java - Returning null as an int permitted with ternary operator but not if statement

Question

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?

Answer 1

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.