通配符可以三种方式使用-

  • 上限通配符  - "list">

  • in variable     - 输入变量将数据提供给代码。例如，copy(src，dest)。此处src充当变量，即要复制的数据。

  • out variable   - 输出变量保存由代码更新的数据。例如，copy(src，dest)。在这里，dest充当具有复制数据的变量。

以下示例说明了上述概念。

package com.learnfk;

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

public class GenericsTester {

   //上限通配符
   //in category
   public static void deleteCat(List<"pln"> extends Cat> catList, Cat cat) {
      catList.remove(cat);
      System.out.println("Cat Removed");
   }

   //下界通配符
   //out category
   public static void addCat(List<"pln"> super RedCat> catList) {
      catList.add(new RedCat("Red Cat"));
      System.out.println("Cat Added");
   }

   //无界通配符
   //UsingObjectmethod toString()
   public static void printAll(List<"pln"> list) {
      for (Object item : list)
         System.out.println(item + " ");
   }

   public static void main(String[] args) {

      List<Animal> animalList= new ArrayList<Animal>();
      List<RedCat> redCatList= new ArrayList<RedCat>();

      //add list of super class Animal of Cat class
      addCat(animalList);
      //add list of Cat class
      addCat(redCatList);  
      addCat(redCatList);  

      //打印所有动物
      printAll(animalList);
      printAll(redCatList);

      Cat cat=redCatList.get(0);
      //删除cat
      deleteCat(redCatList, cat);
      printAll(redCatList); 
   }
}

class Animal {
   String name;
   Animal(String name) { 
      this.name=name;
   }
   public String toString() { 
      return name;
   }
}

class Cat extends Animal { 
   Cat(String name) {
      super(name);
   }
}

class RedCat extends Cat {
   RedCat(String name) {
      super(name);
   }
}

class Dog extends Animal {
   Dog(String name) {
      super(name);
   }
}

这将产生以下输出-

Cat Added
Cat Added
Cat Added
Red Cat 
Red Cat 
Red Cat 
Cat Removed
Red Cat

参考链接

https://www.learnfk.com/java-generics/java-generics-wildcards-guidelines.html