StringBuilder Class in Java

In Java, the StringBuilder class (part of the java.lang package) provides a mutable sequence of characters. Unlike the String class (which is immutable), StringBuilder allows modification of character sequences without creating new objects, making it memory-efficient and faster for frequent string operations.

• It provides similar functionality to StringBuffer, but without thread safety.
• StringBuilder is not synchronized, so it performs better in single-threaded applications.
• Use StringBuffer only when thread safety is required; otherwise, prefer StringBuilder for improved performance.

Declaration:

StringBuilder sb = new StringBuilder("Initial String");

Example: : Basic Demonstration of StringBuilder

public class Geeks {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("GeeksforGeeks");
        System.out.println("Initial StringBuilder: " + sb);

        sb.append(" is awesome!");
        System.out.println("After append: " + sb);
    }
}

Initial StringBuilder: GeeksforGeeks
After append: GeeksforGeeks is awesome!

The append() method adds the given string to the end of the existing sequence without creating a new object. This makes StringBuilder efficient for concatenation operations.

StringBuilder Constructors

StringBuilder class provides multiple constructors for different use cases.

1. StringBuilder() : Creates an empty builder with a default capacity of 16 characters.
2. StringBuilder(int capacity) : Creates an empty builder with a specified initial capacity.
3. StringBuilder(String str) : Initializes the builder with the content of the given String.
4. StringBuilder(CharSequence cs) : Initializes the builder with the given CharSequence (for example, String or StringBuffer).

Example:

public class StringBuilderConstructorsDemo {
    public static void main(String[] args) {
        StringBuilder sb1 = new StringBuilder();
        sb1.append("Hello");
        System.out.println("sb1: " + sb1);

        StringBuilder sb2 = new StringBuilder(50);
        sb2.append("This has initial capacity 50");
        System.out.println("sb2: " + sb2);

        StringBuilder sb3 = new StringBuilder("Geeks");
        sb3.append("ForGeeks");
        System.out.println("sb3: " + sb3);

        CharSequence cs = "Java";
        StringBuilder sb4 = new StringBuilder(cs);
        sb4.append("Programming");
        System.out.println("sb4: " + sb4);
    }
}

sb1: Hello
sb2: This has initial capacity 50
sb3: GeeksForGeeks
sb4: JavaProgramming

Commonly Used Methods in StringBuilder

The StringBuilder class provides various methods for string manipulation.

Example: Using StringBuilder Methods

public class Geeks {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder("GeeksforGeeks");
        System.out.println("Initial: " + sb);

        sb.append(" is awesome!");
        System.out.println("After append: " + sb);

        sb.insert(13, " Java");
        System.out.println("After insert: " + sb);

        sb.replace(0, 5, "Welcome to");
        System.out.println("After replace: " + sb);

        sb.delete(8, 14);
        System.out.println("After delete: " + sb);

        sb.reverse();
        System.out.println("After reverse: " + sb);

        System.out.println("Capacity: " + sb.capacity());
        System.out.println("Length: " + sb.length());

        char c = sb.charAt(5);
        System.out.println("Character at index 5: " + c);

        sb.setCharAt(5, 'X');
        System.out.println("After setCharAt: " + sb);

        String sub = sb.substring(5, 10);
        System.out.println("Substring (5–10): " + sub);

        sb.reverse(); // Revert for search
        System.out.println("Index of 'Geeks': " + sb.indexOf("Geeks"));

        sb.deleteCharAt(5);
        System.out.println("After deleteCharAt: " + sb);

        String result = sb.toString();
        System.out.println("Final String: " + result);
    }
}

Output:

Initial: GeeksforGeeks
After append: GeeksforGeeks is awesome!
After insert: GeeksforGeeks Java is awesome!
After replace: Welcome toforGeeks Java is awesome!
After delete: Welcome eeks Java is awesome!
After reverse: !emosewa si avaJ skee emocleW
Capacity: 60
Length: 29
Character at index 5: e
After setCharAt: !emosXwa si avaJ skee emocleW
Substring (5?10): Xwa s
Index of 'Geeks': -1
After deleteCharAt: Welcoe eeks Java is awXsome!
Final String: Welcoe eeks Java is awXsome!

In the above program, we use different methods of the StringBuilder class to perform different string manipulation operations such as append(), insert(), reverse() and delete().

Important StringBuilder Methods

StringBuilder vs String vs StringBuffer

The table below demonstrates the difference between String, StringBuilder and StringBuffer:

Advantages of StringBuilder

• Performs faster string manipulations in single-threaded environments.
• Reduces memory overhead by modifying content in place.
• Automatically increases capacity when needed.
• Suitable for operations inside loops where strings are frequently changed.

Disadvantages of StringBuilder

• Not synchronized; unsuitable for multi-threaded environments.
• May allocate extra memory if the initial capacity is set too high.
• Requires manual synchronization if used across multiple threads.

StringBuilder and StringBuffer

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