概览

集合框架的源码经常见到“? extends E”、“? super T”。本篇文章以实例+注释讲讲“有限通配符的参数化类型”的创建、存值以及取值。

存值：只要能保证存放类是指定类及其子类即可。null不受“? extends/super T”约束。

取值：“? extends T”取得的默认类型为上界T，“? super T”的默认类型为所有类的父类Object。

Demo

package generic;import java.util.PriorityQueue;public class Extend {    public static void main(String[] args) {        // ? extends T,T为临界类        // extends限定了类的上界        // Type mismatch: cannot convert from PriorityQueue
    
      to PriorityQueue
             //PriorityQueue
      pq = new PriorityQueue
     
      ();        PriorityQueue
       pq = new PriorityQueue
      
       ();                // 无法直接放入，因为无法保证存放类与Son的关系        //The method add(capture#1-of ? extends Parent) in the type PriorityQueue
       
         is not applicable for the arguments (Son)        //pq.add(new Son()));        //The method add(capture#1-of ? extends Parent) in the type PriorityQueue
        
          is not applicable for the arguments (Son)        //pq.add(new Parent());        //null不受类型限定，但PriorityQueue不允许为空，会抛出空指针异常        //pq.add(null);                // 间接存放        PriorityQueue
         
           pqs = new PriorityQueue
          
           (); pqs.add(new Son("1")); pqs.add(new Son("2")); pqs.add(new Son("3")); pqs.add(new Son("4")); PriorityQueue
            pq1 = pqs; //取值 Son s = (Son) pq1.poll(); Parent p = pq1.poll(); //Daughter d = (Daughter) pq1.poll(); // 编译通过，执行报错。类型转换异常。 Person pp= pq1.poll(); System.out.println(s.getName()); System.out.println(p.getName()); System.out.println(pp.getName()); }}
          
         
        
       
      
     
    

package generic;import java.util.PriorityQueue;public class Super {    public static void main(String[] args) {        // ? super T,T为临界类        // super限制了下界        // Type mismatch: cannot convert from PriorityQueue
    
      to PriorityQueue
             //PriorityQueue
      pq = new PriorityQueue
     
      ();        PriorityQueue
       pq = new PriorityQueue
      
       ();                // 可存放临界类的子类，因为任一“? super T”也是其父类        pq.add(new Son("1"));        pq.add(new Daughter("2"));        pq.add(new Parent("3"));        // The method add(capture#4-of ? super Parent) in the type PriorityQueue
       
         is not applicable for the arguments (Person)        //pq.add(new Person("4"));        // 取值（默认Object，类型顺序必须与存放对应或者是其父类，否则类型转换错误）        /*Parent p = (Parent) pq.poll();        Daughter d = (Daughter) pq.poll();        Son s = (Son) pq.poll();*/                Parent p = (Parent) pq.poll();        Parent d = (Parent) pq.poll();        Parent s = (Parent) pq.poll();                System.out.println(d.getName());        System.out.println(p.getName());        System.out.println(s.getName());    }}
       
      
     
    

package generic;public class Person implements Comparable
    
     {    protected String name;    public Person(String name) {        super();        this.name = name;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    @Override    public int compareTo(Person o) {        return o.name.compareTo(this.name);    }}
    

package generic;public class Parent extends Person{    private String name;    public Parent(String name) {        super(name);        this.name = name;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }}

package generic;public class Son extends Parent{    private String name;    public Son(String name) {        super(name);        this.name = name;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }}

package generic;public class Daughter extends Parent{    private String name;    public Daughter(String name) {        super(name);        this.name = name;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }}

说点什么

针对以上特性，java中有“PECS（“Producer Extends，Consumer Super”）”的说法。即如果要用参数化类型表示生产者，就使用<? extends T>；如果表示消费者，就使用<? super T>。

更多有意思的内容，欢迎访问笔者小站：

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