无涯教程-Java泛型 - 通配符准则通配符可以三种方式使用- 上限通配符 - ? 跟extends 搭配使用。

通配符可以三种方式使用-

上限通配符 - ? 跟extends 搭配使用。
下限通配符 - ? 跟super 搭配使用。
无界通配符 - ?

为了确定哪种通配符最适合该条件，无涯教程首先将传递给方法的参数类型分类为 in 和 out 参数。

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<? 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<? 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<?> list) {
for (Object item : list)
System.out.println(item + " ");
}
public static void main(String[] args) {
  </span><span class="typ">List</span><span class="pun">&lt;</span><span class="typ">Animal</span><span class="pun">&gt;</span><span class="pln"> animalList</span><span class="pun">=</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">ArrayList</span><span class="pun">&lt;</span><span class="typ">Animal</span><span class="pun">&gt;();</span><span class="pln">
  </span><span class="typ">List</span><span class="pun">&lt;</span><span class="typ">RedCat</span><span class="pun">&gt;</span><span class="pln"> redCatList</span><span class="pun">=</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">ArrayList</span><span class="pun">&lt;</span><span class="typ">RedCat</span><span class="pun">&gt;();</span><span class="pln">

  </span><span class="com">//add list of super class Animal of Cat class</span><span class="pln">
  addCat</span><span class="pun">(</span><span class="pln">animalList</span><span class="pun">);</span><span class="pln">
  </span><span class="com">//add list of Cat class</span><span class="pln">
  addCat</span><span class="pun">(</span><span class="pln">redCatList</span><span class="pun">);</span><span class="pln">  
  addCat</span><span class="pun">(</span><span class="pln">redCatList</span><span class="pun">);</span><span class="pln">  

  </span><span class="com">//打印所有动物</span><span class="pln">
  printAll</span><span class="pun">(</span><span class="pln">animalList</span><span class="pun">);</span><span class="pln">
  printAll</span><span class="pun">(</span><span class="pln">redCatList</span><span class="pun">);</span><span class="pln">

  </span><span class="typ">Cat</span><span class="pln"> cat</span><span class="pun">=</span><span class="pln">redCatList</span><span class="pun">.</span><span class="kwd">get</span><span class="pun">(</span><span class="lit">0</span><span class="pun">);</span><span class="pln">
  </span><span class="com">//删除cat</span><span class="pln">
  deleteCat</span><span class="pun">(</span><span class="pln">redCatList</span><span class="pun">,</span><span class="pln"> cat</span><span class="pun">);</span><span class="pln">
  printAll</span><span class="pun">(</span><span class="pln">redCatList</span><span class="pun">);</span><span class="pln"> 

}
}
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

无涯教程-Java泛型 - 通配符准则

参考链接