如何检查数组元素是否为空，以避免Java中的NullPointerException异常 [英] How to check if array element is null to avoid NullPointerException in Java

问题描述

我有一些部分填充的对象数组，当我重复遍历它们时，我尝试检查所选对象是否为 null ，然后再进行其他操作。然而，即使是通过 NullPointerException 来检查它是否为 null 的行为。 array.length 将包括所有 null 元素。你如何检查数组中的 null 元素？例如下面的代码将为我抛出一个NPE。

  Object [] [] someArray = new Object [5] [ ]。 （int i = 0; i< = someArray.length-1; i ++）{
 if（someArray [i]！= null）{//做某事
} 
} 
  

解决方案

你比你说的更多。我从你的例子中运行了以下扩展测试：

  public class test {
 
 public static void main （String [] args）{
 Object [] [] someArray = new Object [5] []; 
 someArray [0] = new Object [10]; 
 someArray [1] = null; 
 someArray [2] = new Object [1]; 
 someArray [3] = null; 
 someArray [4] = new Object [5]; （int i = 0; i< = someArray.length-1; i ++）{
 if（someArray [i]！= null）{
 System.out。 println（not null）; 
} else {
 System.out.println（null）; 
} 
} 
} 
} 
  

得到预期的输出：

  $ / cygdrive / c / Program\ Files / Java / jdk1.6.0_03 / bin / java -cp。 test 
 not null 
 null 
 not null 
 null 
 not null 
  

你可能试图检查someArray [index]的长度吗？

I have a partially nfilled array of objects, and when I iterate through them I tried to check to see whether the selected object is null before I do other stuff with it. However, even the act of checking if it is null seem to through a NullPointerException. array.length will include all null elements as well. How do you go about checking for null elements in an array? For example in the following code will throw an NPE for me.

Object[][] someArray = new Object[5][];
for (int i=0; i<=someArray.length-1; i++) {
    if (someArray[i]!=null) { //do something
    } 
}

解决方案

You have more going on than you said. I ran the following expanded test from your example:

public class test {

    public static void main(String[] args) {
        Object[][] someArray = new Object[5][];
        someArray[0] = new Object[10];
        someArray[1] = null;
        someArray[2] = new Object[1];
        someArray[3] = null;
        someArray[4] = new Object[5];

        for (int i=0; i<=someArray.length-1; i++) {
            if (someArray[i] != null) {
                System.out.println("not null");
            } else {
                System.out.println("null");
            }
        }
    }
}

and got the expected output:

$ /cygdrive/c/Program\ Files/Java/jdk1.6.0_03/bin/java -cp . test
not null
null
not null
null
not null

Are you possibly trying to check the lengths of someArray[index]?

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