How to swap two strings without using a third variable in java / Various methods to swap two strings without additional variable

Swapping strings

Swapping two strings means interchanging their values with one another. Example, there are two string variables s1 and s2 with values
s1 = “codippa.com” and
s2 = “the website”,
Swapping the two strings will make their value as
s1 = “the website” and
s2 = “codippa.com”

Common Approach

The most common approach of swapping two strings is to store the value of one of the strings into a temporary variable and then assigning the value of second string to first one. Finally, assigning the value of temporary variable back to the first string as shown below

String s1 = "codippa.com";
String s2 = "the website";
// create a temporary variable
String temp = null;
// assign the value of first string to temporary variable
temp = s1;
// assign the value of second string to first one
s1 = s2;
// assign the value of temporary variable to second string
s2 = temp;
System.out.println(s1); //prints "the website"
System.out.println(s2); //prints "codippa.com"

This is a simple solution and is of no harm. But suppose due to some requirement you are not supposed to use a temporary(or third) variable or this scenario is put before in an interview, then this post will help you out. It lists different methods to swap two string values without a temp variable.

Method 1: Using concatenation and substring

Java string api provides a couple of substring method which enable to extract portions of a string. This method will utilize its following substring methods.

Algorithm

This method can be carried out using the below steps.

Code for the algorithm is given below

String s1 = "codippa.com";
String s2 = "the website";
// calculate the length of first string
int length = s1.length(); // will be 11
s1 = s1 + s2; // result will be codippa.comthewebsite
s2 = s1.substring(0, length); // result will be codippa.com
s1 = s1.substring(length); // result will be thewebsite
// print both strings
System.out.println("First string : "+s1);
System.out.println("Second string : "+s2); 

Above code will print

Method 2: Using concatenation and split

This approach uses split method of java.lang.String class which divides a string on the argument supplied to it and gives the result in an array.

Algorithm

Code for the above algorithm is given below

String s1 = "codippa.com";
String s2 = "the website";
// concatenate the strings with a separator in between
s2 = s1 + "$" + s2;
// split and assign the value at second index to first string
s1 = s2.split("\\$")[1];
// split and assign the value at first index to second string
s2 = s2.split("\\$")[0];
// print both strings
System.out.println("First string : " + s1);
System.out.println("Second string : " + s2);

Since $ is a special character, it needs to be escaped with \\. If you do not want this, then use any other character such as #, %, & etc., or may be a number.

Method 3: Using concatenate and replace

This approach uses replace method of java.lang.String class. This method when called on a string, replaces the sequence of characters in the string with other supplied characters and returns the string after replacement. It takes 2 arguments: string to replace and replacement. Example, "codippa.com".replace(".com", "web"); returns codippaweb.

Algorithm

Code implemented using the above algorithm follows

String s1 = "codippa.com";
String s2 = "the website";
// concatenate the strings
s2 = s1 + s2;
// replace first string with blank value
s1 = s2.replace(s1, "");
// replace second string with blank value
s2 = s2.replace(s1, "");
// print both strings
System.out.println("First string : " + s1);
System.out.println("Second string : " + s2);

Output will be

Method 4: Using a helper method

This is a tricky approach which makes use of a method which accepts two string arguments and returns one of those strings. Out of the two arguments, the first argument is one of the strings while second argument is not only a string but an assignment also.

public static void main(String[] args) {
  String s1 = "codippa.com";
  String s2 = "the website";
  // call the helper method with 2 arguments
  s1 = returnFirstString(s2, s2 = s1);
  // print both strings
  System.out.println("First string : " + s1);
  System.out.println("Second string : " + s2);
}

static String returnFirstString(String first, String second) {
    return first;
}

Output will be

Method returnFirstString accepts two arguments and just returns the first argument. One would say, when the second argument is not required, then why to pass it. Well, there is a reason to it. Notice the line s1 = returnFirstString(s2, s2 = s1); where this method is called. This method call passes the string s2 as first argument. In the second argument, string s1 is assigned to s2 so that s2 now contains the value of s1. Now, without the second argument, it would not be possible to assign s1 to s2.
Also note that in this method call the order of arguments should not be changed since if s2 = s1 is placed as first argument, it will change the value of s2 and the same will be reflected in the second argument.

Note

All the above methods make use of certain java concepts which are highlighted after the method.

Hope this post helped you in better understanding the methods to swap two strings and the logic behind them. Keep visiting for more such stuff.