Is it possible to write swap method in Java?

Question

Here is the question: write a method that swaps two variables. These two variables should be primitives. It doesn't need to be generic e.g. two int variables. Is there a way?!

Answer 1

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).

Answer 2

Without using an array or objects, no, it is not possible to do it within a method.

Answer 3

Check out this JavaWorld article that explains it in detail:

http://www.javaworld.com/javaworld/javaqa/2000-05/03-qa-0526-pass.html

A swap of two primitives will never work because primitives are passed by value in Java. You can't even write a method to swap two objects for that matter.

Like @Thomas said, the only thing you could do is have your primitives contained within other objects/arrays and modify those.

Answer 4

One-liner for any primitive numbers:

a += (b - (b = a));

Answer 5

You can make a generic version of @marcus's swap method that swaps any number of objects of the same type:

<T> T swap(T... args) {   // usage: z = swap(a, a=b, b=c, ... y=z);
    return args[0];
}

b = swap(a, a=b);
z = swap(x, x=y, y=z);

Answer 6

In java5, the closest I can think of, which may help you, is :

The AtomicInteger class (and others) have getAndSet() atomic methods ..

Answer 7

To write a swap method that swaps primitives you'd have to have the concept of "out" variables, i.e. variables whose values are passed up to the calling context. C# has those but you must still specify that they're out variables.

Answer 8

This function will swap two ints

Integer[] swap(int a, int b){
    return new Integer[]{b,a};
}

Answer 9

Here's a method that swaps two primitive variables

private void swap(){
    int a = 1;
    int b = 2;
    int temp = a;
    a = b;
    b = temp;
}

It might not be of much use though ;)

Ok seriously, it could be done if the variables are class level:

public class MyClass{
    // excuse horrible coding practice of public mutable fields
    public int a = 1;
    public int b = 2;

    public void swap(){
        int temp = a;
        a = b;
        b = temp;
    }
}

Again though, I fail to see what the use of this could be

Answer 10

I have read the above answers seeking an explanation as to why it is said that a swapping program cannot be written in java in the way it is written in c++. I did the following way program screenshot

Answer 11

As Thomas Owens said. You could probably do it in C by passing variables by &reference, but afaik not in Java without using objects.

Answer 12

Yes it is possible to swap two variable using a method. But you should declare that method with empty parentheses and then call it by reference(empty parentheses) . Here is an example that illustrates swapping of two variable using a method.

public class Swapping

{

static String A="Apple";
static String B="Bat";

    public static  void swap()
    {
       String k;
        k=A;
        A=B;
        B=k;

    }

  public static void main(String[] args) 
  {
    System.out.println("Before swapping");
    System.out.println("A= "+A);
    System.out.println("B= "+B);
    swap();
    System.out.println("After swapping");
    System.out.println("A= "+A);
    System.out.println("B= "+B);
  }

}

By compiling the above code the output comes as follows:

Before swapping

A= Apple

B= Bat

After swapping

A= Bat

B= Apple

//In case of call by reference original value is changed if we made changes in the called method

Answer 13

public class Swap
{
    public static void main (String[]args)
    {
        int y = 5;
        int x = 4;
        int c;

        System.out.println("y = "+y);
        System.out.println("x = "+x);

        c=x; //c = 4
        x=y; //x = 5;
        y=c;

        System.out.println("\n");
        System.out.println("y= "+y);
        System.out.println("x= "+x);
    }    
}