1.泛型类

public class Holder<T> {
    T a;
    public Holder(T a) {
        this.a = a;
    }
    public static void main(String[] args) {
        new Holder<Shape>(new Circle());///只能存入Shape或者是其子类
        /////new Holder<Circle>(new Shape());  编译错误
    }
}

class Shape{
    void fun(){
        System.out.println("Shape");
    }
}

class Circle extends Shape{
    @Override
    void fun() {
        System.out.println("Circle");
    }
}

2.泛型方法

public class GenericMethod {
    
    public <T> T f(T x){
        return x;
    }
    
    public <T> String v(T x){
        return x.getClass().getName();
    }
    
    public <T> List<T> asList(T... args) {
        List<T> list = new ArrayList<>();
        for(T t:args){
            list.add(t);
        }
        return list;
    }
    
    public static void main(String[] args) {
        GenericMethod g = new GenericMethod();
        
        g.f("fuck"); //////自动类型推断，自动推断只会发生在赋值操作
        
        g.<String>f("you");  /////显式类型说明，一般不用
        
        List<String> list = g.asList("I fuck you".split(" ")); ////可变参数与泛型方法
        System.out.println(list);
        
        System.out.println(g.v(1));
    }
}

3.擦除