Working with objects

Figure 194. Instances of a Class

Figure 195. General class structure

Figure 196. What's a class anyway?

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.

Figure 197. Rectangle objects

Figure 198. A class describing rectangles

public class Rectangle {
  int width;
  int height;

  // solid or dashed:
  boolean hasSolidBorder;
}

Figure 199. Rectangle class and instances

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Figure 200. Generated diagrams

Figure 201. The new operator: Creating rectangle instances

Rectangle dashedRectangle = new Rectangle();

Rectangle solidRectangle = new Rectangle();

...

Figure 202. Syntax creating instances

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

Wording examples:

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

Figure 203. Assigning attribute values to class instances

Rectangle dashedRectangle = new Rectangle();

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

Syntax accessing object attributes:

variable.attributeName = value;

Figure 204. Instance memory representation

Figure 205. References and null

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)

Figure 206. Checking for object presence

Rectangle r;

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

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

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