StringBuffer Class in Java


The StringBuffer and StringBuilder classes are used when there is a necessity to make a lot of modifications to Strings of characters.

Unlike Strings objects of type StringBuffer and Stringbuilder can be modified over and over again with out leaving behind a lot of new unused objects.

The StringBuilder class was introduced as of Java 5 and the main difference between the StringBuffer and StringBuilder is that StringBuilders methods are not thread safe(not Synchronised).

It is recommended to use StringBuilder whenever possible because it is faster than StringBuffer. However if thread safety is necessary the best option is StringBuffer objects.

Difference: String class objects are immutable and hence their contents cannot be modified. StringBuffer class objects are mutable, so they can be modified. The reason is that StringBuffer is mutable and can expand dynamically in memory. To store characters, we can use append() method or insert() method like

object.append(“Hello”); // add “Hello” to object

object.insert(0,”Hello”); // insert “Hello” starting from 0th position in object.


public class Test{

    public static void main(String args[]){
       StringBuffer sBuffer = new StringBuffer(" test");
       sBuffer.append(" String Buffer");

This would produce the following result:

test String Buffer

StringBuffer Methods:

Here is the list of important methods supported by StringBuffer class:

SN Methods with Description
1 public StringBuffer append(String s)
Updates the value of the object that invoked the method. The method takes boolean, char, int, long, Strings etc.
2 public StringBuffer reverse()
The method reverses the value of the StringBuffer object that invoked the method.
3 public delete(int start, int end)
Deletes the string starting from start index until end index.
4 public insert(int offset, int i)
This method inserts an string s at the position mentioned by offset.
5 replace(int start, int end, String str)
This method replaces the characters in a substring of this StringBuffer with characters in the specified String.

Here is the list of other methods (Except set methods ) which are very similar to String class:

SN Methods with Description
1 int capacity()
Returns the current capacity of the String buffer.
2 char charAt(int index)
The specified character of the sequence currently represented by the string buffer, as indicated by the index argument, is returned.
3 void ensureCapacity(int minimumCapacity)
Ensures that the capacity of the buffer is at least equal to the specified minimum.
4 void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin)
Characters are copied from this string buffer into the destination character array dst.
5 int indexOf(String str)
Returns the index within this string of the first occurrence of the specified substring.
6 int indexOf(String str, int fromIndex) 
Returns the index within this string of the first occurrence of the specified substring, starting at the specified index.
7 int lastIndexOf(String str)
Returns the index within this string of the rightmost occurrence of the specified substring.
8 int lastIndexOf(String str, int fromIndex)
Returns the index within this string of the last occurrence of the specified substring.
9 int length()
Returns the length (character count) of this string buffer.
10 void setCharAt(int index, char ch)
The character at the specified index of this string buffer is set to ch.
11 void setLength(int newLength)
Sets the length of this String buffer.
12 CharSequence subSequence(int start, int end) 
Returns a new character sequence that is a subsequence of this sequence.
13 String substring(int start)
Returns a new String that contains a subsequence of characters currently contained in this StringBuffer.The substring begins at the specified index and extends to the end of the StringBuffer.
14 String substring(int start, int end)
Returns a new String that contains a subsequence of characters currently contained in this StringBuffer.
15 String toString()
Converts to a string representing the data in this string buffer.

In Simple words:

String is used to manipulate character strings that cannot be changed (read-only and immutable).

StringBuffer is used to represent characters that can be modified.

Performance wise, StringBuffer is faster when performing concatenations. This is because when you concatenate a String, you are creating a new object (internally) every time since String is immutable.

You can also use StringBuilder which is similar to StringBuffer except it is not synchronized. The maximum size for either of these is Integer.MAX_VALUE (231 – 1 = 2,147,483,647) or maximum heap size divided by 2