Java泛型详解：自定义泛型的extends和super用法

package pers.li.genericity.$2limits.practise; //定义父类 abstract class A { private int s; public int getS() { return s; } public void setS(int s) { this.s = s; } public abstract void thisName(); } //定义子类A1 class A1 extends A { @Override public void thisName() { System.err.println("this is A1"); } } //定义A1的子类A11 class A11 extends A1 { @Override public void thisName() { System.err.println("this is A11"); } } //定义子类A2 class A2 extends A { @Override public void thisName() { System.err.println("this is A2"); } } //定义泛型类 class Fan<T>{ private T fan1; private T fan2; public T getFan1() { return fan1; } public void setFan1(T fan1) { this.fan1 = fan1; } public T getFan2() { return fan2; } public void setFan2(T fan2) { this.fan2 = fan2; } public Fan(T fan1, T fan2) { this.fan1 = fan1; this.fan2 = fan2; } public Fan() { this.fan1 = null; this.fan2 = null; } } public class Test { public static void main(String[] args) { //？号表示任意类型，即Object类型，所以取值需要强转才能调用方法，放值（即set方法）由于不确定类型，所以会报错 Fan<?> fan = new Fan(new A1(),new A2()); Object fan1 = fan.getFan1(); A1 a = (A1)fan1; a.thisName(); // fan.setFan1(new A1()); System.err.println("? super A1只能存放A1及其子类对象并且不能get，因为不知道放进去的对象是哪个+++++++++++++++++++++++++++++==="); //只能存放A1及其子类对象并且不能get，因为不知道放进去的对象是哪个，可以通过强制转换获取结果 Fan<? super A1> fan2 = new Fan<A>(); fan2.setFan1(new A1()); fan2.setFan2(new A11()); Object fan11 = fan2.getFan1(); A1 ss = (A1)fan11; ss.thisName(); // A fan11 = fan2.getFan1(); System.err.println("? extends A只能取值不能放值,因为T必须是继承于A的子类，放值时无法知道到底是哪个子类+++++++++++++++++++++++++++++==="); Fan<? extends A> fan3 = new Fan(new A1(),new A2()); fan3.getFan1().thisName(); fan3.getFan2().thisName(); /* fan3.setFan2(new A1()); fan3.setFan2(new A2()); fan3.setFan2(new A() { @Override public void thisName() { } });*/ System.err.println("真实泛型========================================"); Fan<A> fan4 = new Fan(new A1(),new A2()); fan4.getFan1().thisName(); fan4.getFan2().thisName(); fan4.setFan2(new A2()); } }