String vs StringBuilder vs StringBuffer

String is an integral part of Java programming. String is nothing but a sequence of characters. It is one of the most widely used classes in Java.

Create a String

There are two methods which are usually followed to create a string in java.

Using String Literal

Using the new keyword

These string objects in java are immutable which means, they are constant and cannot be changed once created. But what if, you want to change the values possessed by a string variable for various String operations, like concatenation, substring.

StringBuffer and StringBuilder

Since String is immutable in Java, whenever we perform any kind of String manipulation, it generates a new string discarding the old string for garbage collection. Let me tell you if you don`t know already that these are heavy operations we are talking about and it generates a lot of garbage in bulk.
In order to avoid this problem, Java has provided StringBuffer and StringBuilder class that take care of String manipulations. Even the String concatenate operator uses StringBuffer or StringBuilder class internally. These are mutable objects in Java and they offer various methods for String manipulation. The various methods include:

append()
insert()
delete()
substring()

To understand the difference between String, StringBuffer and StringBuilder let us look at an example.

Example

/*Java program to demonstrate difference between String, StringBuilder and StringBuffer */ class HelloWorld { // Concatenation using String public static void concat1(String s1) { s1 = s1 + "World"; } // Concatenation using StringBuilder public static void concat2(StringBuilder s2) { s2.append("World"); } // Concatenation using StringBuffer public static void concat3(StringBuffer s3) { s3.append("World"); } public static void main(String[] args) { String s1 = "Hello"; concat1(s1); // s1 is not changed System.out.println("String: " + s1); StringBuilder s2 = new StringBuilder("Hello"); concat2(s2); // s2 is changed System.out.println("StringBuilder: " + s2); StringBuffer s3 = new StringBuffer("Hello"); concat3(s3); // s3 is changed System.out.println("StringBuffer: " + s3); } }

Output

Let us look at the output of the above example code.

Fig.1- String vs StringBuilder vs StringBuffer

In the first scenario, since string is immutable, the string passed from main() is not changed.
In the second scenario, with StringBuilder class when we pass a string “Hello” and perform s2.append(“World”), it changes the actual value of the string in main() to “HelloWorld”.
Similar to the second scenario, in the third case, with StringBuffer class when we pass a string “Hello” and perform s2.append(“World”), it changes the actual value of the string in main() to “HelloWorld”.
The second and third case looks almost same, but they have one difference that StringBuffer is thread safe and multiple threads can use it without any issue. We can also point out the difference between these two classes in another manner which is, StringBuffer is mutable and synchronized, whereas StringBuilder is mutable but not synchronized by default.

Synchronization and Thread - Safety

In the above text we talked about synchronization and thread-safety, but what do these term actually mean? Let us talk about threads first.
A thread in Java is the path which is followed when executing a program. Each and every Java program has at least one thread which is known as the main thread. It is created by the Java Virtual Machine (JVM) at the beginning of a program when the main() method is invoked.
Java is a multi-threaded application that allows multiple threads execution at any particular time. It`s nature of multi – threaded environment creates a concept of thread – safety which implies the correct behaviour of a method, variable or field in a program when multiple simultaneous threads are using a resource. Now let us look at synchronization.
When we say something is synchronized in a Java program, it means that multiple threads can access, and modify it without any problem or side effect. StringBuffer is synchronized, so you can use it with multiple threads without any problem.

Summary

If a string is going to remain constant throughout a program, then use the String class.
If a string can change and will be accessed by only one single thread, then use the StringBuilder class. And if it is being accessed by multiple string then use StringBuffer class as it provides thread – safety.
It should be noted that, you should not use StringBuffer unnecessarily, which means, don't use it if only one thread is modifying and accessing it because it has lot of locking and unlocking code for synchronization which will unnecessarily take up CPU time.