How to check if array element is null to avoid NullPointerException in Java

Question

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
    } 
}

Answer 1

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]?

Answer 2

It does not.

See below. The program you posted runs as supposed.

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>