Java Program to Swap Two Strings

🎓 Top 15 Udemy Courses (80-90% Discount): My Udemy Courses - Ramesh Fadatare — All my Udemy courses are real-time and project oriented courses.

▶️ Subscribe to My YouTube Channel (178K+ subscribers): Java Guides on YouTube

▶️ For AI, ChatGPT, Web, Tech, and Generative AI, subscribe to another channel: Ramesh Fadatare on YouTube

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.

Swapping Using a Temporary Variable
Swapping Without Using a Temporary Variable
Swapping Strings Using a Wrapper Class
Complete Example Program
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!

My Top and Bestseller Udemy Courses. The sale is going on with a 70 - 80% discount. The discount coupon has been added to each course below:

Spring Boot + RabbitMQ Course - The Practical Guide