多态Polymorphism
向上转换Upcasting
分析题
Point.java
1 2 3 4 public class Point { public int x; public int y;}
Shape.java
1 2 3 public class Shape { public Point origin;}
Rect.java
1 2 3 4 public class Rect extends Shape { public int width; public int height;}
Circle.java
1 2 3 public class Circle extends Shape { public int radius;}
基于上面的代码，分析下面每段代码是否可以正常运行
1.
1 2 3 public void run() { Point rect = new Rect();}
A
可以正常运行
B
会报错
2.
1 2 3 public void run() { Shape rect = new Rect();}
A
可以正常运行
B
会报错
3.
1 2 3 public void run() { Circle rect = new Rect();}
A
可以正常运行
B
会报错
4.
1 2 3 4 5 6 7 public void run() { draw(new Rect());} public void draw(Shape shape) { ...}
A
可以正常运行
B
会报错
5.
1 2 3 4 5 6 7 public void run() { draw(new Shape());} public void draw(Shape shape) { ...}
A
可以正常运行
B
会报错
变量类型 与 调用保障
分析题
Point.java
1 2 3 4 public class Point { public int x; public int y;}
Shape.java
1 2 3 4 5 6 7 public class Shape { public Point origin; public void func1() { ... }}
Rect.java
1 2 3 4 5 6 7 8 public class Rect extends Shape { public int width; public int height; public void func2() { }}
基于上面的代码，分析下面每段代码是否可以正常运行
1.
1 2 3 4 public void run() { Rect rect = new Rect(); rect.func1();}
A
可以正常运行
B
会报错
2.
1 2 3 4 public void run() { Rect rect = new Rect(); rect.func2();}
A
可以正常运行
B
会报错
3.
1 2 3 4 public void run() { Shape shape = new Rect(); shape.func1();}
A
可以正常运行
B
会报错
4.
1 2 3 4 public void run() { Shape shape = new Rect(); shape.func2();}
A
可以正常运行
B
会报错
向下转换Downcasting
分析题
Point.java
1 2 3 4 public class Point { public int x; public int y;}
Shape.java
1 2 3 4 5 6 7 public class Shape { public Point origin; public void func1() { ... }}
Rect.java
1 2 3 4 5 6 7 8 public class Rect extends Shape { public int width; public int height; public void func2() { }}
Circle.java
1 2 3 4 5 6 7 public class Circle extends Shape { public int radius; public void func2() { }}
基于上面的代码，分析下面每段代码是否可以正常运行
1.
1 2 3 4 public void run() { Shape shape = new Rect(); Rect rect = shape;}
A
可以正常运行
B
编译时报错
C
运行时报错
2.
1 2 3 4 public void run() { Shape shape = new Rect(); Rect rect = (Rect)shape;}
A
可以正常运行
B
编译时报错
C
运行时报错
3.
1 2 3 4 public void run() { Shape shape = new Shape(); Rect rect = (Rect)shape;}
A
可以正常运行
B
编译时报错
C
运行时报错
4.
1 2 3 4 public void run() { Shape shape = new Rect(); Circle circle = (Circle)shape;}
A
可以正常运行
B
编译时报错
C
运行时报错
5.
1 2 3 public void run() { Circle circle = new Rect();}
A
可以正常运行
B
编译时报错
C
运行时报错
类型检测
分析题
Point.java
1 2 3 4 public class Point { public int x; public int y;}
Shape.java
1 2 3 4 5 6 7 public class Shape { public Point origin; public void func1() { ... }}
Rect.java
1 2 3 4 5 6 7 8 public class Rect extends Shape { public int width; public int height; public void func2() { }}
Circle.java
1 2 3 4 5 6 7 public class Circle extends Shape { public int radius; public void func2() { }}
基于上面的代码，分析下面每段代码的运行结果是什么
1.
1 2 3 4 public void run() { Rect rect = new Rect(); Console.println(rect instanceof Rect);}
A
True
B
False
2.
1 2 3 4 public void run() { Rect rect = new Rect(); Console.println(rect instanceof Shape);}
A
True
B
False
3.
1 2 3 4 public void run() { Shape shape = new Rect(); Console.println(shape instanceof Rect);}
A
True
B
False
4.
1 2 3 4 public void run() { Shape shape = new Rect(); Console.println(shape instanceof Shape);}
A
True
B
False
5.
1 2 3 4 public void run() { Rect rect = new Rect(); Console.println(rect.getClass() == Rect.class);}
A
True
B
False
6.
1 2 3 4 public void run() { Rect rect = new Rect(); Console.println(rect.getClass() == Shape.class);}
A
True
B
False
7.
1 2 3 4 public void run() { Shape shape = new Rect(); Console.println(shape.getClass() == Rect.class);}
A
True
B
False
8.
1 2 3 4 public void run() { Shape shape = new Rect(); Console.println(shape.getClass() == Shape.class);}
A
True
B
False
方法覆盖Method Overriding
分析题
Point.java
1 2 3 4 public class Point { public int x; public int y;}
Shape.java
1 2 3 4 5 6 7 public class Shape { public Point origin; public void draw() { Console.println("Shape draw"); }}
基于上面的代码，分析下面每段代码的运行结果是什么
1.
Rect.java
1 2 3 4 public class Rect extends Shape { public int width; public int height;}
Main.java
1 2 3 4 5 6 public class Main { public void run() { Rect rect = new Rect(); rect.draw(); }}
Shape draw
2.
Rect.java
1 2 3 4 5 6 7 8 public class Rect extends Shape { public int width; public int height; public void draw() { Console.println("Rect draw"); }}
Main.java
1 2 3 4 5 6 public class Main { public void run() { Rect rect = new Rect(); rect.draw(); }}
Rect draw
3.
Rect.java
1 2 3 4 5 6 7 8 public class Rect extends Shape { public int width; public int height; public void draw() { Console.println("Rect draw"); }}
Main.java
1 2 3 4 5 6 public class Main { public void run() { Shape rect = new Rect(); rect.draw(); }}
Rect draw
4.
Rect.java
1 2 3 4 5 6 7 8 9 public class Rect extends Shape { public int width; public int height; public void draw() { Console.println("Rect draw"); super.draw(); }}
Main.java
1 2 3 4 5 6 public class Main { public void run() { Rect rect = new Rect(); rect.draw(); }}
Rect draw Shape draw
继承 x 多态Inheritance x Polymorphism
代码题
Graph 继承提升
Q36 E06
