Stupid Things to do in Java #1

Here’s the first post, in hopefully what will be a relatively infrequent series. It’s going to cover stupid things I’ve done in Java, so you won’t make the same mistakes.

In this first post I’m covering a scenario where I was getting a NullPointerException on a field that is initialised on construction, i.e. it should never be null. Below is a set of classes that recreates the situation (I don’t know if it compiles or runs), see if you can spot the mistake:

public class X {
    private String someVar;

    public X(String someVar) {
        this.someVar = someVar;
    }

    public void doSomething() {
        // Does something with someVar
        this.someVar.length();
    }
}   

public class Y extends X {
    private static final Y INSTANCE = new Y();

    private static String SOME_CONSTANT = "blah";

    private Y() {
        super(SOME_CONSTANT);
    }

    public static Y getInstance() { 
        return INSTANCE;
    }
}

public class Main {
    public static void main(String[] args) {
        Y y = Y.getInstance();
        y.doSomething();    // NullPointerException happens here
    }
}

Did you spot it? In this example X.someVar will always be null because Y.INSTANCE is created before Y.SOME_CONSTANT, meaning doSomething() will throw a NullPointerException. It’s an odd bug because it makes you think about class initialisation and mixing static and non-static fields in your class. My mistake was thinking that constants (i.e. static final fields) are always set before you can instantiate a class, but in this example you can use it before it is set. I don’t think the compiler catches it because the first use of SOME_CONSTANT appears after it’s definition, but the declaration of INSTANCE jumps ahead of it causing the problem. So if you have a singleton, make sure you create it after the rest of your static fields.

Update 1: Dmitry spotted a couple of bugs in my code, and one that in fact shows the above case cannot happen. A variable declared as static final always gets initialised first, so the code I was working on must have omitted the final declaration hence causing the NullPointerException.

Update 2: I’ve modified the code above to now break as I described since some people are running it. See the comments for explanations on the full set of conditions for this bug to occur.

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This entry was posted on Sunday, November 19th, 2006 at 5:17 pm and is filed under Java. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.