Java Program to Swap Two Strings

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Introduction

Swapping two strings is a common task in programming. While swapping primitive types like integers is straightforward, swapping objects like strings requires a bit more attention, especially considering that strings in Java are immutable. This blog post will explore different methods of swapping two strings in Java.

Table of Contents

1. Swapping Using a Temporary Variable
2. Swapping Without Using a Temporary Variable
3. Swapping Strings Using a Wrapper Class
4. Complete Example Program
5. Conclusion

1. Swapping Using a Temporary Variable

The most straightforward method to swap two strings is by using a temporary variable.

Example:

public class SwapStringsUsingTemp {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = "World";

        System.out.println("Before swap:");
        System.out.println("str1: " + str1);
        System.out.println("str2: " + str2);

        // Swapping using a temporary variable
        String temp = str1;
        str1 = str2;
        str2 = temp;

        System.out.println("After swap:");
        System.out.println("str1: " + str1);
        System.out.println("str2: " + str2);
    }
}

Output:

Before swap:
str1: Hello
str2: World
After swap:
str1: World
str2: Hello

Explanation:

• A temporary variable temp is used to hold the value of str1.
• The value of str2 is assigned to str1.
• The value of temp (original value of str1) is assigned to str2.

2. Swapping Without Using a Temporary Variable

Swapping strings without using a temporary variable is also possible by concatenating and splitting the strings.

Example:

public class SwapStringsWithoutTemp {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = "World";

        System.out.println("Before swap:");
        System.out.println("str1: " + str1);
        System.out.println("str2: " + str2);

        // Swapping without using a temporary variable
        str1 = str1 + str2; // str1 becomes "HelloWorld"
        str2 = str1.substring(0, str1.length() - str2.length()); // str2 becomes "Hello"
        str1 = str1.substring(str2.length()); // str1 becomes "World"

        System.out.println("After swap:");
        System.out.println("str1: " + str1);
        System.out.println("str2: " + str2);
    }
}

Output:

Before swap:
str1: Hello
str2: World
After swap:
str1: World
str2: Hello

Explanation:

• The strings are concatenated and stored in str1.
• str2 is then assigned the substring of str1 from the start to the length of the original str1.
• str1 is assigned the substring from the end of str2's length to the end of the concatenated string.

3. Swapping Strings Using a Wrapper Class

When you need to swap strings in a method, a wrapper class can be used to simulate pass-by-reference behavior in Java.

Example:

public class SwapStringsUsingWrapper {
    public static void main(String[] args) {
        StringWrapper str1 = new StringWrapper("Hello");
        StringWrapper str2 = new StringWrapper("World");

        System.out.println("Before swap:");
        System.out.println("str1: " + str1.value);
        System.out.println("str2: " + str2.value);

        // Swapping using a method and wrapper class
        swapStrings(str1, str2);

        System.out.println("After swap:");
        System.out.println("str1: " + str1.value);
        System.out.println("str2: " + str2.value);
    }

    public static void swapStrings(StringWrapper str1, StringWrapper str2) {
        String temp = str1.value;
        str1.value = str2.value;
        str2.value = temp;
    }
}

class StringWrapper {
    String value;

    StringWrapper(String value) {
        this.value = value;
    }
}

Output:

Before swap:
str1: Hello
str2: World
After swap:
str1: World
str2: Hello

Explanation:

• A StringWrapper class is created to hold the string value.
• The swapStrings method swaps the values of the StringWrapper objects.
• This simulates pass-by-reference behavior, allowing the strings to be swapped within a method.

4. Complete Example Program

Here is a complete program that demonstrates all the methods discussed above to swap two strings.

Example Code:

public class SwapStringsExample {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = "World";

        // Using a Temporary Variable
        System.out.println("Using Temporary Variable:");
        swapUsingTemp(str1, str2);

        // Using Concatenation and Substring
        System.out.println("\nUsing Concatenation and Substring:");
        swapWithoutTemp(str1, str2);

        // Using Wrapper Class
        System.out.println("\nUsing Wrapper Class:");
        StringWrapper sw1 = new StringWrapper("Hello");
        StringWrapper sw2 = new StringWrapper("World");
        swapUsingWrapper(sw1, sw2);
        System.out.println("After swap:");
        System.out.println("str1: " + sw1.value);
        System.out.println("str2: " + sw2.value);
    }

    public static void swapUsingTemp(String str1, String str2) {
        System.out.println("Before swap:");
        System.out.println("str1: " + str1);
        System.out.println("str2: " + str2);

        String temp = str1;
        str1 = str2;
        str2 = temp;

        System.out.println("After swap:");
        System.out.println("str1: " + str1);
        System.out.println("str2: " + str2);
    }

    public static void swapWithoutTemp(String str1, String str2) {
        System.out.println("Before swap:");
        System.out.println("str1: " + str1);
        System.out.println("str2: " + str2);

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

        System.out.println("After swap:");
        System.out.println("str1: " + str1);
        System.out.println("str2: " + str2);
    }

    public static void swapUsingWrapper(StringWrapper str1, StringWrapper str2) {
        System.out.println("Before swap:");
        System.out.println("str1: " + str1.value);
        System.out.println("str2: " + str2.value);

        String temp = str1.value;
        str1.value = str2.value;
        str2.value = temp;

        System.out.println("After swap:");
        System.out.println("str1: " + str1.value);
        System.out.println("str2: " + str2.value);
    }
}

class StringWrapper {
    String value;

    StringWrapper(String value) {
        this.value = value;
    }
}

Output:

Using Temporary Variable:
Before swap:
str1: Hello
str2: World
After swap:
str1: World
str2: Hello

Using Concatenation and Substring:
Before swap:
str1: Hello
str2: World
After swap:
str1: World
str2: Hello

Using Wrapper Class:
Before swap:
str1: Hello
str2: World
After swap:
str1: World
str2: Hello

5. Conclusion

Swapping two strings in Java can be accomplished in several ways, each with its own advantages. Using a temporary variable is straightforward and easy to understand, while concatenation and substring manipulation provide an alternative approach without additional variables. Using a wrapper class simulates pass-by-reference behavior, allowing swaps within methods.

By understanding these different methods, you can choose the one that best fits your needs and coding style. Happy coding!