Difference Between String, StringBuilder, and StringBuffer in Java

If you're learning Java, one of the most common interview questions is the difference between String, StringBuilder, and StringBuffer.

This is one of the most frequently asked topics in Java interviews.

Written by Shivkumar Udas – Engineering student sharing practical Java guides for beginners.

💡 What is String in Java?

String is an immutable class in Java, which means its value cannot be changed after creation.


String str = "Java";
str.concat(" Programming"); // new object created

Key Point: Every modification creates a new object.

StringBuilder is a mutable class, meaning its value can be changed without creating a new object.


StringBuilder sb = new StringBuilder("Java");
sb.append(" Programming");
System.out.println(sb);

Key Point: Faster and efficient for modifications.

StringBuffer is similar to StringBuilder but is thread-safe.


StringBuffer sb = new StringBuffer("Java");
sb.append(" Programming");
System.out.println(sb);

Key Point: Thread-safe but slower than StringBuilder.

Feature	String	StringBuilder	StringBuffer
Mutability	Immutable	Mutable	Mutable
Performance	Slow	Fast	Moderate
Thread Safety	No	No	Yes
Usage	Simple strings	Frequent modifications	Multi-threaded environment

Use String for fixed text like names.

Use StringBuilder when modifying text frequently like building a message.

Use StringBuffer when multiple threads access the same data.

Always remember: String is immutable, while StringBuilder and StringBuffer are mutable.

In most real-world applications, StringBuilder is preferred because it is faster.

Create a program to append text using String, StringBuilder, and StringBuffer. Compare performance.

Understanding the difference between String, StringBuilder, and StringBuffer helps you write efficient Java code.

Choose the right class based on your use case and performance needs.