🔄 Java: How to Reverse a String (Step-by-Step Guide)

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🧲 Introduction – Why Reverse a String?

Reversing a string is a common interview question and an important part of learning string manipulation in Java. It’s often used in problems related to palindromes, data encryption, and algorithm practice.

✅ In this tutorial, you’ll learn:

• How to reverse a string using different methods
• Manual reversal using loops
• Built-in Java APIs (StringBuilder, Collections)
• Edge case handling and performance tips

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🔁 Method 1: Using StringBuilder.reverse()

public class ReverseString {
    public static void main(String[] args) {
        String original = "Java";
        String reversed = new StringBuilder(original).reverse().toString();
        System.out.println("Reversed: " + reversed);
    }
}

✅ Explanation:

• StringBuilder(original) creates a mutable version of the string.
• reverse() reverses the characters.
• toString() converts it back to an immutable String.

💡 Best for quick and efficient solutions.

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🔄 Method 2: Using a for Loop (Manual Reversal)

public class ReverseStringLoop {
    public static void main(String[] args) {
        String original = "Developer";
        String reversed = "";

        for (int i = original.length() - 1; i >= 0; i--) {
            reversed += original.charAt(i);
        }

        System.out.println("Reversed: " + reversed);
    }
}

✅ Explanation:

• Starts from the end of the string and builds a new one in reverse order.
• Demonstrates string concatenation logic.

⚠️ Warning: Not recommended for large strings (inefficient due to immutability of String).

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🔄 Method 3: Using Character Array

public class ReverseStringArray {
    public static void main(String[] args) {
        String original = "Program";
        char[] chars = original.toCharArray();

        for (int i = 0, j = chars.length - 1; i < j; i++, j--) {
            char temp = chars[i];
            chars[i] = chars[j];
            chars[j] = temp;
        }

        System.out.println("Reversed: " + new String(chars));
    }
}

✅ Explanation:

• Converts string to array, then swaps characters from both ends.
• Memory-efficient and faster for large strings.

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🔁 Method 4: Using Collections.reverse() (List-Based)

import java.util.*;

public class ReverseWithCollections {
    public static void main(String[] args) {
        String original = "Collection";
        List<Character> list = new ArrayList<>();

        for (char c : original.toCharArray()) {
            list.add(c);
        }

        Collections.reverse(list);

        StringBuilder reversed = new StringBuilder();
        for (char c : list) {
            reversed.append(c);
        }

        System.out.println("Reversed: " + reversed);
    }
}

✅ Explanation:

• Useful if you’re already working with character collections.
• Collections.reverse() reverses the order of elements in-place.

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🧼 Best Practices

• ✅ Use StringBuilder.reverse() for simplicity and speed.
• ✅ Use manual methods for learning or specific custom needs.
• ⚠️ Avoid + operator inside loops for large strings.
• ✅ Validate for null or empty strings before reversing.

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📌 Summary – String Reversal in Java

Reversing a string is a core programming task that helps you understand string manipulation, memory management, and loop constructs in Java. Java provides both built-in solutions and manual techniques for different use cases.

🧾 Key Takeaways:

• Use StringBuilder.reverse() for quick results.
• Use loops or char arrays for custom logic and learning.
• Always handle empty or null strings safely.

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❓FAQs – Java String Reversal

❓ What is the fastest way to reverse a string in Java?

Use StringBuilder.reverse() — it’s built-in, efficient, and readable.

❓ Can strings be reversed without using additional memory?

No. Since String is immutable, a new object is always created.

❓ How to reverse a string without using StringBuilder?

Use a loop or convert to a character array and swap characters manually.

❓ Is it possible to reverse words in a sentence, not characters?

Yes. Split the string by space, then reverse the array of words.

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