Java泛型与通配符 [英] Java Generics with wildcard

问题描述

 有什么办法可以解决这种情况（我试图尽可能简单地处理场景） public class Test {
 
 public static void main（String [] args）{
 $ b $ * / $ 
 *这里我想指出CollectionGeneric可以是
 *扩展Animal的东西（但构造函数不允许
 *通配符）
 * / 
 CollectionGeneric< ;?延伸动物> animalsCollectionGeneric = new CollectionGeneric< Animal>（）; 
列表< ;?延伸动物>动物= getAnimals（）; 
 / *为什么我不能这样做？ * / 
 animalsCollectionGeneric.setBeans（animals）; 
} 
 
 private static List <？延伸动物> getAnimals（）{
返回新的ArrayList< Dog>（）; 
} 
} 
 
 class CollectionGeneric< T> {
私人列表< T>豆; 
 
 public List< T> getBeans（）{
 return（beans！= null）？ beans：new ArrayList< T>（）; 
} 
 
 public void setBeans（List< T> bean）{
 this.beans = beans; 
 
 
 
 interface Animal {} 
 
 class Dog implements Animal {} 
  

这种情况给了我下一个错误：

 在类型
 CollectionGeneric< capture＃2-of？中设置setBeans（List< capture＃2-of？extends Animal>）延伸动物>不适用于
的参数（List< capture＃3-of？extends Animal>）* 
  

我不确定是否有一种方法可以用泛型来做到这一点， 解决方案

这意味着两个集合不能被证明具有相同的类型边界：

  CollectionGeneric< ;?延伸动物> animalsCollectionGeneric = 
新CollectionGeneric< Animal>（）; 
列表< ;?延伸动物>动物= getAnimals（）
  

第一个可能在运行时具有 CollectionGeneric< Tiger> ; 和第二个列表< Gnu> 。混合这些将意味着你失去了类型安全性（更不用说大屠杀）。

因此你需要向编译器证明这两者是相关的，所以你的通用签名应为：

  public void setBeans（List< ;? extends T> beans）{} 
 public List< T> getBeans（）; 
  

并用作：

 列表与LT ;?延伸动物> beans = getBeans（）; 
 GenericCollection< Animal>动物=新的GenericCollection< Animal>（）; 
 animals.add（beans）; 
  

There is any way to fix this situation (I have try to simplyfy the scenario as much as i could):

public class Test {

    public static void main(String[] args) {

        /*
         * HERE I would like to indicate that the CollectionGeneric can be of
         * something that extends Animal (but the constructor doesn't allow
         * wildcards)
         */
        CollectionGeneric<? extends Animal> animalsCollectionGeneric = new CollectionGeneric<Animal>();
        List<? extends Animal> animals = getAnimals();
        /* Why I cannt do that? */
        animalsCollectionGeneric.setBeans(animals);
    }

    private static List<? extends Animal> getAnimals() {
        return new ArrayList<Dog>();
    }
}

class CollectionGeneric<T> {
    private List<T> beans;

    public List<T> getBeans() {
        return (beans != null) ? beans : new ArrayList<T>();
    }

    public void setBeans(List<T> beans) {
        this.beans = beans;
    }
}

interface Animal {}

class Dog implements Animal{}

this scenario is giving me the next error:

The method setBeans(List<capture#2-of ? extends Animal>) in the type    
CollectionGeneric<capture#2-of ? extends Animal> is not applicable for
the arguments (List<capture#3-of ? extends Animal>)*

I am not sure about if there is a way to do this with generics,

解决方案

What this means is that the two collections can not be proved to have the same type bounds:

    CollectionGeneric<? extends Animal> animalsCollectionGeneric = 
             new CollectionGeneric<Animal>(); 
    List<? extends Animal> animals = getAnimals()

The first one might at runtime have CollectionGeneric<Tiger> and the second one List<Gnu>. Mixing those would mean you lose the type safety ( not to mention the carnage ).

Therefore you need to prove to the compiler that those two are related, so your generic signatures should be:

public void setBeans(List<? extends T> beans) {}
public List<T> getBeans();

and used as:

List<? extends Animal> beans = getBeans();
GenericCollection<Animal> animals = new GenericCollection<Animal>();
animals.add(beans);

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