Tricky ternary operator in Java

Let's look at the simple Java code in the following snippet:

public class Main {

    private int temp() {
        return true ? null : 0;
        // No compiler error - the compiler allows a return value of null
        // in a method signature that returns an int.
    }

    private int same() {
        if (true) {
            return null;
            // The same is not possible with if,
            // and causes a compile-time error - incompatible types.
        } else {
            return 0;
        }
    }

    public static void main(String[] args) {
        Main m = new Main();
        System.out.println(m.temp());
        System.out.println(m.same());
    }
}

In this simplest of Java code, the temp() method issues no compiler error even though the return type of the function is int , and we are trying to return the value null (through the statement return true ? null : 0; ). When compiled, this obviously causes the run time exception NullPointerException .

However, it appears that the same thing is wrong if we represent the ternary operator with an if statement (as in the same() method), which does issue a compile-time error! Why?

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The compiler interprets null as a null reference to an Integer , applies the autoboxing/unboxing rules for the conditional operator (as described in the Java Language Specification, 15.25), and moves happily on. This will generate a NullPointerException at run time, which you can confirm by trying it.

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I think, the Java compiler interprets true ? null : 0 true ? null : 0 as an Integer expression, which can be implicitly converted to int , possibly giving NullPointerException .

For the second case, the expression null is of the special null type see, so the code return null makes type mismatch.

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Actually, its all explained in the Java Language Specification.

The type of a conditional expression is determined as follows:

If the second and third operands have the same type (which may be the null type), then that is the type of the conditional expression.

Therefore the "null" in your (true ? null : 0) gets an int type and then is autoboxed to Integer.

Try something like this to verify this (true ? null : null) and you will get the compiler error.

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