当使用Collections.addAll（）时，Java集合何时抛出NullPointerException异常 [英] When does Java Collections throw a NullPointerException while using Collections.addAll()

问题描述

Collections.addAll（）方法会在什么条件下抛出 NullPointerException ？官方文档提到：

NullPointerException - 如果指定的集合包含空元素，并且此集合不允许空元素， collection is null p>

  public class CollectionImpl {
 
 public void checkList（）{
 
 List< String> var1 = new ArrayList< String>（）; 
 var1.add（One）; 
 var1.add（Two）; 
 var1.add（Three）; 
 
 System.out.println（var1：+ var1）; 
 
 try {
 if（Collections.addAll（var1，Four，Five，null，1））{
 System.out.println ）; 
} 
} 
 catch（NullPointerException e）{
 System.out.println（Caught Null Pointer Exception+ e）; 
} 
 catch（IllegalArgumentException e）{
 System.out.println（Caught IllegalArgument Exception+ e）; 
} 
 finally {
 System.out.println（var1：+ var1）; 
} 
} 
  

OUTPUT

  var1：[1,2，null] 
 True 
 var1：[1,2， 5，null，6] 
  

解决方案

code> null 元素是集合实现本身的一个属性。它不是可以在集合的特定实例上更改的设置。正如其他人提到的，集合是否允许 null 应该总是记录在类的规范中。

ArrayList 允许 null 元素，因此调用 addAll（） null 的值将总是成功地将它们添加到列表中。

Collection 不允许 null 值为 ArrayDeque 。如果您按以下方式更改 var1 的声明：

  Collection< String> ; var1 = new ArrayDeque< String>（）; 
  

那么你会得到一个 NullPointerException 从调用 addAll（）。

Under what condition does the Collections.addAll() method throw a NullPointerException? The official docs mention:

NullPointerException - if the specified collection contains a null element and this collection does not permit null elements, or if the specified collection is null

How do I make sure that this "Collection does not permit null elements"

public class CollectionImpl {

public void checkList(){

    List<String> var1 = new ArrayList<String>();
     var1.add("One");
     var1.add("Two");
     var1.add("Three");

     System.out.println("var1 : " + var1);

     try{
         if(Collections.addAll(var1,"Four" , "Five" , null , "1")){
            System.out.println("True"); 
         }
     }
     catch(NullPointerException e){
         System.out.println("Caught Null Pointer Exception" + e);
     }
     catch(IllegalArgumentException e){
         System.out.println("Caught IllegalArgument Exception" + e);
     }
     finally{
         System.out.println("var1 : " + var1);
     }
}

OUTPUT

var1 : [1, 2, null]
True
var1 : [1, 2, null, 4, 5, null, 6]

解决方案

Whether a collection supports null elements is a property of the collection implementation itself. It's not a setting that can be changed on a particular instance of a collection. As others have mentioned, whether a collection permits null should always be documented in the class's specification.

Your example uses ArrayList, which permits null elements, so calling addAll() with null values will always successfully add them to the list.

An example of a Collection that doesn't permit null values is ArrayDeque. If you change your declaration of var1 as follows:

Collection<String> var1 = new ArrayDeque<String>();

then you'll get a NullPointerException thrown from the call to addAll().

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