Chapter 6: Inheritance

Transcription

1 Chapter 6: Inheritance EECS 1030 moodle.yorku.ca

2 State of an object final int WIDTH = 3; final int HEIGTH = 4; final int WEIGHT = 80; GoldenRectangle rectangle = new GoldenRectangle(WIDTH, HEIGHT, WEIGHT); Problem Draw the memory diagram.

3 Memory diagram final int WIDTH = 3; final int HEIGTH = 4; final int WEIGHT = 80; GoldenRectangle rectangle = new GoldenRectangle(WIDTH, HEIGHT, WEIGHT); 100 main invocation WIDTH HEIGHT WEIGHT rectangle

4 Memory diagram final int WIDTH = 3; final int HEIGTH = 4; final int WEIGHT = 80; GoldenRectangle rectangle = new GoldenRectangle(WIDTH, HEIGHT, WEIGHT); 100 main invocation 3 WIDTH HEIGHT WEIGHT rectangle

5 Memory diagram final int WIDTH = 3; final int HEIGTH = 4; final int WEIGHT = 80; GoldenRectangle rectangle = new GoldenRectangle(WIDTH, HEIGHT, WEIGHT); 100 main invocation 3 WIDTH 4 HEIGHT WEIGHT rectangle

6 Memory diagram final int WIDTH = 3; final int HEIGTH = 4; final int WEIGHT = 80; GoldenRectangle rectangle = new GoldenRectangle(WIDTH, HEIGHT, WEIGHT); 100 main invocation 3 WIDTH 4 HEIGHT 80 WEIGHT rectangle

7 Memory diagram final int WIDTH = 3; final int HEIGTH = 4; final int WEIGHT = 80 GoldenRectangle rectangle = new GoldenRectangle(WIDTH, HEIGHT, WEIGHT); 100 main invocation 3 WIDTH 4 HEIGHT 80 WEIGHT rectangle 200 GoldenRectangle object width height weight

8 Memory diagram final int WIDTH = 3; final int HEIGTH = 4; final int WEIGHT = 80 GoldenRectangle rectangle = new GoldenRectangle(WIDTH, HEIGHT, WEIGHT); 100 main invocation 3 WIDTH 4 HEIGHT 80 WEIGHT rectangle 200 GoldenRectangle object 3 width 4 height 80 weight

9 Memory diagram final int WIDTH = 3; final int HEIGTH = 4; final int WEIGHT = 80 GoldenRectangle rectangle = new GoldenRectangle(WIDTH, HEIGHT, WEIGHT); 100 main invocation 3 WIDTH 4 HEIGHT 80 WEIGHT 200 rectangle 200 GoldenRectangle object 3 width 4 height 80 weight

10 State of an object Note The private attributes width and height of the Rectangle class are part of the state of a GoldenRectangle object, but are not inherited. As a result, the private attributes width and height of the Rectangle class cannot be accessed by their name in the GoldenRectangle class.

11 The extends keyword To specify that the GoldenRectangle class is a subclass of the Rectangle class, we use the following class header: public class GoldenRectangle extends Rectangle

12 Attributes Question Which attributes do we introduce in the GoldenRectangle class? Provide names and types.

13 Attributes Question Which attributes do we introduce in the GoldenRectangle class? Provide names and types. Answer private int weight;

14 Attributes Question Which attributes do we introduce in the GoldenRectangle class? Provide names and types. Answer private int weight; Note We do not introduce attributes width and height in the GoldenRectangle class, since they are already present in the super class Rectangle.

15 Constructors Question Where do we initialize the state of an object?

16 Constructors Question Where do we initialize the state of an object? Answer In the constructors.

17 Constructors Question Where do we initialize the state of an object? Answer In the constructors. Question Which attributes are part of the state of a GoldenRectangle object?

18 Constructors Question Where do we initialize the state of an object? Answer In the constructors. Question Which attributes are part of the state of a GoldenRectangle object? Answer width, height and weight.

19 Constructors Problem In the constructors of the GoldenRectangle class, we have to initialize the attributes width, height and weight. However, we cannot access width and height by their name, that is, cannot use this.width and this.height in the constructors of the GoldenRectangle class.

20 Constructors Problem In the constructors of the GoldenRectangle class, we have to initialize the attributes width, height and weight. However, we cannot access width and height by their name, that is, cannot use this.width and this.height in the constructors of the GoldenRectangle class. Solution Delegate to a constructor of the Rectangle class.

21 Memory diagram final int WIDTH = 3; final int HEIGTH = 4; final int WEIGHT = 80 GoldenRectangle rectangle = new GoldenRectangle(WIDTH, HEIGHT, WEIGHT); 100 main invocation 3 WIDTH 4 HEIGHT 80 WEIGHT rectangle 200 GoldenRectangle object width height weight

22 Memory diagram final int WIDTH = 3; final int HEIGTH = 4; final int WEIGHT = 80 GoldenRectangle rectangle = new GoldenRectangle(WIDTH, HEIGHT, WEIGHT); 100 main invocation 3 WIDTH 4 HEIGHT 80 WEIGHT rectangle 200 GoldenRectangle object width height weight 300 GoldenRectangle invocation 200 this 3 width 4 height 80 weight

23 Memory diagram final int WIDTH = 3; final int HEIGTH = 4; final int WEIGHT = 80 GoldenRectangle rectangle = new GoldenRectangle(WIDTH, HEIGHT, WEIGHT); 100 main invocation 3 WIDTH 4 HEIGHT 80 WEIGHT rectangle 200 GoldenRectangle object width height weight 300 GoldenRectangle invocation 200 this 3 width 4 height 80 weight 400 Rectangle invocation 200 this 3 width 4 height

24 Memory diagram this.width = width; this. height = height; 100 main invocation 3 WIDTH 4 HEIGHT 80 WEIGHT rectangle 200 GoldenRectangle object width height weight 300 GoldenRectangle invocation 200 this 3 width 4 height 80 weight 400 Rectangle invocation 200 this 3 width 4 height

25 Memory diagram this.width = width; this. height = height; 100 main invocation 3 WIDTH 4 HEIGHT 80 WEIGHT rectangle 200 GoldenRectangle object 3 width height weight 300 GoldenRectangle invocation 200 this 3 width 4 height 80 weight 400 Rectangle invocation 200 this 3 width 4 height

26 Memory diagram this.width = width; this.height = height; 100 main invocation 3 WIDTH 4 HEIGHT 80 WEIGHT rectangle 200 GoldenRectangle object 3 width 4 height weight 300 GoldenRectangle invocation 200 this 3 width 4 height 80 weight 400 Rectangle invocation 200 this 3 width 4 height

27 Memory diagram... this.weight = weight; 100 main invocation 3 WIDTH 4 HEIGHT 80 WEIGHT rectangle 200 GoldenRectangle object 3 width 4 height weight 300 GoldenRectangle invocation 200 this 3 width 4 height 80 weight

28 Memory diagram... this.weight = weight; 100 main invocation 3 WIDTH 4 HEIGHT 80 WEIGHT rectangle 200 GoldenRectangle object 3 width 4 height 80 weight 300 GoldenRectangle invocation 200 this 3 width 4 height 80 weight

29 Delegation Question How do we delegate in the constructor of the GoldenRectangle class to the constructor of the Rectangle class?

30 Delegation Question How do we delegate in the constructor of the GoldenRectangle class to the constructor of the Rectangle class? Answer Although it may not be the most intuitive syntax, we use super(width, height);

31 Delegation Question How do we delegate in the constructor of the GoldenRectangle class to the constructor of the Rectangle class? Answer Although it may not be the most intuitive syntax, we use super(width, height); Note super has an implicit parameter, namely this.

32 Delegation Question How do we delegate in the constructor of the GoldenRectangle class to the constructor of the Rectangle class? Answer Although it may not be the most intuitive syntax, we use super(width, height); Note super has an implicit parameter, namely this. Rule If we delegate to the constructor of the super class, then this has to be done first, that is, super needs to be the first statement of the constructor.

33 Delegation Problem Implement the constructor of the GoldenRectangle class.

34 Methods Question Which methods are inherited by the GoldenRectangle class from the Rectangle class?

35 Methods Question Which methods are inherited by the GoldenRectangle class from the Rectangle class? Answer compareto, getarea, getheight, getwidth, scale, setheight and setwidth.

36 Methods Question Which methods are inherited by the GoldenRectangle class from the Rectangle class? Answer compareto, getarea, getheight, getwidth, scale, setheight and setwidth. Question Which methods are overridden of the Rectangle class in the GoldenRectangle class?

37 Methods Question Which methods are inherited by the GoldenRectangle class from the Rectangle class? Answer compareto, getarea, getheight, getwidth, scale, setheight and setwidth. Question Which methods are overridden of the Rectangle class in the GoldenRectangle class? Answer equals, hashcode and tostring.

38 Methods Question Which methods of the GoldenRectangle class are new?

39 Methods Question Which methods of the GoldenRectangle class are new? Answer getweight.

40 Inherited methods Nothing needs to be done.

41 New methods Problem Implement getweight method.

42 Overridden methods Problem Implement equals, hashcode and tostring.