Note: The two sample classes were adapted from the code featured in David Hovemeyer and William Pugh's article "Finding More Null Pointer Bugs, But Not Too Many," which is available at http://findbugs.cs.umd.edu/papers/MoreNullPointerBugs07.pdf.

Listing 1

public class TestFields {

	Object x;
	TestFields(Object x) {
		this.x = x;
	}
    int falsePositive1(int level) {
        x = null;
        if (level > 0)
            x = new Object();
        if (level > 4)
            return x.hashCode();
        return 0;
    }

    int truePositive1(int level) {
        x = null;
        if (level > 0)
            x = new Object();
        if (level < 4)
            return x.hashCode();
        return 0;
    }
    int falsePositive2(boolean b) {
        x = null;
        if (b)
            x = new Object();
        if (b)
            return x.hashCode();
        return 0;
    }
    int truePositive2(boolean b) {
        x = null;
        if (b)
            x = new Object();
        if (!b)
            return x.hashCode();
        return 0;
    }

    int falsePositive3(boolean b) {
        Object y = null;
        if (x != null)
            y = new Object();
        if (y != null)
            return x.hashCode() + y.hashCode();
        else
            return 0;
    }
    int truePositive3(boolean b) {
       	Object y = null;
        if (x != null)
            y = new Object();
        if (y != null)
            return x.hashCode() + y.hashCode();
        else
            return x.hashCode();
    }
    int falsePositive4(boolean a, boolean b) {
       	x = null;
        Object y = null;
        if (a) x = "x";
        if (b) y = "y";
        if (y != null)
            return x.hashCode() + y.hashCode();
        else
            return 0;
    }

    int truePositive4(boolean a, boolean b) {
    	x = null;
        Object y = null;
        if (a) x = "x";
        if (b) y = "y";
        if (y != null)
            return x.hashCode() + y.hashCode();
        else
            return x.hashCode();
    }

    int truePositive5() {
    	if (x == null) return x.hashCode();
    	return 0;
    }

    int truePositive6() {
    	if (x == null) {
    		Object y = x;
    		return y.hashCode();
    	}
    	return 0;
    }

    int ifalsePositive1(boolean b) {
        x = null;
        if (!b)x = new Object();
        return LocalHelper.helper1(x, b);
    }

    int itruePositive1(boolean b) {
        x = null;
        if (b) x = new Object();
        return LocalHelper.helper1(x, b);
    }

    int itruePositive2() {
    	x = null;
        return LocalHelper.helper2(x);
    }

    int itruePositive3(boolean b) {
    	x = null;
        if (b) x = "x";
        return LocalHelper.helper3(x);
    }

}


Listing 2

public class TestLocal {

    int falsePositive1(int level) {
        Object x = null;
        if (level > 0)
            x = new Object();
        if (level > 4)
            return x.hashCode();
        return 0;
    }

    int truePositive1(int level) {
        Object x = null;
        if (level > 0)
            x = new Object();
        if (level < 4)
            return x.hashCode();
        return 0;
    }

    int falsePositive2(boolean b) {
        Object x = null;
        if (b)
            x = new Object();
        if (b)
            return x.hashCode();
        return 0;
    }

    int truePositive2(boolean b) {
        Object x = null;
        if (b)
            x = new Object();
        if (!b)
            return x.hashCode();
        return 0;
    }

    int falsePositive3(Object x, boolean b) {
        Object y = null;
        if (x != null)
            y = new Object();
        if (y != null)
            return x.hashCode() + y.hashCode();
        else
            return 0;
    }

    int truePositive3(Object x, boolean b) {
       	Object y = null;
        if (x != null)
            y = new Object();
        if (y != null)
            return x.hashCode() + y.hashCode();
        else
            return x.hashCode();
    }

    int falsePositive4(boolean a, boolean b) {
       	Object x = null;
        Object y = null;
        if (a) x = "x";
        if (b) y = "y";
        if (y != null)
            return x.hashCode() + y.hashCode();
        else
            return 0;
    }

    int truePositive4(boolean a, boolean b) {
    	Object x = null;
        Object y = null;
        if (a) x = "x";
        if (b) y = "y";
        if (y != null)
            return x.hashCode() + y.hashCode();
        else
            return x.hashCode();
    }

    int truePositive5(Object x) {
    	if (x == null) {
    		return x.hashCode();
    	}
    	return 0;
    }

    int truePositive6(Object x) {
    	if (x == null) {
    		Object y = x;
    		return y.hashCode();
    	}
    	return 0;
    }

    int ifalsePositive1(boolean b) {
        Object x = null;
        if (!b) x = new Object();
        return LocalHelper.helper1(x, b);
    }

    int itruePositive1(boolean b) {
        Object x = null;
        if (b) x = new Object();
        return LocalHelper.helper1(x, b);
    }

    int itruePositive2() {
        return LocalHelper.helper2(null);
    }

    int itruePositive3(boolean b) {
    	Object x = null;
        if (b) x = "x";
        return LocalHelper.helper3(x);
    }

}

public class LocalHelper {

//	 Bug when x is null and b is false
    public static int helper1(Object x, boolean b) {
        if (b) return 0;
        return x.hashCode();
    }

    public static int helper2(Object x) {
        return x.hashCode();
    }

    public static int helper3(Object x) {
        return x.hashCode();
    }

}