Java的泛型

public class GenericDemo {
    public static void main(String[] args) {
        Collection collection =new ArrayList();

        //添加元素
        collection.add("java");
        collection.add(12);

        System.out.println("-----------------");
        System.out.println(collection);
        for (Object o: collection) {
            System.out.println(o);
        }

        System.out.println("--------------------");
        Iterator iterator=collection.iterator();
        while (iterator.hasNext()){
            Object next = iterator.next();
            System.out.println(next);
        }
    }
}

这是没有使用泛型的情况，可以发现任何对象都可以装进去，但是一旦我们修改为

Collection collection =new ArrayList();

就表示只能接收String对象,而不是所有都能装进去(12就不能装进去，会报错)

public class Generic {
    private T t;

    public T getT() {
        return t;
    }

    public void setT(T t) {
        this.t = t;
    }
}

//测试类
public class GenericDemo {
    public static void main(String[] args) {
       Generic g1=new Generic<>();
       g1.setT("王五");
       System.out.println(g1.getT());

        Generic g2=new Generic<>();
        g2.setT(12);
        System.out.println(g2.getT());
    }
}

public class Generic {
    public void show(T t){
        System.out.println(t);
    }
}

//测试类
public class GenericDemo {
    public static void main(String[] args) {
       Generic generic=new Generic<>();
       generic.show("zhangsan");

       Generic generic1=new Generic<>();
       generic1.show(12);
    }
}

//public class Generic {
//    public void show(T t){
//        System.out.println(t);
//    }
//}

public class Generic {
    public void show(T t){
        System.out.println(t);
    }
}

//测试类
public class GenericDemo {
    public static void main(String[] args) {
       Generic generic=new Generic();
        generic.show("zhangsan");
        generic.show(15);
    }
}

public interface Generic {
    void show(T t);
}

public class GenericImpl implements Generic{
    @Override
    public void show(T t) {
        System.out.println(t);
    }
}

public class test {
    public static void main(String[] args) {
        GenericImpl generic=new GenericImpl();
        generic.show("wangwu");

        Generic generic1=new GenericImpl<>();
        generic1.show(12);
    }
}

public class GenericDemo {
    public static void main(String[] args) {
        //类型通配符:?
        List list=new ArrayList