第十九课，尝试使用继承的方法定义其他类（被称为父类）的子类。如何理解继承？列如水果和苹果的关系，我们可以说苹果继承了水果。检查该程序，Rectangle类有字段width,height和构造函数，在main()方法中创建对象并打印对象的面积。
public class Main
{
public static void main(String[] args)
{
Rectangle rect = new Rectangle(100, 200);
System.out.println(rect.getArea());
}
}
class Rectangle
{
private int width;
private int height;
public Rectangle(int width, int height)
{
this.width = width;
this.height = height;
}
public int getArea()
{
return width * height;
}
}
在Rectangle类下面添加类class Square {}
public class Main
{
public static void main(String[] args)
{
Rectangle rect = new Rectangle(100, 200);
System.out.println(rect.getArea());
}
}
class Rectangle
{
private int width;
private int height;
public Rectangle(int width, int height)
{
this.width = width;
this.height = height;
}
public int getArea()
{
return width * height;
}
}
class Square {
}
在类名Square之后写下extends Rectangle，以使Square类继承Rectangle类。
public class Main
{
public static void main(String[] args)
{
Rectangle rect = new Rectangle(100, 200);
System.out.println(rect.getArea());
}
}
class Square extends Rectangle
{
}
class Rectangle
{
private int width;
private int height;
public Rectangle(int width, int height)
{
this.width = width;
this.height = height;
}
public int getArea()
{
return width * height;
}
}
该错误表示我们必须为类Square提供构造函数
我们仅仅添加了一个参数size
public class Main
{
public static void main(String[] args)
{
Rectangle rect = new Rectangle(100, 200);
System.out.println(rect.getArea());
}
}
class Square extends Rectangle
{
public Square(int size)
{
super(size, size);
}
}
class Rectangle
{
private int width;
private int height;
public Rectangle(int width, int height)
{
this.width = width;
this.height = height;
}
public int getArea()
{
return width * height;
}
}
我们在Rectangle类中调用super构造函数并以size作为宽和高
public class Main
{
public static void main(String[] args)
{
Rectangle rect = new Rectangle(100, 200);
System.out.println(rect.getArea());
}
}
class Square extends Rectangle
{
public Square(int size)
{
super(size, size);
}
}
class Rectangle
{
private int width;
private int height;
public Rectangle(int width, int height)
{
this.width = width;
this.height = height;
}
public int getArea()
{
return width * height;
}
}
在main()方法中以100作为参数新建一个Square对象，并将其分配给变量Square square
public class Main
{
public static void main(String[] args)
{
Rectangle rect = new Rectangle(100, 200);
System.out.println(rect.getArea());
Square square=new Square(100);
}
}
class Square extends Rectangle
{
public Square(int size)
{
super(size, size);
}
}
class Rectangle
{
private int width;
private int height;
public Rectangle(int width, int height)
{
this.width = width;
this.height = height;
}
public int getArea()
{
return width * height;
}
}
调用square.getArea()获得正方形面积并打印Square类中的getArea()方法继承自Rectangle类
public class Main
{
public static void main(String[] args)
{
Rectangle rect = new Rectangle(100, 200);
System.out.println(rect.getArea());
Square square=new Square(100);
System.out.println(square.getArea());
}
}
class Square extends Rectangle
{
public Square(int size)
{
super(size, size);
}
}
class Rectangle
{
private int width;
private int height;
public Rectangle(int width, int height)
{
this.width = width;
this.height = height;
}
public int getArea()
{
return width * height;
}
}
现在改变变量rect为Rectangle类型尽管实际的对象是一个Square类型，您可以使用Rectangle继承类作为变量的类型，这个概念被称为多态。
public class Main
{
public static void main(String[] args)
{
Rectangle rect = new Rectangle(100, 200);
System.out.println(rect.getArea());
Rectangle square = new Square(100);
System.out.println(square.getArea());
}
}
class Square extends Rectangle
{
public Square(int size)
{
super(size, size);
}
}
class Rectangle
{
private int width;
private int height;
public Rectangle(int width, int height)
{
this.width = width;
this.height = height;
}
public int getArea()
{
return width * height;
}
}
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