Skip to content

Java StringBuilder Class

StringBuffer Class in Java

Last Updated : 18 Apr, 2025

The 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.

Features of StringBuffer Class

The key features of StringBuffer class are listed below:

Unlike String, we can modify the content of the StringBuffer without creating a new object.
StringBuffer has an initial capacity, and it can also be adjusted later with the help of the ensureCapacity() method.
With the help of the append() method, we can add characters, strings, or objects at the end of the StringBuffer.
With the help of the insert() method, we can insert characters, strings, or objects at a specified position in the StringBuffer.
With the help of the delete() method, we can remove characters from the StringBuffer.
With the help of reverse() method, we can reverse the order of characters in the StringBuffer.

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

Java

//Demonstrating String Buffer 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

Advantages of using StringBuffer in Java

The advanatages of StringBuffer class are listed below:

Mutable: StringBuffer are mutable it means that we can change the content after the object has been created, on the other hand String are immutable once it created it can not be modified.
Efficient: Since StringBuffer objects are mutable, it is suitable in scenarios where we need to modify the string multiple times. If we do the same thing with string, everytime a new object is created and the old one is deleted, which is very bad in terms of performance and memory.

Note: Both String and StringBuffer objects are thread safe, but in different ways. StringBuffer is synchronized it means it is thread-safe but keep in mind that this synchronization can cause performance issues if accessed by multiple threads at the same time. On the other hand immutable objects like String are thread-safe because their state can not be modified once they are created.

Constructors of StringBuffer Class

Constructor	Description	Syntax
StringBuffer()	It reserves room for 16 characters without reallocation	StringBuffer s = new StringBuffer();
StringBuffer(int size)	It accepts an integer argument that explicitly sets the size of the buffer.	StringBuffer s = new StringBuffer(20);
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.	StringBuffer s = new StringBuffer("GeeksforGeeks");

Methods of Java StringBuffer Class

Methods	Action Performed
append()	Used to add text at the end of the existing text.
length()	The length of a StringBuffer can be found by the length( ) method.
capacity()	the total allocated capacity can be found by the capacity( ) method.
charAt()	This method returns the char value in this sequence at the specified index.
delete()	Deletes a sequence of characters from the invoking object.
deleteCharAt()	Deletes the character at the index specified by the loc.
ensureCapacity()	Ensures capacity is at least equal to the given minimum.
insert()	Inserts text at the specified index position.
length()	Returns the length of the string.
reverse()	Reverse the characters within a StringBuffer object.
replace()	Replace one set of characters with another set inside a StringBuffer object.

Examples of Java StringBuffer Method

1. append() Method

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

Example:

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

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

Example:

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

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

Example:

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

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

Example:

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

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

Example:

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

The 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.

Example:

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

Some Interesting Facts about the StringBuffer Class

Do keep the following points in the back of your mind:

java.lang.StringBuffer extends (or inherits from) Object class.
All Implemented Interfaces of StringBuffer classes are Serializable, Appendable, CharSequence.
public final class StringBuffer extends Object implements Serializable, CharSequence, Appendable.
String buffers are safe for use by multiple threads. The methods can be synchronized wherever necessary so that all the operations on any particular instance behave as if they occur in some serial order.
Whenever an operation occurs involving a source sequence (such as appending or inserting from a source sequence) this class synchronizes only on the string buffer performing the operation, not on the source.
It inherits some of the methods from the Object class which such as clone(), equals(), finalize(), getClass(), hashCode(), notifies(), notifyAll().

Remember: StringBuilder, J2SE 5 adds a new string class to Java’s already powerful string handling capabilities. This new class is called StringBuilder. It is identical to StringBuffer except for one important difference: it is not synchronized, which means that it is not thread-safe.
The advantage of StringBuilder is faster performance. However, in cases in which you are using multithreading, you must use StringBuffer as it is thread-safe rather than StringBuilder.

Other Methods in Java StringBuffer

These auxiliary methods are as follows:

Methods	Description	Syntax
ensureCapacity()	It is used to increase the capacity of a StringBuffer object. The new capacity will be set to either the value we specify or twice the current capacity plus two (i.e. capacity+2), whichever is larger. Here, capacity specifies the size of the buffer.	void ensureCapacity(int capacity)
appendCodePoint(int codePoint)	This method appends the string representation of the codePoint argument to this sequence.	public StringBuffer appendCodePoint(int codePoint)
charAt(int index)	This method returns the char value in this sequence at the specified index.	public char charAt(int index)
IntStream chars()	This method returns a stream of int zero-extending the char values from this sequence.	public IntStream chars()
int codePointAt(int index)	This method returns the character (Unicode code point) at the specified index.	public int codePointAt(int index)
int codePointBefore(int index)	This method returns the character (Unicode code point) before the specified index.	public int codePointBefore(int index)
int codePointCount(int beginIndex, int endIndex)	This method returns the number of Unicode code points in the specified text range of this sequence.	public int codePointCount(int beginIndex, int endIndex)
IntStream codePoints()	This method returns a stream of code point values from this sequence.	public IntStream codePoints()
void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)	In this method, the characters are copied from this sequence into the destination character array dst.	public void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)
int indexOf(String str)	This method returns the index within this string of the first occurrence of the specified substring.	public int indexOf(String str) public int indexOf(String str, int fromIndex)
int lastIndexOf(String str)	This method returns the index within this string of the last occurrence of the specified substring.	public int lastIndexOf(String str) public int lastIndexOf(String str, int fromIndex)
int offsetByCodePoints(int index, int codePointOffset)	This method returns the index within this sequence that is offset from the given index by codePointOffset code points.	public int offsetByCodePoints(int index, int codePointOffset)
void setCharAt(int index, char ch)	In this method, the character at the specified index is set to ch.	public void setCharAt(int index, char ch)
void setLength(int newLength)	This method sets the length of the character sequence.	public void setLength(int newLength)
CharSequence subSequence(int start, int end)	This method returns a new character sequence that is a subsequence of this sequence.	public CharSequence subSequence(int start, int end)
String substring(int start)	This method returns a new String that contains a subsequence of characters currently contained in this character sequence.	public String substring(int start) public String substring(int start,int end)
String toString()	This method returns a string representing the data in this sequence.	public String toString()
void trimToSize()	This method attempts to reduce storage used for the character sequence.	public void trimToSize()

Note: Above we only have discussed the most widely used methods and do keep a tight bound around them as they are widely used in programming geeks.

Examples of the above Methods

Example 1: length() and capacity() Methods

Java

// Java Program to Illustrate StringBuffer class // via length() and capacity() methods 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         int q = s.capacity();          // Printing the length and capacity of         // above generated input string on console         System.out.println("Length of string GeeksforGeeks="                            + p);         System.out.println("Capacity of string GeeksforGeeks="                            + q);     } }

Output

Length of string GeeksforGeeks=13 Capacity of string GeeksforGeeks=29

Example 2: append()

It is used to add text at the end of the existing text. Here are a few of its forms:

StringBuffer append(String str)
StringBuffer append(int num)

Illustration:

Java

// Java Program to Illustrate StringBuffer class // via append() method  import java.io.*;  class Geeks {   	public static void main(String[] args){                // Creating an object of StringBuffer class and         // passing random string         StringBuffer s = new StringBuffer("Geeksfor");          // Usage of append() method         s.append("Geeks");          System.out.println(s);          s.append(1);                  System.out.println(s);     } }

Output

GeeksforGeeks GeeksforGeeks1

Example 3: insert()

It is used to insert text at the specified index position. Syntax of method is mentioned below:

StringBuffer insert(int index, String str)
StringBuffer insert(int index, char ch)

Here, the index specifies the index at which point the string will be inserted into the invoking StringBuffer object.

Illustration:

Java

// Java Program to Illustrate StringBuffer class // via insert() method import java.io.*;  class Geeks {     public static void main(String[] args) {                // Creating an object of StringBuffer class         StringBuffer s = new StringBuffer("GeeksGeeks");          // Inserting element and position as an arguments         s.insert(5, "for");           System.out.println(s);          s.insert(0, 5);          System.out.println(s);          s.insert(3, true);          System.out.println(s);          s.insert(5, 41.35d);          System.out.println(s);          s.insert(8, 41.35f);           System.out.println(s);          // Declaring and initializing character array         char geeks_arr[] = { 'p', 'a', 'w', 'a', 'n' };          // Inserting character array at offset 9         s.insert(2, geeks_arr);          System.out.println(s);     } }

Output

GeeksforGeeks 5GeeksforGeeks 5GetrueeksforGeeks 5Getr41.35ueeksforGeeks 5Getr41.41.3535ueeksforGeeks 5Gpawanetr41.41.3535ueeksforGeeks

Example 4: reverse( )

It can reverse the characters within a StringBuffer object using reverse( ). This method returns the reversed object on which it was called.

Java

// Java Program to Illustrate StringBuffer class // via reverse() method import java.io.*;  class Geeks {     public static void main(String[] args){                // Creating a string via creating         // object of StringBuffer class         StringBuffer s = new StringBuffer("GeeksGeeks");          // Invoking reverse() method         s.reverse();          System.out.println(s);     } }

Output

skeeGskeeG

Example 5: delete( ) and deleteCharAt()

It can delete characters within a StringBuffer by using the methods delete( ) and deleteCharAt( ). The delete( ) method deletes a sequence of characters from the invoking object. Here, the start Index specifies the index of the first character to remove, and the end Index specifies an index one past the last character to remove. Thus, the substring deleted runs from start Index to endIndex–1. The resulting StringBuffer object is returned. The deleteCharAt( ) method deletes the character at the index specified by loc. It returns the resulting StringBuffer object.

Syntax:

StringBuffer delete(int startIndex, int endIndex)
StringBuffer deleteCharAt(int loc)

Illustration:

Java

// Java Program to Illustrate StringBuffer class // via delete() and deleteCharAt() Methods  import java.io.*;  class Geeks {     public static void main(String[] args) {                StringBuffer s = new StringBuffer("GeeksforGeeks"); 	    s.delete(0, 5);             System.out.println(s);          s.deleteCharAt(7);          System.out.println(s);     } }

Output

forGeeks forGeek

Example 6: replace()

It can replace one set of characters with another set inside a StringBuffer object by calling replace( ). The substring being replaced is specified by the indexes start Index and endIndex. Thus, the substring at start Index through endIndex–1 is replaced. The replacement string is passed in str. The resulting StringBuffer object is returned.

Syntax:

StringBuffer replace(int startIndex, int endIndex, String str)

Illustration:

Java

// Java Program to Illustrate StringBuffer class // via replace() method import java.io.*;  class Geeks {     public static void main(String[] args) {                StringBuffer s = new StringBuffer("GeeksforGeeks");         s.replace(5, 8, "are");          System.out.println(s);     } }

Output

GeeksareGeeks

Java StringBuilder Class

L

Lokesh Todwal

Improve

Article Tags :

Practice Tags :

Similar Reads