• Inheritance

final double x = 2, y = 3;
final Shape shape = new Shape(x, y);

final double width = 2, height = 5;
final Rectangle r = new Rectangle(x, y , width, height);

final double radius = 2.5;
final Circle circle = new Circle(x, y , radius);

/**
  * Creating a shape located at center coordinate.
  * @param x The center's x component.
  * @param y The center's y component.
  */
public Shape(double x,double y) {
  this.x = x;
  this.y = y;
}

final Rectangle r = 
   new Rectangle(x, y ❶,
                 width, height ❷);

Solution: Nested constructor call. Coming soon ...

/**
  * Creating a rectangle at (x|y) of given width and height.
  * @param x Center's x component.
  * @param y Center's y component.
  * @param width Rectangle's width.
  * @param height Rectangle's height.
  */
public Rectangle(double x, double y,
             double width, double height) {
  super(x, y) ❶;
  this.width = width; this.height = height ❷;
}

• Inheritance
  • ➟ Overriding equals() and hashCode()

public abstract class Shape {
  private double x, y;
     ...
  @Override ❶ public boolean equals(final Object o) {
    if (o instanceof Shape s ❷) { 
      return x == s.x && y == s.y; ❸
    } else {
      return false; ❹
    } ...

public class Rectangle extends Shape {
  ...
  @Override public boolean equals(final Object o) {
    if (o instanceof Rectangle r) {
      return super.equals(o) ❶ &&
             width == r.width && height == r.height ❷;
    } else {
      return false;
    } ...

168. Let me pass, please!
169. Why is == correctly comparing enum instances?

• Inheritance
  • ➟ Overriding toString()

#public class Rectangle extends Shape {
  @Override public String toString() {
    return "Rectangle at " + super.toString() ❶ +
      ", width= " + width + ", height=" + height; ❷
  } ...

public class Circle extends Shape {
  /**
   * Creating a circle of given center and radius
   * @param x Center's x component.
   * @param y Center's y component.
   * @param radius The circle's radius.
   */
  public Circle(double x,double y, double radius) {
    super(x, y);
    this.radius = radius;
  }
  @Override public String toString() {
    return "Circle at " + super.toString() +", radius= " + radius;
  }
  private double radius;
}

170. String vs. StringBuffer
171. Alternate implementation of opposite directions

• Inheritance
  • ➟ final methods

public class Shape {
  /**
   * Move by a given translation vector
   * @param xTrans Translation's x component
   * @param yTrans Translation's y component
   */
  public void move(final int xTrans, final int yTrans) {
    x += xTrans;
    y += yTrans;
  } ...

public class Rectangle extends Shape {
  @Override public void move(int xTrans, int yTrans) {
    // I'm so dumb!
    ...
  }

public abstract class Shape {
... public final void move(final int xTrans, final int yTrans) {
      x += xTrans;
      y += yTrans;
    }...

public class Rectangle extends Shape {
  // Syntax error: 'move(int, int)' cannot override
  // 'move(int, int)' in 'inherit.Shape'; overridden method is final
  @Override public void move(int xTrans, int yTrans) {...

• Inheritance
  • ➟ Abstract methods

final Shape[] shapes ❶ = {
    new Circle(1, 1, 2.) ❷,
    new Rectangle(1, -1, 2., 3.)❷};

for (final Shape s : shapes) {
  System.out.println(s.toString() + ": area = " + s.getArea()); ❸
}

Circle at (1.0|1.0), radius= 2.0: area = 12.566370614359172
Rectangle at (1.0|-1.0), width= 2.0, height=3.0: area = 6.0

Solution: Abstract method getArea() in superclass Shape.

final Shape s =
   new Shape(1., 2.); // 'Shape' is abstract; cannot be instantiated

// Error: Class 'Circle' must either be declared abstract or
//        implement abstract method 'getArea()' in 'Shape'
public class Circle extends Shape {

  public Circle(double x,double y, double radius) {
    super(x, y);
    this.radius = radius;
  }
  private double radius;
}

• Inheritance
  • ➟ Abstract methods
    • ➟ Geometry classes reconsidered

172. Defining a Shape class hierarchy
173. Scaling shapes
174. Providing toString() methods

• Inheritance
  • ➟ protected access

package model;
public abstract class Shape ❶{

  final protected long creationTime ❷= System.nanoTime();
...
}
------------------------------------------------
package model.sub;
public class Rectangle ❸extends Shape {
  static final Logger log = LogManager.getLogger(Rectangle.class);

  @Override public double getArea() {
    log.info("Rectangle creation time:" + creationTime ❹);
    return width * height;
  } ...
}

175. protected vs. “package private”
176. protected access involving different instances

• Inheritance
  • ➟ final classes

public final class Shape { ... }

-------------------------
public class Rectangle
  extends Shape { // Error: final class cannot be extended
...

• Inheritance
  • ➟ The instanceof operator

Rectangle r1 = new Rectangle(1, 2, 5, 4),
          r2 = new Rectangle(1, 2, 1, 7),
          r3 = new Rectangle(1, 2, 5, 4);

Circle c = new Circle(-2, 3, 5);

System.out.print(r1.equals("Hi"));//false: Differing classes Rectangle and String.
System.out.print(r1.equals(r2));  //false: Differing width and height.
System.out.print(r3.equals(r1));  //true:  Two rectangles having identical
                                  //       (x|y), width and height.
System.out.print(r1.equals(c));   //false: Differing classes Rectangle and Circle.
System.out.print(c.equals(c));    //true: Object equal to itself.

Two Shape instances shall be considered equal if:

public abstract class Shape {
  private double x, y;

  protected boolean equalCenter(final Shape o) {
      return Math.abs(o.x - x) + Math.abs(o.y - y) < 1.E-15;
  }
...

public class Rectangle extends Shape {
    @Override public boolean equals(Object o) {
    if (o instanceof Rectangle r) {

      final Rectangle oRectangle = (Rectangle) o; // Cast is «legal»
      return super.equalCenter(r) &&
             Math.abs(r.width- width) +
                 Math.abs(r.height- height) < 1.E-15;
    }
    return false;
  }
 ...

For o == null the expression o instanceof Rectangle evaluates to false.

public class Circle extends Shape {
  @Override public boolean equals(final Object o) {
    if (o instanceof Circle c){
      return super.equalCenter(c) &&
        Math.abs(c.radius - radius) < 1.E-15;
    }
    return false;
  } ...

• Inheritance
  • ➟ The @Override annotation.

public class Shape {

  double x, y;
 ...
  @Override  // Promise: Subsequent method overrides Object.toString();
  public String toString() {
    return "(" + x + "|" + y + ")";
  }
}

public class Shape {

  double x, y;
 ...
  @Override  // Error: method does not override a method from a supertype
  public String toString(int value) {
    return "(" + x + "|" + y + ")";
  }
}

Explanation: The given method does not override Object.toString().

• Inheritance
  • ➟ Runtime polymorphism