Is it possible to write swap method in Java? [duplicate]

While it is not possible to write a function that simply swaps two variables, it is possible to write a helper function that allows you to:

• Swap two variables using only one statement
• Without temporary variables in the caller’s code
• Without ‘boxing’ primitives
• With a few overloads (one of them using generics), it works for any type

That’s how you could do it:

int returnFirst(int x, int y) {
    return x;
}
<T> T returnFirst(T x, T y) {
    return x;
}
// other overloads as needed
int a = 8, b = 3;
a = returnFirst(b, b = a); // try reading this as a = b; b = a;
System.out.println("a: " + a + ", b: " + b); // prints a: 3, b: 8

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 assign the result to a

If returnFirst is too long, you can choose a shorter name to make code more compact (e.g. a = sw(b, b = a)).

Suppose you need to swap many variables of different types one after the other. By using returnFirst there’s no need for intAux, objAux, etc. There’s less risk of mistakenly using the wrong variable somewhere, because there are no extra variables (in the caller, at least).