创建一个数组来存储Java中的泛型类型 [英] Creating an array to store generic types in Java

问题描述

假设我必须创建一个数组来存储整数的ArrayList，并且数组大小为10。

下面的代码将执行此操作：

  ArrayList< Integer> [] pl2 = new ArrayList [10]; 
  

问题1：

在我看来，更合适的代码是

  ArrayList< Integer> [] pl2 = new ArrayList< Integer> [10]; 
  

为什么这不起作用？

问题2：

以下两种编译

1. ArrayList< Integer> [] pl2 = new ArrayList [10];


2. ArrayList [] pl3 = new ArrayList [10];

参考声明的区别是什么？ c $ c> pl2 和 pl3 是否值得关注？

解决方案 div>

问题1：

基本上，这是Java语言禁止的。这包括在泛型的Java语言规范中。

当您使用

  ArrayList< Integer> [] pl2 = new ArrayList [10]; //警告
  

你会得到编译器警告，因为下面的例子会编译（为每行产生警告）：

$ $ $ $ $ $ $ $ $ $ $ ArrayList wrongRawArrayList = new ArrayList（）; //警告
wrongRawArrayList.add（string1）; //警告
wrongRawArrayList.add（string2）; //警告

pl2 [0] = wrongRawArrayList; //警告

但现在你的数组，应该包含 ArrayList 整数，包含完全错误 的ArrayList 字符串


问题2：





， p12 声明为您提供编译时检查，并在从 ArrayList 中获取项目时使用cast来释放。

稍微修改过的例子：

  ArrayList< Integer> [] pl2 = new ArrayList [10]; //警告
 
 ArrayList< String> wrongArrayList = new ArrayList< String>（）; // 好！ 
 wrongArrayList.add（string1）; // 好！ 
 wrongArrayList.add（string2）; // 好！ 
 
 pl2 [0] = wrongArrayList; //错误
  

现在，由于您使用的是泛型，因此无法编译。
但是如果你使用

$ pre $ Array $ [] pl2 = new ArrayList [10];


您将获得与第一个示例中相同的结果。

Suppose I have to create an array which stores ArrayList's of Integers and the array size is 10.

The below code will do it:

ArrayList<Integer>[] pl2 = new ArrayList[10]; 

Question 1:

In my opinion the more appropriate code would be

ArrayList<Integer>[] pl2 = new ArrayList<Integer>[10];    

Why does this not work?

Question 2:

Both of the below compile

1. ArrayList<Integer>[] pl2 = new ArrayList[10];
2. ArrayList[] pl3 = new ArrayList[10];

What is the difference as far as the reference declaration of pl2 and pl3 is concerned?

解决方案

Question 1:

Basically, this is forbidden by Java language. This is covered in Java Language Specification for generics.

When you use

ArrayList<Integer>[] pl2 = new ArrayList[10];    // warning

you get the compiler warning, because the following example will compile (generating warning for every line of code):

ArrayList wrongRawArrayList = new ArrayList();      // warning
wrongRawArrayList.add("string1");                   // warning 
wrongRawArrayList.add("string2");                   // warning  

pl2[0] = wrongRawArrayList;                         // warning 

but now you array, that supposed to contain ArrayList of Integer, contains totally wrong ArrayList of String objects.

Question 2:

As it was already answered, declaration of p12 provides you with compile time checking and frees you from using casting when getting items from your ArrayList.

Slightly modified previous example:

ArrayList<Integer>[] pl2 = new ArrayList[10];                // warning 

ArrayList<String> wrongArrayList = new ArrayList<String>();  // OK!
wrongArrayList.add("string1");                               // OK! 
wrongArrayList.add("string2");                               // OK!

pl2[0] = wrongArrayList;                                     // ERROR

Now, since you are using generics, this won't compile. But if you use

ArrayList[] pl2 = new ArrayList[10]; 

you will get the same result as in the first example.

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