The embodiment of generics in inheritance and wildcards

List<String> And List<Object>, There is no parent-child relationship

List<Object> And ArrayList<Object> There is a parent-child relationship

wildcard ：

<?>

Requirements for reading and writing wildcards ：

about list<?> Cannot add data in ： list.add() no way , In addition to adding Null outside

        Read ：（ Can only read to Object）

The use of restricted wildcards ：

        

 

package TongPeiFu;

import org.junit.Test;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class test {
    @Test
    public void test(){
        List<Object> list1 = null;
        List<String> list2 = null;

        List<?> list = null;

        list = list1;
        list = list2;

        List<String > list3 = new ArrayList<>();
        list3.add("aa");
        list3.add("bb");
        list3.add("cc");
        list = list3;

    }
    public void show(List<?> list){
        Iterator<?> iterator = list.iterator();
        while (iterator.hasNext()){
            Object obj = iterator.next();
            System.out.println(obj);
        }
    }

    @Test
    public void test2(){
        List<? extends Person> list1 = null;
        List<? super Person> list2 = null;

        List<Person> list3 = null;
        List<Student> list4 = null;
        List<Object> list5 = null;

//        ? extends Person  here ？ finger Person Subclass of class or Person Class itself , When you can't Person The parent of a class （Object）
        list1 = list3;
        list1 = list4;
//        list1 = list5;

//        ? super Person Empathy ;

//         Receive data 
        list3 = new ArrayList<Person>();
        list4 = new ArrayList<Student>();
        list5 = new ArrayList<Object>();

        list1= list4;
//        Student st = list1.get(0); list1 The generics of are  ?extends Person Of , So at least you can only use Person receive 

        list2 = list3;
//        list2  The generic structure is <? super Person> Only use Object Object acceptance of 
        Object obj = list2.get(0);


//         Write data 
//             about list1 Speaking of :  Its generic structure is Person Subclasses of , So I'm not sure 
//        list1.add(new Student());    Can not be 
//        list1.add(new Person());     Can not be 

//             about list2 Speaking of : Its generic structure is Person Parent class of , therefore Person Class and its subclasses can be written 
        list2.add(new Person());
        list2.add(new Student());
//        list2.add(new Object());   Can not be 
    }
}


class Person{}
class Student extends Person{}