1. L EC. 07: I NHERITANCE 0

2. 2015 S PRING C ONTENT  Inheritance basics  Member access and inheritance  Constructors and inheritance  Superclass references and subclass objects  Hiding and overriding inherited members  Method overriding  Polymorphism [ 程設 4]  Abstract classes and methods [ 程設 4]  Final [ 程設 4]  Blueprint of java.lang.Object class [ 程設 4] 1

3. E XAMPLE 4 class A { A(){ System.out.println("In a's 1st constructer..."); } A(String s) { System.out.println("In a's 2 nd constructer..."); System.out.println(s); } class B extends A { B(String s) { super(s); // must appear at the 1st line System.out.println("In b's constructer..."); System.out.println(s); } class App223a { public static void main(String[] args) { B obj = new B("Hello from Java!"); } 2 In a's 2 nd constructer... Hello from Java! In b's constructer... Hello from Java! + A() + A(String) is-a A + B(String) B

4. E XAMPLE 5 class A { //A(int i) { System.out.print("A"); } A() { System.out.print("A"); } class B extends A { B() { // error. Require Int. System.out.print("B"); } class C extends B { C() { System.out.print("C"); } public class App223b { public static void main(String[] args) { C obj = new C(); } 3 ABC ♥

5. E XAMPLE 4 class TwoDShape { private double width; private double height; TwoDShape(double w, double h) { width = w; height = h; } double getWidth() { return width; } double getHeight() { return height; } void setWidth(double w) { width = w; } void setHeight(double h) { height = h; } void showDim() { System.out.println(width + " " + height); } - double width - double height TwoDShape(double, double) void showDim() is-a TwoDShape private String style Triangle(String, double, double) double area() void showStyle() Triangle ♥

6. 5 class Triangle extends TwoDShape { private String style; Triangle(String s, double w, double h) { super(w, h); style = s; } double area() { return getWidth() * getHeight() / 2; } void showStyle() { System.out.println("Triangle is " + style); } class Shapes4 { public static void main(String args[]) { Triangle t1 = new Triangle(“filled", 8.0, 12.0); t1.showStyle(); System.out.println(t1.area()); } - double width - double height TwoDShape(double, double) void showDim() is-a TwoDShape - String style Triangle(String, double, double) double area() void showStyle() Triangle Triangle is filled 48.0

7. E XERCISE 2 A  重複 Ex2. 利用 class Volume 來計算一個長方體的體積. 但是請利用建構子來給定初值. 6

8. E XAMPLE  Executing the following program will output BCAD. 7 class Some { Some() { System.out.print(“A”); } Some(int i) { System.out.print(“B”); } class Foo extends Some { Foo() { super(3); System.out.print(“C”); } Foo(double d) { System.out.print(“D”); //super() is inserted } public static void main(String… args) { Foo obj; obj = new Foo(); obj = new Foo(3.3); } ♥

9. I LLEGAL EXAMPLE class Some { Some() { System.out.print(“A”); } Some(int i) { System.out.print(“B”); } class Foo extends Some { Foo() { Some(3); // illegal ( 不能被繼承 ), should use super(3) } Foo(int i) { super(3); super(); // illegal, super call must be the first statement } Foo(double d) { super(d); // illegal, no matched constructor } 8

10. E XAMPLE  Executing the following program will output ADCAD. 9 class Some { Some() { System.out.print(“A”); } Some(int i) { System.out.print(“B”); } class Foo extends Some { Foo() { this(3.3); // must be the first statement System.out.print(“C”); } Foo(double d) { System.out.print(“D”); } public static void main(String… args) { Foo obj; obj = new Foo(); obj = new Foo(3.3); }

11. I LLEGAL EXAMPLE class Some { Some() { System.out.print(“A”); } Some(int i) { System.out.print(“B”); } } class Foo extends Some { Foo() { Foo(3); // illegal, should use this(3) } Foo(int i) { super(3); this(); // illegal, this call must be the first statement } Foo(double d) { this(“ABC”); // illegal, no matched constructor in Foo } 10

12. S UPERCLASS REFERENCES AND SUBCLASS OBJECTS  A reference variable of a superclass can be assigned a reference to an object of any subclass derived from that superclass.  In other words, a superclass reference can refer to a subclass object.  Remember that a subclass “is-a” superclass, so a subclass object is a superclass object.  Conversely, a reference variable of a subclass cannot be assigned a reference to an object of its superclass. 11

13. E XAMPLE 1 class X { int a; X(int i) { a = i; } } class Y extends X { int b; Y(int i, int j) { super(j); b = i; } class SupSubRef { public static void main(String args[]) { X x = new X(10); Y y = new Y(5, 6); X x2 = new Y(5, 6); System.out.println(x2.a); //System.out.println(x2.b); } 12 6 int a is-a X int b Y SupSubRef.java:23: error: cannot find symbol System.out.println(x2.b); ^ symbol: variable b location: variable x2 of type X 1 error 別墅設計圖 別墅 + 游泳池 地址

14. E XAMPLE 2 class X { int a; X(int i) { a = i; } } class Y extends X { int b; Y(int i, int j) { super(j); b = i; } class SupSubRef { public static void main(String args[]) { Y y2 = new X(10); // Error, incompatible types } 13 int a is-a X int b Y 別墅設計圖 別墅 + 游泳池

15. E XERCISE 2B  參考下圖繼承關系. Vehicle 類別的 reference 是否能叫 用 Car 實體的 drive method? 14 + start() Vehicle + drive() Car

16. N OTES ON S UPERCLASS REFERENCES AND SUBCLASS OBJECTS  The type of the reference variable — not the type of the object that it refers to — determines what members can be accessed.  When a reference to a subclass object is assigned to a superclass reference variable, you will have access only to those parts of the object defined by the superclass.  So, objX2.b is illegal.  The above rules apply to all the hidden members.  In fact, the binding for hidden members is static binding and compiler has no knowledge of the object type. 15

17. T ESTING CLASS TO WHICH AN OBJECT BELONGS  The operator instanceof is used to check if an object is instantiated from a specific class.  Example Number obj = new Integer(3); Then the following two expressions are true obj instanceof Number obj instanceof Integer But, the following expression is false obj instanceof Double 16 Number IntegerDouble is-a [Ex2B] Car c = new Car(); System.out.println(c instanceof Car); System.out.println(c instanceof Vehicle); 2 hr