Question

Possible Duplicate:
NullPointerException through auto-boxing-behavior of Java ternary operator

The following code uses simple conditional operators.

public class Main
{
    public static void main(String[] args)
    {
        Integer exp1 = true ? null : 5;
        Integer exp2 = true ? null : true ? null : 50;

        System.out.println("exp1 = " +exp1+" exp2 = "+exp2);

        Integer exp3 = false ?  5 : true ? null: 50; //Causes the NullPointerException to be thrown.

        System.out.println("exp3 = "+exp3);
    }
}

This code compiles fine. All the expressions ultimately attempt to assign null to Integer type variables exp1, exp2 and exp3 respectively.

The first two cases don’t throw any exception and produce exp1 = null exp2 = null which is obvious.

The last case however, if you go through it carefully, you will see it also attempts to assign null to Integer type variable exp3 and looks something similar to the preceding two cases but it causes the NulllPointerException to be thrown. Why does it happen?

Before I posted my question, I have referred to this nice question but in this case, I couldn’t find what rules as specified by JLS are applied here.

Answer 1

The null is being unboxed to an int, because the assignment is due to the 5 an int, not an Integer. However, a null cannot be represented as an int, hence the NullPointerException.

If you replace 5 by new Integer(5), then it’ll work.

Integer exp3 = false ? new Integer(5) : true ? null : 50;

See also:

• Java Language Specification – Chapter 15.25 – Conditional Operator ? :
• Booleans, conditional operators and autoboxing