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StringBuilder Class in Java

Last Updated : 8 May, 2026

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:


Output
Initial StringBuilder: GeeksforGeeks
After append: GeeksforGeeks is awesome!

Explanation : This example demonstrates the use of the append() method in StringBuilder to add text at the end of an existing string.

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.

Hierarchy of StringBuilder in Java

It extends the AbstractStringBuilder class and implements the CharSequence and Serializable interfaces.

👁 stringbuilder
StringBuilder Class

StringBuilder Constructors

StringBuilder class provides multiple constructors for different use cases.

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

Example:


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

Explanation : This program demonstrates different constructors of the StringBuilder class and how to append data using the append() method.

Commonly Used Methods in StringBuilder

The StringBuilder class provides various methods for string manipulation.

Example: Using StringBuilder Methods

Output:

👁 aa

Explanation : This program demonstrates various StringBuilder methods such as append(), insert(), replace(), delete(), reverse(), substring(), and setCharAt() to perform dynamic string manipulation efficiently in Java.

Important StringBuilder Methods

MethodDescriptionExample
append(String str)Appends the specified string to the end of the StringBuilder.sb.append("Geeks");
insert(int offset, String)Inserts the specified string at the given position in the StringBuilder.sb.insert(5, " Geeks");
replace(int start, int end, String)Replaces characters in a substring with the specified string.sb.replace(6, 11, "Geeks");
delete(int start, int end)Removes characters in the specified range.sb.delete(5, 11);
reverse()Reverses the sequence of characters in the StringBuilder.sb.reverse();
capacity()Returns the current capacity of the StringBuilder.int cap = sb.capacity();
length()Returns the number of characters in the StringBuilder.int len = sb.length();
charAt(int index)Returns the character at the specified index.char ch = sb.charAt(4);
setCharAt(int index, char)Replaces the character at the specified position with a new character.sb.setCharAt(0, 'G');
substring(int start, int end)Returns a new String that contains characters from the specified range.String sub = sb.substring(0, 5);
ensureCapacity(int minimum)Ensures the capacity of the StringBuilder is at least equal to the specified minimum.sb.ensureCapacity(50);
deleteCharAt(int index)Removes the character at the specified position.sb.deleteCharAt(3);
indexOf(String str)Returns the index of the first occurrence of the specified string.int idx = sb.indexOf("Geeks");
lastIndexOf(String str)Returns the index of the last occurrence of the specified string.int idx = sb.lastIndexOf("Geeks");
toString()Converts the StringBuilder object to a String.String result = sb.toString();

StringBuilder vs String vs StringBuffer

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

Features

String

StringBuilder

StringBuffer

Mutability

String are immutable(creates new objects on modification)

StringBuilder are mutable(modifies in place)

StringBuffer are mutable (modifies in place)

Thread-Safe

It is thread-safe

It is not thread-safe

It is thread-safe

Performance

It is slow because it creates an object each time

It is faster (no object creation)

It is slower due to synchronization overhead

Use Case

Fixed, unchanging strings

Single-threaded string manipulation

Multi-threaded string manipulation

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.

Limitations of StringBuilder

  • Not thread-safe because it is not synchronized.
  • May consume extra memory if large capacity is allocated unnecessarily.
  • Requires additional synchronization for safe use in multi-threaded applications.
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