Conditional operator in concatenated string

Question

I'd like know why the following program throws a NPE

public static void main(String[] args) {
    Integer testInteger = null;
    String test = "test" + testInteger == null ? "(null)" : testInteger.toString();
}

while this

public static void main(String[] args) {
    Integer testInteger = null;
    String test = "test" + (testInteger == null ? "(null)" : testInteger.toString());
}

doesn't. It's certainly a priority problem and I'm curious how the concatenation works inside.

Answer 1

This is an example of the importance of understanding operator precedence.

You need the parentheses otherwise it is interpreted as follows:

String test = ("test" + testInteger) == null ? "(null)" : testInteger.toString();

See here for a list of operators and their precedence. Also note the warning at the top of that page:

Note: Use explicit parentheses when there is even the possibility of confusion.

Answer 2

Without the brackets it's doing this effectively: String test = ("test" + testInteger) == null ? "(null)" : testInteger.toString(); Which results in an NPE.