Java StringBuffer class is used to create mutable (modifiable) string. The StringBuffer class in java is same as String class except it is mutable i.e. it can be changed.
Note: Java StringBuffer class is thread-safe i.e. multiple threads cannot access it simultaneously. So it is safe and will result in an order.

Table of Contents

Important methods of StringBuffer class

Constructor	Description
StringBuffer()	creates an empty string buffer with the initial capacity of 16.
StringBuffer(String str)	creates a string buffer with the specified string.
StringBuffer(int capacity)	creates an empty string buffer with the specified capacity as length.

Important methods of StringBuffer class

Modifier and Type	Method	Description
public synchronized StringBuffer	append(String s)	is used to append the specified string with this string. The append() method is overloaded like append(char), append(boolean), append(int), append(float), append(double) etc.
public synchronized StringBuffer	insert(int offset, String s)	is used to insert the specified string with this string at the specified position. The insert() method is overloaded like insert(int, char), insert(int, boolean), insert(int, int), insert(int, float), insert(int, double) etc.
public synchronized StringBuffer	replace(int startIndex, int endIndex, String str)	is used to replace the string from specified startIndex and endIndex.
public synchronized StringBuffer	delete(int startIndex, int endIndex)	is used to delete the string from specified startIndex and endIndex.
public synchronized StringBuffer	reverse()	is used to reverse the string.
public int	capacity()	is used to return the current capacity.
public void	ensureCapacity(int minimumCapacity)	is used to ensure the capacity at least equal to the given minimum.
public char	charAt(int index)	is used to return the character at the specified position.
public int	length()	is used to return the length of the string i.e. total number of characters.
public String	substring(int beginIndex)	is used to return the substring from the specified beginIndex.
public String	substring(int beginIndex, int endIndex)	is used to return the substring from the specified beginIndex and endIndex.

What is mutable string

A string that can be modified or changed is known as mutable string. StringBuffer and StringBuilder classes are used for creating mutable string.

1) StringBufferappend() method

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

1. class StringBufferExample{ 
2. public static void main(String args[]){ 
3. StringBuffer sb=new StringBuffer("Hello "); 
4. sb.append("Java");//now original string is changed 
5. System.out.println(sb);//prints Hello Java 
6. } 
7. } 
2) StringBufferinsert() method
The insert() method inserts the given string with this string at the given position.
1. class StringBufferExample2{ 
2. public static void main(String args[]){ 
3. StringBuffer sb=new StringBuffer("Hello "); 
4. sb.insert(1,"Java");//now original string is changed 
5. System.out.println(sb);//prints HJavaello 
6. } 
7. }

3) StringBufferreplace() method

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

1. class StringBufferExample3{ 
2. public static void main(String args[]){ 
3. StringBuffer sb=new StringBuffer("Hello"); 
4. sb.replace(1,3,"Java"); 
5. System.out.println(sb);//prints HJavalo 
6. } 
7. }

4) StringBufferdelete() method

The delete() method of StringBuffer class deletes the string from the specified beginIndex to endIndex.

1. class StringBufferExample4{ 
2. public static void main(String args[]){ 
3. StringBuffer sb=new StringBuffer("Hello"); 
4. sb.delete(1,3); 
5. System.out.println(sb);//prints Hlo 
6. } 
7. }

5) StringBufferreverse() method

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

1. class StringBufferExample5{ 
2. public static void main(String args[]){ 
3. StringBuffer sb=new StringBuffer("Hello"); 
4. sb.reverse(); 
5. System.out.println(sb);//prints olleH 
6. } 
7. }

6) StringBuffercapacity() method

The capacity() method of StringBuffer class returns the current capacity of the buffer. The default capacity of the buffer is 16. If the number of character increases from its current capacity, it increases the capacity by (oldcapacity*2)+2. For example if your current capacity is 16, it will be (16*2)+2=34.

1. class StringBufferExample6{ 
2. public static void main(String args[]){ 
3. StringBuffer sb=new StringBuffer(); 
4. System.out.println(sb.capacity());//default 16 
5. sb.append("Hello"); 
6. System.out.println(sb.capacity());//now 16 
7. sb.append("java is my favourite language"); 
8. System.out.println(sb.capacity());//now (16*2)+2=34 i.e (oldcapacity*2)+2 
9. } 
10. }

7) StringBufferensureCapacity() method

The ensureCapacity() method of StringBuffer class ensures that the given capacity is the minimum to the current capacity. If it is greater than the current capacity, it increases the capacity by (oldcapacity*2)+2. For example if your current capacity is 16, it will be (16*2)+2=34.

1. class StringBufferExample7{ 
2. public static void main(String args[]){ 
3. StringBuffer sb=new StringBuffer(); 
4. System.out.println(sb.capacity());//default 16 
5. sb.append("Hello"); 
6. System.out.println(sb.capacity());//now 16 
7. sb.append("java is my favourite language"); 
8. System.out.println(sb.capacity());//now (16*2)+2=34 i.e (oldcapacity*2)+2 
9. sb.ensureCapacity(10);//now no change 
10. System.out.println(sb.capacity());//now 34 
11. sb.ensureCapacity(50);//now (34*2)+2 
12. System.out.println(sb.capacity());//now 70 
13. } 
14. }

Java StringBuilder class

Java StringBuilder class is used to create mutable (modifiable) string. The Java StringBuilder class is same as StringBuffer class except that it is non-synchronized. It is available since JDK 1.5.

Important Constructors of StringBuilder class

Constructor	Description
StringBuilder()	creates an empty string Builder with the initial capacity of 16.
StringBuilder(String str)	creates a string Builder with the specified string.
StringBuilder(int length)	creates an empty string Builder with the specified capacity as length.

Important methods of StringBuilder class

Method	Description
public StringBuilder append(String s)	is used to append the specified string with this string. The append() method is overloaded like append(char), append(boolean), append(int), append(float), append(double) etc.
public StringBuilder insert(int offset, String s)	is used to insert the specified string with this string at the specified position. The insert() method is overloaded like insert(int, char), insert(int, boolean), insert(int, int), insert(int, float), insert(int, double) etc.
public StringBuilder replace(int startIndex, int endIndex, String str)	is used to replace the string from specified startIndex and endIndex.
public StringBuilder delete(int startIndex, int endIndex)	is used to delete the string from specified startIndex and endIndex.
public StringBuilder reverse()	is used to reverse the string.
public int capacity()	is used to return the current capacity.
public void ensureCapacity(int minimumCapacity)	is used to ensure the capacity at least equal to the given minimum.
public char charAt(int index)	is used to return the character at the specified position.
public int length()	is used to return the length of the string i.e. total number of characters.
public String substring(int beginIndex)	is used to return the substring from the specified beginIndex.
public String substring(int beginIndex, int endIndex)	is used to return the substring from the specified beginIndex and endIndex.

Java StringBuilder Examples

Let’s see the examples of different methods of StringBuilder class.

1) StringBuilder append() method

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

1. class StringBuilderExample{ 
2. public static void main(String args[]){ 
3. StringBuilder sb=new StringBuilder("Hello "); 
4. sb.append("Java");//now original string is changed 
5. System.out.println(sb);//prints Hello Java 
6. } 
7. }

2) StringBuilder insert() method

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

1. class StringBuilderExample2{ 
2. public static void main(String args[]){ 
3. StringBuilder sb=new StringBuilder("Hello "); 
4. sb.insert(1,"Java");//now original string is changed 
5. System.out.println(sb);//prints HJavaello 
6. } 
7. }

3) StringBuilder replace() method

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

1. class StringBuilderExample3{
2. public static void main(String args[]){
3. StringBuilder sb=new StringBuilder(“Hello”);
4. sb.replace(1,3,”Java”);
5. System.out.println(sb);//prints HJavalo
6. }
7. }

4) StringBuilder delete() method

The delete() method of StringBuilder class deletes the string from the specified beginIndex to endIndex.

1. class StringBuilderExample4{ 
2. public static void main(String args[]){ 
3. StringBuilder sb=new StringBuilder("Hello"); 
4. sb.delete(1,3); 
5. System.out.println(sb);//prints Hlo 
6. } 
7. }

5) StringBuilder reverse() method

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

1. class StringBuilderExample5{ 
2. public static void main(String args[]){ 
3. StringBuilder sb=new StringBuilder("Hello"); 
4. sb.reverse(); 
5. System.out.println(sb);//prints olleH 
6. } 
7. }

6) StringBuilder capacity() method

The capacity() method of StringBuilder class returns the current capacity of the Builder. The default capacity of the Builder is 16. If the number of character increases from its current capacity, it increases the capacity by (oldcapacity*2)+2. For example if your current capacity is 16, it will be (16*2)+2=34.

1. class StringBuilderExample6{ 
2. public static void main(String args[]){ 
3. StringBuilder sb=new StringBuilder(); 
4. System.out.println(sb.capacity());//default 16 
5. sb.append("Hello"); 
6. System.out.println(sb.capacity());//now 16 
7. sb.append("java is my favourite language"); 
8. System.out.println(sb.capacity());//now (16*2)+2=34 i.e (oldcapacity*2)+2 
9. } 
10. }

7) StringBuilder ensureCapacity() method

The ensureCapacity() method of StringBuilder class ensures that the given capacity is the minimum to the current capacity. If it is greater than the current capacity, it increases the capacity by (oldcapacity*2)+2. For example if your current capacity is 16, it will be (16*2)+2=34.

1. class StringBuilderExample7{ 
2. public static void main(String args[]){ 
3. StringBuilder sb=new StringBuilder(); 
4. System.out.println(sb.capacity());//default 16 
5. sb.append("Hello"); 
6. System.out.println(sb.capacity());//now 16 
7. sb.append("java is my favourite language"); 
8. System.out.println(sb.capacity());//now (16*2)+2=34 i.e (oldcapacity*2)+2 
9. sb.ensureCapacity(10);//now no change 
10. System.out.println(sb.capacity());//now 34 
11. sb.ensureCapacity(50);//now (34*2)+2 
12. System.out.println(sb.capacity());//now 70 
13. } 
14. }

So, this brings us to the end of blog. This Tecklearn ‘StrinngBuffer and StringBuilder Class in Java’ blog helps you with commonly asked questions if you are looking out for a job in Java Programming. If you wish to learn Java and build a career Java Programming domain, then check out our interactive, Java and JEE Training, that comes with 24*7 support to guide you throughout your learning period. Please find the link for course details:

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