How to swap two string variables in Java without using a third variable

Question

How do I swap two string variables in Java without using a third variable, i.e. the temp variable?

String a = \"one\"
String b = \"two\"
String temp = null;
temp         


        

Answer 1

// taken from this answer: https://stackoverflow.com/a/16826296/427413

String returnFirst(String x, String y) {
    return x;
}

String a = "one"
String b = "two"
a = returnFirst(b, b = a); // If this is confusing try reading as a=b; b=a;

This works because the Java language guarantees (Java Language Specification, Java SE 7 Edition, section 15.12.4.2) that all arguments are evaluated from left to right (unlike some other languages, where the order of evaluation is undefined), so the execution order is:

1. The original value of b is evaluated in order to be passed as the first argument to the function
2. The expression b = a is evaluated, and the result (the new value of b) is passed as the second argument to the function
3. The function executes, returning the original value of b and ignoring its new value
4. You assing the result to a
5. Now the values have been swapped and you didn't need to declare temp. The parameter x works as temp, but it looks cleaner because you define the function once and you can use it everywhere.