oop-Inheritance & Polymorphism

本文主要作为java学习笔记，方便以后查看，大部分内容都源于以下网站：

　　http://www.ntu.edu.sg/home/ehchua/programming/index.html#Game

本文的内容是根据自己理解而归类，有不准确的地方，敬请谅解。

本文主要内容：

　　1.继承(inheritance)

　　2.多态(polymorphism)

　　3.抽象类(abstract_class)

　　4.接口(interface)

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1.继承

修饰符：extends

继承只能是单继承，一个子类只能继承一个父类，不能继承多个父类。

The "@Override" is known as annotation (introduced in JDK 1.5), which asks compiler to check whether there is such a method in the superclass to be overridden. This helps greatly if you misspell the name of the method to be overridden. For example, suppose that you wish to override method toString() in a subclass. If @Override is not used and toString() is misspelled as TOString(), it will be treated as a new method in the subclass, instead of overriding the superclass. If @Override is used, the compiler will signal an error.@Override annotation is optional, but certainly nice to have.
Annotations are not programming constructs. They have no effect on the program output. It is only used by the compiler, discarded after compilation, and not used by the runtime.

“@Override”作为标注，告诉编译器去检查父类中是否存在子类将要重载的同名函数。注意这里的Override开头的‘O’需要大写。

Point.java

public class Point {
    private int x;
    private int y;

    public Point() {
        x = 0;
        y = 0;
    }   
    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }   

    public int getX() {
        return x;
    }   
    public void setX(int x) {
        this.x = x;
    }   
    public int getY() {
        return y;
    }   
    public void setY(int y) {
        this.y = y;
    }   

    public String toString() {
        return "(" + x + ", " + y + ")";
    }   
}

Point3D.java

public class Point3D extends Point {
    private int z;

    public Point3D() {
        super();　　　　//调用父类的构造函数
        z = 0;
    }   
    public Point3D(int x, int y, int z) {
        super(x, y); 　　//调用父类的构造函数
        this.z = z;
    }   

    public int getZ() {
        return z;
    }   
    public void setZ(int z) {
        this.z = z;
    }   

    @Override
    public String toString() {
        return "(" + super.getX() + ", " + super.getY() + ", " + z + ")";
    }   
}

TestPoint3D.java

运行结果：

(1, 2, 3)
1
2
3
(4, 5, 6)

Java provides a binary operator called instanceof which returns true if an object is an instance of a particular class. The syntax is as follows:
anObject instanceof aClass

Circle c1 = new Circle();
System.out.println(c1 instanceof Circle);  // true

A subclass instance processes all the attributes operations of its superclass. When a superclass instance is expected, it can be substituted by a subclass instance. In other words, a reference to a class may hold an instance of that class or an instance of one of its subclasses - it is called substitutability.
    If a subclass instance is assign to a superclass reference, you can invoke the methods defined in the superclass only. You cannot invoke methods defined in the subclass.
    However, the substituted instance retains its own identity in terms of overridden methods and hiding variables. If the subclass overrides methods in the superclass, the subclass's version will be executed, instead of the superclass's version.

2.多态

Shape.java

public class Shape {
    private String color;

    public Shape (String color) {
        this.color = color;
    }   

    @Override
    public String toString() {
        return "Shape of color="" + color + """;
    }   
    public double getArea() {
        System.out.println("shape unknown! Cannot compute area!");
        return 0;
    }   
}

Rectangle.java

public class Rectangle extends Shape {
    private int length;
    private int width;

    public Rectangle(String color, int length, int width) {
        super(color);
        this.length = length;
        this.width = width;
    }  
　　//重载父类的方法 
    @Override
    public String toString() {
        return "Rectangle of length=" + length + " and width=" 
            + width + ", subclass of " + super.toString();
    }   
　　//重载父类的方法
    @Override
    public double getArea() {
        return length * width;
    }   
}

Triangle.java

public class Triangle extends Shape {
    private int length;
    private int width;

    public Triangle(String color, int length, int width) {
        super(color);
        this.length = length;
        this.width = width;
    }   
　　//重载父类的方法
    @Override
    public String toString() {
        return "Rectangle of length=" + length + " and width=" 
            + width + ", subclass of " + super.toString();

    } 
　　//重载父类的方法
    @Override
    public double getArea() {
        return 0.5 * length * width;
    }   
}

TestShape.java

public class TestShape {
    public static void main(String[] args) {
        Shape s1 = new Rectangle("red", 4, 5); 　　　　//多态的体现，可以将子类复制给父类，应为子类具有父类的属性
        System.out.println(s1);
        System.out.println("Area is " + s1.getArea());　　//根据赋值前的情况，将会调用Rectangle中的getArea函数

        Shape s2 = new Triangle("blue", 4, 5); 
        System.out.println(s2);
        System.out.println("Area is " + s2.getArea());　　//根据赋值前的情况，将会调用Rectangle中的getArea函数
    }   
}

运行结果：

Rectangle of length=4 and width=5, subclass of Shape of color="red"
Area is 20.0
Rectangle of length=4 and width=5, subclass of Shape of color="blue"
Area is 10.0

3.抽象类（abastract class)

修饰符：abstract

抽象类只有接口，但是其中的函数并没有实现。

An abstract class is incomplete in its definition, since the implementation of its abstract methods is missing. Therefore, an abstract class cannot be instantiated. In other words, you cannot create instances from an abstract class (otherwise, you will have an incomplete instance with missing method's body).

To use an abstract class, you have to derive a subclass from the abstract class. In the derived subclass, you have to override the abstract methods and provide implementation to all the abstract methods. The subclass derived is now complete, and can be instantiated. (If a subclass does not provide implementation to all the abstract methods of the superclass, the subclass remains abstract.)

This property of the abstract class solves our earlier problem. In other words, you can create instances of the subclasses such as Triangle and Rectangle, and upcast them to Shape (so as to program and operate at the interface level), but you cannot create instance of Shape, which avoid the pitfall that we have faced. For example,

Shape.java

abstract public class Shape {
    private String color;

    public Shape(String color) {
        this.color = color;
    }   

    @Override
    public String toString() {
        return "Shape of color"" + color + """;
    }   
　　//其中的方法不必实现，实现由其子类来完成
    abstract public double getArea();
}

Rectangle.java和Triangle.java使用上面的。子类继承父类，并实现器中的未实现的函数。抽象类不能够实例化，即通过new操作符，新建一个instance。

TestShape.java

public class TestShape {
    public static void main(String[] args) {
        Shape s1 = new Rectangle("red", 4, 5); 
        System.out.println(s1);
        System.out.println("Area is " + s1.getArea());

        Shape s2 = new Triangle("blue", 4, 5); 
        System.out.println(s2);
        System.out.println("Area is " + s2.getArea());

        //wrong
        //Shape s3 = new Shape("green");
    }   
}

4.接口（interface)

A Java interface is a 100% abstract superclass which define a set of methods its subclasses must support. An interface contains only public abstract methods (methods with signature and no implementation) and possibly constants (public static final variables). You have to use the keyword "interface" to define an interface (instead of keyword "class" for normal classes). The keyword public and abstract are not needed for its abstract methods as they are mandatory.

An interface is a contract for what the classes can do. It, however, does not specify how the classes should do it.

Movable.java

public interface Movable {
    public void moveUp();
    public void moveDown();
    public void moveLeft();
    public void moveRight();
}

MovablePoint.java

public class MovablePoint implements Movable {
    private int x, y;

    public MovablePoint(int x, int y) {
        this.x = x;
        this.y = y;
    }   

    @Override
    public String toString() {
        return "Point at (" + x + ", " + y + ")";
    }   
    @Override
    public void moveUp() {
        y++;
    }   
    @Override
    public void moveDown() {
        y--;
    }   
    @Override
    public void moveLeft() {
        x--;
    }   
    @Override
    public void moveRight() {
        x++;
    }   
}

TestMovable.java

public class TestMovable {
    public static void main(String[] args) {
        Movable m1 = new MovablePoint(7,7);

        m1.moveUp();
        System.out.println(m1);
        m1.moveDown();
        System.out.println(m1);
        m1.moveLeft();
        System.out.println(m1);
        m1.moveDown();
        System.out.println(m1);
    }   
}

运行结果

Point at (7, 8)
Point at (7, 7)
Point at (6, 7)
Point at (6, 6）

接口的出现，使得java可以间接实现多继承，因为一个类可以通过修饰符implements来实现多个接口。

As mentioned, Java supports only single inheritance. That is, a subclass can be derived from one and only one superclass. Java does not support multiple inheritance to avoid inheriting conflicting properties from multiple superclasses. Multiple inheritance, however, does have its place in programming.

A subclass, however, can implement more than one interfaces. This is permitted in Java as an interface merely defines the abstract methods without the actual implementations and less likely leads to inheriting conflicting properties from multiple interfaces. In other words, Java indirectly supports multiple inheritances via implementing multiple interfaces. For example,

public class Circle extends Shape implements Movable, Displayable {  // One superclass but implement multiple interfaces
   .......
}

The formal syntax for declaring interface is:

[public|protected|package] interface interfaceName
[extends superInterfaceName] {
   // constants
   static final ...;

   // abstract methods' signature
   ...
}