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 = a;
a = b;
b = temp;

But here there is a third variable. We need to eliminate the use of the third variable.

Answer 1

Do it like this without using a third variable:

String a = "one";
String b = "two";

a = a + b;
b = a.substring(0, (a.length() - b.length()));
a = a.substring(b.length());

System.out.println("a = " + a);
System.out.println("b = " + b);

Answer 2

// 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.

Answer 3

String a="one";
String b="two";

a = a.concat("#" + b);
b = a.split("#")[0];
a = a.split("#")[1];

This will work as long as your string doesn't contain the # character in them. Feel free to use any other character instead.

You could use a possible Unicode character, like "\u001E" instead of the #.

Answer 4

public class SwapStringVariable {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        String a = "test";
        String b = "paper";

        a = a + b;
        b = a.substring(0, a.length()  - b.length());
        a = a.substring(b.length(), a.length());

        System.out.println(a + " " + b);


    }

}

Answer 5

The simplest way is given below:

String a = "one";
String b = "two";
System.out.println("Before swap: " a + " " + b);
int len = a.length();
a = a + b;
b = a.substring(0, len);
a = a.substring(len);
System.out.println("After swap: " a + " " + b);

Answer 6

String str1 = "first";
String str2 = "second";

str1 = str2+str1;
str2 = str1.substring(str2.length(),str1.length());
str1 = str1.substring(0,(str1.length() - str2.length()));

System.out.println(str1);
System.out.println(str2);

Answer 7

String name = "george";
String name2 = "mark";
System.out.println(name+" "+name2);
System.out.println(name.substring(name.length()) + name2 +  " "+ name );

Here substring(); method returns empty string. hence , names can be appended.

Answer 8

Here you go. Try this:

String a = "one";
String b = "two";
//String temp=null;
int i = a.length();
a += b;
b = a.substring(0, i);
a = a.substring(i, a.length());
System.out.println(a + " " + b);

Take any value in as string in a variable. It will be swap.

Answer 9

• Actually, the code in your question did not swap String a,b.

  You should see this issue: Swap two strings in Java, by passing them to a utility function, but without returning objects or using wrapper classes

• And Java is passing by value: Is Java "pass-by-reference" or "pass-by-value"?

Answer 10

Jj Tuibeo's solution works if you add replaceFirst() and use a regular expression:

a += b;
b = a.replaceFirst(b + "$", "");
a = a.replaceFirst("^" + b, "");

Answer 11

For String Method 1:

public class SwapWithoutThirdVar{  
public static void main (String args[]){    
 String s1 ="one";
 String s2 ="two";
 s1= s1+s2;
 s2 = s1.substring(0,(s1.length()-s2.length()));
 s1 = s1.substring(s2.length()); 
 System.out.println("s1 == "+ s1);
 System.out.println("s2 == "+ s2);

}
}

Method 2:

public class SwapWithoutThirdVar
{
public static void main (String[] args) throws java.lang.Exception
 {
 String s1 = "one";            
 String s2 ="two";          
 s1=s2+s1;
 s2=s1.replace(s2,"");
 s1=s1.replace(s2,"");         
 System.out.println("S1 : "+s1); 
 System.out.println("S2 : "+s2);
  }
}

For Integers

public class SwapWithoutThirdVar {

public static void main(String a[]){
    int x = 10;
    int y = 20;       
    x = x+y;
    y=x-y;
    x=x-y;
    System.out.println("After swap:");
    System.out.println("x value: "+x);
    System.out.println("y value: "+y);
}
}

Answer 12

You can do in this way.

public static void main(String[] args) {
        // TODO Auto-generated method stub

        String a = "one";
        String b = "two";

        System.out.println(a);
        System.out.println(b);

        a = a+b;
        b = "";

        System.out.println("*************");
         b = a.substring(0, 3);
         a = a.substring(3, 6);

         System.out.println(a);
         System.out.println(b);

    }

Answer 13

package com.core;

public class SwappingNoTemp {

public static void main(String[] args) {

    String a = "java";
    String b = "c";

    a = a + b;

    b = a.substring(0, a.length() - b.length());
    a = a.substring(b.length());

    System.out.println("swapping is a.." + a);
    System.out.println("swapping  is b.." + b);

}

}

Answer 14

String a = "one";//creates "one" object on heap
String b = "two";// creates "two" object on heap
System.out.printf("a is %s , b is %s%n",a,b);
a = "two";// will not create new "two" object & now a is pointing to "two" object
b = "one";// will not create new "one" object & now b is pointing to "one" object
System.out.printf("a is %s , b is %s%n",a,b);

Answer 15

You can also do this by using a temp variable but in a different way:

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

temp=a.concat(b);
b=temp.substring(0,a.length());
a=temp.substring(a.length(),temp.length());

System.out.println("After Swapping A:"+a+"B:"+b);

Answer 16

I'd just like to point out, just in case anyone looks through this thread. There is no way of swapping two strings without using a third variable. In the java examples, since strings are immutable, a=a+b creates a third string, and doesn't reassign a. Doing the reverseString, creates a new variable, but this time on the stack frame as the program goes into a new scope. The use of Xor swapping might work, but Strings are immutable in java, and so xor swaps will also create a temporary variable. So in fact, unless the language enables you to reassign to the same memory space, it isn't possible to swap strings without creating new variables.

Answer 17

Swap with a multidimension assignment:

var a = "one";
var b = "two";
console.log("a =",a);
console.log("b =",b);
a = [b, b = a][0];
console.log("after swap a =",a);
console.log("after swap b =",b);

Answer 18

For strings:

String a = "one"
String b = "two"

a = a + b;
b = a.replace(b, "");
a = a.replace(b, "");

Answer 19

public class ex{
    public static void main(String[] args){

        String s1="abc";
        String s2="def";

        System.out.println(s1);
        System.out.println(s2);

        s3=s2.replaceAll(s1,s2=s1);

        System.out.println(s1);
        System.out.println(s2);

    }
}