Java通用集合，无法将列表添加到列表 [英] Java generic collection, cannot add list to list

问题描述

为什么如下

  public class ListBox {
 private随机随机= new Random 
 private List< ;? extends Collection< Object>>框; 
 
 public ListBox（）{
 box = new ArrayList<>（）; 
} 
 
 public void addTwoForks（）{
 int sizeOne = random.nextInt（1000）; 
 int sizeTwo = random.nextInt（1000）; 
 
 ArrayList< Object> one = new ArrayList<>（sizeOne）; 
 ArrayList< Object> two = new ArrayList<>（sizeTwo）; 
 
 box.add（one）; 
 box.add（two）; 
} 
 
 public static void main（String [] args）{
 new ListBox（）。addTwoForks（）; 
} 
} 
  

不工作？只是为了学习的目的玩仿制药，我希望我能够插入任何扩展Collection在那里，但我得到这个错误：

 在类型List< capture＃2-of？中的方法add（capture＃2-of？extends Collection< Object& extends Collection< Object>>不适用于参数（ArrayList< Object>）
在List< capture＃3-of？中的方法add（capture＃3-of？extends Collection< Object& extends Collection< Object>>不适用于ListBox.addTwoForks（ListBox.java:23）
在ListBox.main（ListBox.java:28）
上的参数（ArrayList< Object>）
 
  




解决方案

您已宣布 $ c>是 的 $ c>。但是根据Java编译器，它可以是扩展 Collection 的任何，即 List< Vector< Object> 。因此，它必须禁止使用通用类型参数的添加操作。它不能让你添加 ArrayList< Object> 到列表，可以列表< Vector< Object>> 。

尝试删除通配符：

< p $ p> private List< Collection< Object>>框;


这应该工作，因为你可以添加 ArrayList< Object& code>到 集合< Object> 的列表。

Why does the following

public class ListBox {
    private Random random = new Random();
    private List<? extends Collection<Object>> box;

public ListBox() {
    box = new ArrayList<>();
}

public void addTwoForks() {
    int sizeOne = random.nextInt(1000);
    int sizeTwo = random.nextInt(1000);

    ArrayList<Object> one = new ArrayList<>(sizeOne);
    ArrayList<Object> two = new ArrayList<>(sizeTwo);

    box.add(one);
    box.add(two);
}

public static void main(String[] args) {
    new ListBox().addTwoForks();
}
}

Not work? Just toying around with generics for the purpose of learning and I expected that I would be able to insert anything that extends Collection in there but I get this error:

The method add(capture#2-of ? extends Collection<Object>) in the type List<capture#2-of ? extends Collection<Object>> is not applicable for the arguments (ArrayList<Object>)
The method add(capture#3-of ? extends Collection<Object>) in the type List<capture#3-of ? extends Collection<Object>> is not applicable for the arguments (ArrayList<Object>)

at ListBox.addTwoForks(ListBox.java:23)
at ListBox.main(ListBox.java:28)

解决方案

You've declared box to be a List of something that extends Collection of Object. But according to the Java compiler, it could be anything that extends Collection, i.e. List<Vector<Object>>. So it must disallow add operations that take the generic type parameter for this reason. It can't let you add an ArrayList<Object> to a List that could be List<Vector<Object>>.

Try removing the wildcard:

private List<Collection<Object>> box;

This should work because you can certainly add an ArrayList<Object> to a List of Collection<Object>.

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