refresh java logo

StringBuffer and StringBuilder in Java

Apart from String class java has given two more classes to handle string values in java, StringBuffer and StringBuilder. These classes were added with some purpose, this tutorial explains the purpose to add these classes, the difference between both and when to use StringBuffer or StringBuilder.

String in java is immutable, which means you can not change string value once created. Whenever we do string manipulation like concatenation, substring, replace etc, it generates a new string and leaves the original string as it is in memory which becomes a garbage value. For an example the code given below will generate a garbage value as "refresh java" in memory.

  String str = "refresh java";
  str = str + " tutorial";			 

The concatenation operation above will create a new strings as "refresh java tutorial" in string pool memory and assigns it's reference in str but this operation won't change the original string "refresh java", it will still exist in pool memory as a garbage value.

what is garbage value ?

Any value or object inside the memory that is not being referred by program or application is known as garbage. The process of collecting and destroying such values from memory is known as garbage collection.

The string manipulation operations doesn't cost much if it is not happening frequently, but if it's happening frequently and at many number of places in your application, this may cause serious memory leak and performance issue as it will generate a lot of garbage in memory. To solve this problem java designer has given two more classes, StringBuffer and StringBuilder.

The string values stored in StringBuffer or StringBuilder data type are mutable. In other words objects of StringBuffer and StringBuilder classes are mutable which means if you apply any operation on such strings, it will change the original string itself rather than creating a new string. As a result it will not generate garbage in memory. The Syntax of declaring StringBuffer and StringBuilder is :

 StringBuffer var_name = new StringBuffer("StringBuffer value");

 StringBuilder var_name = new StringBuilder("StringBuilder value");
 
 // Example
 StringBuffer strbuff = new StringBuffer("refresh java");
 StringBuilder strbdr = new StringBuilder("java is easy");

Here StringBuffer and StringBuilder are the data type(classes), var_name is the name of variable given as per the programmers choice and the value of variable is given inside "".

Java considers string literal(value given in "") as String type, so you can not assign string literal directly to StringBuffer and StringBuilder variable as given below. You must have to use new keyword to create StringBuffer and StringBuilder.

 // Both line will throw compilation error.
 StringBuffer strbuff = "refresh java";
 StringBuilder strbdr = "java is easy";

You can also not use + operator to concat string on StringBuffer and StringBuilder variables.

 StringBuffer strbuff = new StringBuffer("refresh java");
 StringBuilder strbdr = new StringBuilder("java is easy");
 // Both line will throw compilation error.
 strbuff = strbuff + " another value";
 strbdr = strbdr + " another value";

How can I add another string value in StringBuffer or StringBuilder variable ?

You need to use append() method of these classes to add another string. Refer the program given below to see the use of this method.

Java Program of StringBuffer and StringBuilder

 class	StringBufferAndBuilder
  {
    public static void main(String args[]) 
      {
         String str = "refresh java";
         String str2 = str;
         StringBuffer strbuff = new StringBuffer("StringBuffer on refresh java");
         StringBuffer strbuff2 = strbuff;
         StringBuilder strbuild = new StringBuilder("StringBuilder on refresh java");
         StringBuilder strbuild2 = strbuild;

         str = str + " tutorial";
         strbuff = strbuff.append(" tutorial");
         strbuild = strbuild.append(" tutorial");
         
         System.out.println("str = "+str);
         System.out.println("str2 = "+str2);
         System.out.println("strbuff = "+strbuff);
         System.out.println("strbuff2 = "+strbuff2);
         System.out.println("strbuild = "+strbuild);
         System.out.println("strbuild2 = "+strbuild2);
      }
  }			 

Output:

str = refresh java tutorial
str2 = refresh java
strbuff = StringBuffer on refresh java tutorial
strbuff2 = StringBuffer on refresh java tutorial
strbuild = StringBuilder on refresh java tutorial
strbuild2 = StringBuilder on refresh java tutorial

As you can see in the output, both String variable have different value while both the StringBuffer and StringBuilder variable have same value, that's because String is immutable. Once you add a string literal in String variable, java creates a new string object and assigns the reference of it in String variable while in-case of StringBuffer and StringBuilder, the same string is modified.

Table below shows some of the useful methods of StringBuffer and StringBuilder class which is commonly used for string manipulation. Both the classes have same methods, the only difference is that methods of StringBuffer class is synchronized. Below table assumes we have a variable s1 of type StringBuffer.

Name Description Example
append(String str) It appends the string represented by str at the end of given string. s1 = new StringBuffer("refresh");
s1.append("Java");
// s1 = "refreshJava"
insert(int offset, String str) Used to insert string represented by str at index specified by offset in given string. s1 = new StringBuffer("refresh");
s1.insert(2, "java");
// s1 = "rejavafresh"
delete (int startIndex, int endIndex) It deletes characters from startIndex to (endIndex-1) from given string. s1 = new StringBuffer("refresh java");
s1.delete(2,7);
// s1 = "re java"
replace (int start, int end, String str) It replaces characters from start to (end-1) index with string represented by str in given string. s1 = new StringBuffer("refresh java");
s1.replace(0,7,"tutorial");
// s1 = "tutorial java"
reverse() It reverses the sequence of characters of given string. s1 = new StringBuffer("refresh java");
s1.reverse();
// s1 = "avaj hserfer"
setCharAt(int index, char ch) Used to replace the character at given index with character represented by ch in given string. s1 = new StringBuffer("refresh");
s1.setCharAt(0,'s');
// s1 = "sefresh"

Conversion from String to StringBuffer/StringBuilder and vice-versa

You can not directly assign a String variable in StringBuffer/StringBuilder or vice-versa. You need to use conversion like below to store it in other string data type variable.

 String str = "refresh java";
 // Both line will throw compilation error.
 // StringBuffer strbuff = str;
 // StringBuilder strbdr = str;
 // Conversion from String to StringBuffer and StringBuilder.
 StringBuffer strbuff = new StringBuffer(str);
 StringBuilder strbdr = new StringBuilder(str);
 
 // Both line will throw compilation error.
 // String str2 = strbuff;
 // String str3 = strbdr;
 // Conversion from StringBuffer/StringBuilder to String.
 String str2 = strbuff.toString();
 String str3 = strbdr.toString();

String vs StringBuffer vs StringBuilder

  • The String object is immutable while objects of StringBuffer and StringBuilder are mutable.
  • StringBuffer is thread safe as it's methods are synchronized while StringBuilder is not, so use StringBuffer in multi-threaded environment where thread safety is needed.
  • StringBuffer is not as faster as StringBuilder for string manipulation operations, since methods of StringBuffer class is synchronized.
  • StringBuilder was added in java 1.5, before that there was only String and StringBuffer classes for string handling.
  • In single threaded environment, prefer String over StringBuilder if you don't need mutable objects as String is more easy and convenient to use. Use StringBuilder if mutable objects needed.

What is multi-thread environment ?

In a multi-threaded environment multiple programs or different parts(thread) of a program runs simultaneously.

What is synchronized in java ?

synchronized is a keyword in java, use to achieve thread safety. A synchronized method can not be accessed simultaneously by multiple threads.

In most of the scenarios we don't use String data type in multithreaded environment where thread safety is needed, as it may cause inconsistent behavior. For example if a thread makes any changes in String variable, the other thread may not get the updated value which may cause inconsistent behavior of your program. To avoid such type of problem you should use StringBuffer class where thread safety is needed.

★★★
  • Refer java oracle docs or java source code of StringBuffer and StringBuilder class to get details of all methods.
  • There are many methods in String and StringBuffer/StringBuilder classes that are available in each other but not all.
  • Digits can also be stored as string in StringBuffer and StringBuilder class in java, for example StringBuffer str = new StringBuffer("12345"); is a valid string.
  • The '+' operator, which performs addition on primitives (such as int and double), is overloaded to perform concatenation on String objects. The compiler, internally uses the append method of StringBuffer/StringBuilder class to implement string concatenation.