如何检查数组元素是否为null以避免Java中的NullPointerException

Question

我有一个部分填充的对象数组,当我遍历它们时,我试图检查所选对象是否null在我用它做其他东西之前.然而,甚至检查它是否null似乎是通过a NullPointerException.array.length将包括所有null元素.你如何检查null数组中的元素？例如,在下面的代码中将为我抛出一个NPE.

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

Answer 1

你说的比你说的更多.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];

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

得到了预期的输出:

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

您是否可能尝试检查someArray [index]的长度？

Answer 2

它不是.

见下文.您发布的程序按预期运行.

C:\oreyes\samples\java\arrays>type ArrayNullTest.java
public class ArrayNullTest {
    public static void main( String [] args ) {
        Object[][] someArray = new Object[5][];
            for (int i=0; i<=someArray.length-1; i++) {
                 if (someArray[i]!=null ) {
                     System.out.println("It wasn't null");
                 } else {
                     System.out.printf("Element at %d was null \n", i );
                 }
             }
     }
}


C:\oreyes\samples\java\arrays>javac ArrayNullTest.java

C:\oreyes\samples\java\arrays>java ArrayNullTest
Element at 0 was null
Element at 1 was null
Element at 2 was null
Element at 3 was null
Element at 4 was null

C:\oreyes\samples\java\arrays>