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StringBuffer Class in Java

Last Updated : 27 May, 2026

StringBuffer class in Java represents a sequence of characters that can be modified, which means we can change the content of the StringBuffer without creating a new object every time. It represents a mutable sequence of characters.

Unlike String, we can modify the content of the StringBuffer without creating a new object.
All methods of StringBuffer are synchronized, making it safe to use in multithreaded environments.
Ideal for scenarios with frequent modifications like append, insert, delete or replace operations.

Example: Here is an example of using StringBuffer to concatenate strings.

Java

public class Geeks {
    public static void main(String[] args){
      
      	// Creating StringBuffer
        StringBuffer s = new StringBuffer();
      
      	// Adding elements in StringBuffer
        s.append("Hello");
        s.append(" ");
        s.append("world");
        
      	// String with the StringBuffer value
      	String str = s.toString();
        System.out.println(str);
    }
}

Output

Hello world

Interface Hierarchy Implemented by StringBuffer

The diagram shows the interfaces implemented by StringBuffer and illustrates its relationship with Appendable, CharSequence, and Serializable in Java.

Constructors of StringBuffer Class

StringBuffer(): It reserves room for 16 characters without reallocation
StringBuffer(int size): It accepts an integer argument that explicitly sets the size of the buffer.
StringBuffer(String str): It accepts a string argument that sets the initial contents of the StringBuffer object and reserves room for 16 more characters without reallocation.

Example:

Java

public class Geeks {
    public static void main(String[] args) {

        // 1. Using default constructor
        StringBuffer sb1 = new StringBuffer();
        sb1.append("Hello");
        System.out.println("Default Constructor: " + sb1);

        // 2. Using constructor with specified capacity
        StringBuffer sb2 = new StringBuffer(50);
        sb2.append("Java Programming");
        System.out.println("With Capacity 50: " + sb2);

        // 3. Using constructor with String
        StringBuffer sb3 = new StringBuffer("Welcome");
        sb3.append(" to Java");
        System.out.println("With String: " + sb3);
    }
}

Output

Default Constructor: Hello
With Capacity 50: Java Programming
With String: Welcome to Java

Implementation of Java StringBuffer Method

1. append() Method

append() method concatenates the given argument with this string.

Java

import java.io.*;

class Geeks {
    public static void main(String args[])
    {
        StringBuffer sb = new StringBuffer("Hello ");
        sb.append("Java"); // now original string is changed
        System.out.println(sb);
    }
}

Output

Hello Java

2. insert() Method

insert() method inserts the given string with this string at the given position.

Java

import java.io.*;

class Geeks {
    public static void main(String args[])
    {
        StringBuffer sb = new StringBuffer("Hello ");
        sb.insert(1, "Java");
        
        // Now original string is changed
        System.out.println(sb);
    }
}

Output

HJavaello

3. replace() Method

replace() method replaces the given string from the specified beginIndex and endIndex-1.

Java

import java.io.*;

class Geeks {
    public static void main(String args[]) {
      
        StringBuffer sb = new StringBuffer("Hello");
        sb.replace(1, 3, "Java");
        System.out.println(sb);
    }
}

Output

HJavalo

4. delete() Method

delete() method is used to delete the string from the specified beginIndex to endIndex-1.

Java

import java.io.*;

class Geeks {
    public static void main(String args[]) {
      
        StringBuffer sb = new StringBuffer("Hello");
        sb.delete(1, 3);
        System.out.println(sb);
    }
}

Output

Hlo

5. reverse() Method

reverse() method of the StringBuffer class reverses the current string.

Java

import java.io.* ;

class Geeks {
    public static void main(String args[]) {
        StringBuffer sb = new StringBuffer("Hello");
        sb.reverse();
        System.out.println(sb);
    }
}

Output

olleH

6. capacity() Method

capacity() method of the StringBuffer class returns the current capacity of the buffer. The default capacity of the buffer is 16. If the number of characters increases from its current capacity, it increases the capacity by (oldcapacity*2)+2.

For example, if the current capacity is 16, it will be (16*2)+2=34.

Java

import java.io.*;

class Geeks {
    public static void main(String args[])
    {
        StringBuffer sb = new StringBuffer();
      
      	// default 16
        System.out.println(sb.capacity()); 
        sb.append("Hello");
      
      	// now 16
        System.out.println(sb.capacity()); 
        sb.append("java is my favourite language");
        
      	// (oldcapacity*2)+2
      	System.out.println(sb.capacity());
    }
}

Output

16
16
34

7. length()

This method return the number of character in given string.

Java

import java.io.*;

class Geeks {
    public static void main(String[] args) {

        // Creating and storing string by creating object of StringBuffer
        StringBuffer s = new StringBuffer("GeeksforGeeks");

        // Getting the length of the string
        int p = s.length();

        // Getting the capacity of the string
        System.out.println("Length of string GeeksforGeeks=" + p);
    } 
}

Output

Length of string GeeksforGeeks=13

Advantages:

Mutable: Its content can be modified after creation while String remains immutable.
Better for Repeated Updates: Frequent concatenations perform efficiently because no new objects are created for each change.
Thread-Safe: All methods are synchronized, making it safe in multithreaded environments.

Limitations:

Performance Overhead: Synchronization makes operations slower compared to non-synchronized alternatives.
Not Ideal for Single-Threaded Applications: When thread safety is unnecessary, it adds extra processing cost.
Lower Performance Than StringBuilder: StringBuilder is preferred in non-threaded scenarios because it is faster.

StringBuffer all Methods

Methods	Description	Syntax
append()	Used to add text at the end of the existing text.	sb.append(String str);
length()	The length of a StringBuffer can be found by the length( ) method.	int len = sb.length();
capacity()	the total allocated capacity can be found by the capacity( ) method.	int cap = sb.capacity();
charAt()	This method returns the char value in this sequence at the specified index.	char ch = sb.charAt(int index);
delete()	Deletes a sequence of characters from the invoking object.	sb.delete(int start, int end);
deleteCharAt()	Deletes the character at the index specified by the loc.	sb.deleteCharAt(int index);
ensureCapacity()	Ensures capacity is at least equal to the given minimum.	sb.ensureCapacity(int minimumCapacity);
insert()	Inserts text at the specified index position.	sb.insert(int offset, String str);
reverse()	Reverse the characters within a StringBuffer object.	sb.reverse();
replace()	Replace one set of characters with another set inside a StringBuffer object.	sb.replace(int start, int end, String str);
ensureCapacity()	Increases StringBuffer capacity to the specified value	void ensureCapacity(int capacity)
appendCodePoint(int codePoint)	Appends code point as a string to the sequence.	public StringBuffer appendCodePoint(int codePoint)
charAt(int index)	Returns the char at the specified index.	public char charAt(int index)
IntStream chars()	Returns a stream of int values from zero-extended chars in the sequence..	public IntStream chars()
codePointAt()	Returns the character (Unicode code point) at the specified index.	public int codePointAt(int index)
codePointBefore()	Returns the Unicode code point before the given index.	public int codePointBefore(int index)
codePointCount()	Returns the count of Unicode code points in the specified text range.	public int codePointCount(int beginIndex, int endIndex)
IntStream codePoints()	Returns a stream of code points from the sequence.	public IntStream codePoints()
getChars()	Copies characters from the sequence into the destination array.	public void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)
indexOf()	Returns the index of the first occurrence of the specified substring.	public int indexOf(String str) public int indexOf(String str, int fromIndex)
lastIndexOf()	Returns the index of the last occurrence of the specified substring.	public int lastIndexOf(String str) public int lastIndexOf(String str, int fromIndex)
offsetByCodePoints()	Returns the index offset by a specified number of code points from the given index..	public int offsetByCodePoints(int index, int codePointOffset)
setCharAt()	In this method, the character at the specified index is set to ch.	public void setCharAt(int index, char ch)
setLength()	This method sets the length of the character sequence.	public void setLength(int newLength)
subSequence()	Returns index offset by given code points from the specified index.	public CharSequence subSequence(int start, int end)
substring()	Returns a new String containing a subsequence of this character sequence.	public String substring(int start) public String substring(int start,int end)
toString()	Returns a string representing the data in the sequence.	public String toString()
trimToSize()	Attempts to minimize storage used by the character sequence.	public void trimToSize()

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