrays.toString(names));

Arrays.fill(names, "N.N");

IO.println("toString: " + 
    Arrays.toString(names));

toString: [Eve, Aaron, Paul, Mandy]
toString: [N.N, N.N, N.N, N.N]

final String[] names = {"Eve", "Aaron", "Paul", "Mandy"};

final String[] lastTwoNames = Arrays.copyOfRange(names, 2, 6);

IO.println("toString: " + Arrays.toString(lastTwoNames));

Result:

toString: [Paul, Mandy, null, null]

final String[]
  l1 = {new String("Eve"), new String("Aaron"), 
        new String("Paul"), new String("Mandy")},

  l2 = {new String("Eve"), new String("Aaron"), 
        new String("Paul"), new String("Mandy")},

  l3 = {new String("Eve"), new String("Aaron"), 
        new String("Paul"), new String("Mobile")};

IO.println("l1.equals(l2):" + Arrays.equals(l1, l2));
IO.println("l1.equals(l3):" + Arrays.equals(l1, l3));

Result:

l1.equals(l2):true
l1.equals(l3):false

Objects and Classes
- ➟ Arrays reconsidered
  - ➟ Extending arrays
    
    Dealing with fixed size.

final String[] member = {"Eve", "John", "Peter", "Jill"};

final String newCourseMember = "Ernest";

member.length = 5; // Error: Size unchangeable

member[4] =  newCourseMember;

static void main() {
 ❶ String[] member = {"Eve", "John", "Peter", "Jill"};
  final String newMember = "Ernest";
  member ❷= append(member, newMember);
}
static String[] append (final String[] values, final String newValue) {
  final String[] copy = ❸ new String[values.length + 1];
  for (int i = 0; i < values.length; i++) { ❹
    copy[i] = values[i]; ❺
  }
  copy[copy.length - 1] = newValue; ❻
  return copy;
}

final String[] member = {"Eve", "John", "Peter", "Jill"};
IO.println("Original array: " + Arrays.toString(member));
final String newMember = "Ernest";
member = append(member, newMember);
IO.println("Extended array: " + Arrays.toString(member));

Original array: [Eve, John, Peter, Jill]
Extended array: [Eve, John, Peter, Jill, Ernest]

static void main() {
    final int [] start = {1,7,-4},
    added = append(start, 77);
    IO.println("added: " + Arrays.toString(added));
  }
static public int[] append(final int[] values, final int newValue) {
  final int[] result = Arrays.copyOf(values, values.length + 1);
  result[values.length] = newValue;
  return result;}

Result:

added: [1, 7, -4, 77]

Objects and Classes
- ➟ Arrays reconsidered
  - ➟ Understanding static int main(String[] args)
    
    main(...) and its array argument.
    
    Implementing parameter passing.

package myapp;
public class Cmd {
  static void main(String[] args) {
    for (int i = 0; i < args.length; i++) {
      IO.println("Parameter " + (i + 1) + ": " + args[i]);
    }
  }
}

java myapp.Cmd 21 334 -13
Parameter 1: 21
Parameter 2: 334
Parameter 3: -13

❶	Entry class `myapp.Cmd` containing `... main(String[] args)`.
❷	Parameters `21`, `334` and `-13` being passed to `String[] args`.

<artifactId>maven-shade-plugin</artifactId>
...
<transformer ...>
  <manifestEntries>
    <Main-Class>myapp.Cmd</Main-Class>
  </manifestEntries>...

unzip ww-1.0-SNAPSHOT.jar
cat  tmp/META-INF/MANIFEST.MF
...
Created-By: Apache Maven 3.5.0
Build-Jdk: 1.8.0_151
Main-Class: myapp.Cmd

mvn package ...
java -jar target/ww-1.0-SNAPSHOT.jar 21 334 -13
Parameter 1: 21
Parameter 2: 334
Parameter 3: -13

Objects and Classes
- ➟ Method calls, the details
  
  Method calls, behind the scenes.
  
  Stack and heap memory.
  
  Stack frames and value propagation. Using the debugger.

public class Circle {
  static final double
    PI = 3.141592653589793;
  double r;
  /** Change a circle's area
   *  @param area The desired new area
   *  @return The circle's new radius */
  double setArea(final double area ❶) {
    double val ❷ = area  / PI ❸;
    return ❹ r ❺ = Math.sqrt(val);
  }
}

❶	Passing arguments.
❷	Defining method local variables.
❸	Accessing class variable.

❹	returning values.

❺	Accessing instance variable.

public class Circle {

  static final double
    PI ❶ = 3.141592653589793;

  double r ❷ ;

  double setArea(final double area ❸) {
    double val ❸ ...
    ...
  }
}

❶	Class scope.
❷	Instance scope
❸	Method scope

Class scope (`static`)	Application process
Instance scope	Object lifetime: `new (...)` until being garbage collected.
Method scope	Method invocation until `return`.

Heap memory

Allocation of class or array instances:

new String()

new float[200]

De-allocation subject to garbage collection.

Execution stack

One instance per process thread.
Hosting variables (values or references)

Method	Description	Precondition	Access type
`push(...)`	Add object on top	-	Modify
`pop()`	Read + remove topmost object	Stack not empty	Modify
`top()`	Read topmost object	Stack not empty	Read only
`empty()`	`true` if and only if stack is empty	-	Read only

Objects and Classes
- ➟ Enumerations (enum)

Modeling finite sets of discrete states.
Examples:

A room's door: {OPEN, CLOSED}

State of matter: {SOLID, LIQUID, GASEOUS}
No dynamic change of state set.

Objects and Classes
- ➟ Enumerations (enum)
  - ➟ Enumeration by integer representation
    
    Example: Weekdays Monday till Sunday represented by constant int values.
    
    Common pitfall examples.
    
    Insufficient means of language protection against self - inflicted problems.

public class Lecture {

    public final int dayHeld; /* e.g. to be held on Tuesdays */

    public final String title; /* e.g. «PHP introduction» */

    public Lecture(final int dayHeld, final String title) {
        this.dayHeld = dayHeld;
        this.title = title;
    }
}

Quick and dirty:

Class Driver:

final Lecture
  phpIntro     = new Lecture(1 /* Monday */, "PHP introduction"),
  advancedJava = new Lecture(5 /* Friday */, "Advanced Java");

Error prone:

Weeks start on Mondays?
Index starts with 0 or 1?

public class Day {

  static public final int
    MONDAY    = 1,
    TUESDAY   = 2,
    WEDNESDAY = 3,
    THURSDAY  = 4,
    FRIDAY    = 5,
    SATURDAY  = 6,
    SUNDAY    = 7;
}

Class Driver:

final Lecture
  phpIntro     = new Lecture(Day.MONDAY, "PHP introduction"),
  advancedJava = new Lecture(Day.FRIDAY, "Advanced Java");

public class Day {
...
  public static String getDaysName(final int day) {
    switch (day) {
      case MONDAY:    return "Monday";
      case TUESDAY:   return "Tuesday";
      case WEDNESDAY: return "Wednesday";
      case THURSDAY:  return "Thursday";
      case FRIDAY:    return "Friday";
      case SATURDAY:  return "Saturday";
      case SUNDAY:    return "Sunday";

      default:        return "Illegal day's code: " + day;
    }
  }
}

public class Lecture {
  public final int dayHeld;
    ...

  public String toString() {
    return "Lecture «" + title + "» being held each " +
           Day.getDaysName(dayHeld);
  }
}

// Class Driver
final Lecture 

  phpIntro = new Lecture(
    Day.MONDAY, "PHP introduction"),

  advancedJava = new Lecture(
    Day.FRIDAY, "Advanced Java");

IO.println(phpIntro);
IO.println(advancedJava);

Lecture «PHP introduction» 
  being held each Monday
Lecture «Advanced Java» 
  being held each Friday

// Class Screwed

final Lecture phpIntro =
    new Lecture(99, "PHP introduction");

IO.println(phpIntro);

Lecture «PHP introduction» being
held each Illegal day's code: 99

Bad: Not even a compiler warning message!

/**
* Charge double prices on weekends
* @param day Day of week
* @param amount Product costs
* @return the effective amount for
*         given day of week.
*/
static public int getPrice(
   final int day, final int amount) {
  switch (day) {
    case Day.SATURDAY:
    case Day.SUNDAY: return 2 * amount;

    default: return amount;
  }
}

// Correct
IO.println(
   getPrice(Day.SUNDAY,  2));

// Argument mismatch
IO.println(
   getPrice(2, Day.SUNDAY));

4
7

Bad: Not even a compiler warning message!

Objects and Classes
- ➟ Enumerations (enum)
  - ➟ Enumeration by dedicated class
    
    Representing enumeration values by instances gaining type safety.
    
    Partial problem solution.

Roadmap:

Define a dedicated enumeration representing class.
Create exactly one class instance per enumeration value.
Enumeration value equality comparison by virtue of the == operator.

public class Day {

  static public final Day
    MONDAY    = new Day(),
    TUESDAY   = new Day(),
    WEDNESDAY = new Day(),
    THURSDAY  = new Day(),
    FRIDAY    = new Day(),
    SATURDAY  = new Day(),
    SUNDAY    = new Day();
}

Note: Class without instance attributes.

Reverting to if .. else if ... required 🙄

public static String getDaysName(final Day day) {
  if (MONDAY == day) {  // Switch no longer possible, sigh!
    return "Monday";
  } else if (TUESDAY == day) {
      ...
  } else if (SUNDAY == day) {
    return "Sunday";
  } else {
    return "Illegal day instance: " + day;
  }
}

/**
* Charge double prices on weekends
* @param day Day of week
* @param amount Product costs
* @return the effective amount for given day of week.
*/
static public int getPrice(final Day day, final int amount) {
  if (Day.SATURDAY == day || Day.SUNDAY == day) {
    return 2 * amount;
  } else {
    return amount; }}

IO.println(Screwed2.getPrice(Day.SUNDAY,  2));   // Class Driver, o.K.
IO.println(Screwed2.getPrice(2, Day.SUNDAY));  // Argument mismatch, compile time error

Class Screwed:

final Day PAST_SUNDAY = new Day();

final Lecture phpIntro = new Lecture(
     PAST_SUNDAY, "PHP introduction");

IO.println(phpIntro.toString());

Lecture «PHP introduction» being
held each Illegal day instance: 
de.hdm_stuttgart.mi.sd1.
class_wrapper.Day@63961c42

Objects and Classes
- ➟ Enumerations (enum)
  - ➟ Defining a private constructor
    
    Clearing the mess.
    
    Sad: switch still not working.

public class Day {

  // Disallow object creation outside class

  private Day() {} 

  static public final Day
    MONDAY    = new Day(),
    TUESDAY   = new Day(),
  ...
    SUNDAY    = new Day();
}

Class Screwed:

Day PAST_SUNDAY = new Day();

Lecture phpIntro = new Lecture(
     PAST_SUNDAY, "PHP introduction");

IO.println(phpIntro.toString());

Compile time error:

'Day()' has private access in
'de.hdm_stuttgart.mi.sd1.
class_wrapper_private.Day'

public class Day {

  static public final Day
    MONDAY    = new Day("Monday"),
  ...
    SUNDAY    = new Day("Sunday");

  public final String name;

  private Day(final String name) {
    this.name = name;
  }

  public String toString() { return name; }
}

Objects and Classes
- ➟ Enumerations (enum)
  - ➟ enum replacing class
    
    Introducing enum as kind of restricted class.
    
    Makes switch working again!

public class Day {

  static public final Day
    MONDAY    = new Day("Monday"),
             ...
    SUNDAY    = new Day("Sunday");

  public final String name;

  private Day(final String name) {       
    this.name = name;
  }
    public String toString() { 
      return name; }
}

public enum Day {

  MONDAY("Monday"),
  ...
  SUNDAY("Sunday");

  final String name;

  Day(final String name) {
     this.name = name;
  }
  public String toString() {       
    return name;}
}

public enum Day {
...
  public static String getItalianDayName(final Day day) {
    switch (day) {
      case MONDAY:    return "Lunedì";
      case TUESDAY:   return "Martedì";
   ...
      case SUNDAY:    return "Domenica";
    }
    return null; // Actually unreachable, but static compiler code analysis is limited
  }
}

public enum Day {
  ...
  private Day(final String name)
    { ...

Compile time warning:

Modifier 'private' is redundant for enum constructors

public enum Day {
  ...
  public Day(final String name)
    { ...

Compile time error:

Modifier 'public' not allowed here

Prohibits enum external instance creation.

Overview

A method adding two values

Syntax of method definition

The static modifier matters!

Documentation: Javadoc™ is your friend!

Separating usage and definition

Followup exercises

Overview

Instances of a Class

General class structure

What's a class anyway?

Rectangle objects

A class describing rectangles

Rectangle class and instances

Rectangle class and instances

Rectangle class and instances

Rectangle class and instances

Rectangle class and instances

Generated diagrams

The new operator: Creating rectangle instances

Syntax creating instances

Assigning attribute values to class instances

Instance memory representation

References and null

Checking for object presence

Overview

Why packages ?

Rules and conventions

Fully qualified class name vs. import

Don't be too lazy!

Special: Classes in package java.lang

Class, package and file system

Source hierarchy view

Overview

Object methods

Get a rectangle's area

Get a rectangle's area

Get a rectangle's area

Get a rectangle area method coding

Scaling a rectangle

Scaling a rectangle

Scaling a rectangle

Scaling method implementation

Scaling method signature

Using the scale(...) method

Method definition syntax

A rectangle's perimeter

A rectangle's perimeter

A rectangle's perimeter

getPerimeter() method implementation

Using Rectangle.getPerimeter()

Followup exercises

Overview

Access control: Overall objectives

Example: Two ways implementing a day's time

private modifier access violation

Absent access modifier, access violation

Access rules

“Tips on Choosing an Access Level”

Followup exercises

Overview

Direct access vs. setter method

Why adding setter methods?

Implementation change: Minutes only, no hours

Followup exercise

Overview

Defining type signatures

Type signature examples

Defining method signatures

Method signature examples: Ignore return type!

Followup exercise

Overview

Method overloading: Same name, different signature

Overloading, alternate names

No overloading in »C«

»C« requires unique function names

No distinction on return type

Method signatures rationale

Method signatures rationale

Followup exercise

The `static` modifier matters!

The `new` operator: Creating rectangle instances

References and `null`

Fully qualified class name vs. `import`

Using the `scale(...)` method

`getPerimeter()` method implementation

Using `Rectangle`.`getPerimeter()`

`private` modifier access violation

`Circle` and variable scopes

`Circle` and variable scopes

`Circle` and variable scopes

`Documenting classes and methods`

`Generated Javadoc`

`Generated Javadoc`

`Generated Javadoc`

`Generated Javadoc`

`Generated Javadoc`

`Refactoring «r» ⇒ «radius»`

`Refactoring «r» ⇒ «radius»`

`Refactoring «r» ⇒ «radius»`

`Scope assignment problem`

`this` overriding method scope

`this` overriding method scope

`this` overriding method scope

`this` overriding method scope

`this` overriding method scope

Finally understanding `IO.print(ln)`