<T> List<T>前面<T>的意思

import java.util.*;
 
class Fruit {     public String toString() {    return "Fruit"; } }
 
class Apple extends Fruit {    public String toString(){ return "Apple";    } }
 
class Person {    public String toString(){    return "Person";    } }
 
class ClassName<T> {//主类，文件名ClassName.java
      
 
    void show_1(T t){
        System.out.println("show_1  "+ t.toString());
    }
     
    <E> void show_2(E e){
        System.out.println("show_2  "+e.toString());
    }
     
    <T> void show_3(T t){
        System.out.println("show_3  "+t.toString());
    }
     
     
 
    public static void main(String[] args) {
        ClassName<Fruit> o = new ClassName<Fruit>();
        Fruit f = new Fruit();
        Apple a = new Apple();
        Person p = new Person();
        System.out.println("show_1 演示________________________");
        o.show_1( f );
        o.show_1( a );
//        o.show_1( p );  这行代码是不能编译通过的。因为在
//        ClassName<Fruit>中已经限定了全局的T为Fruit，所以不能再加入Person;
        System.out.println("show_2 演示________________________");
        o.show_2( f );
        o.show_2( a );
        o.show_2( p );
        System.out.println("show_3 演示________________________");
        o.show_3( f );
        o.show_3( a );
        o.show_3( p );
         
    }
}

show_1 演示________________________
show_1  Fruit
show_1  Apple
show_2 演示________________________
show_2  Fruit
show_2  Apple
show_2  Person
show_3 演示________________________
show_3  Fruit
show_3  Apple
show_3  Person