Differences Between String, StringBuffer, and StringBuilder in Java

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Question :

What are the differences between String, StringBuffer, and StringBuilder? What are their respective usage scenarios?

Réponse :

In Java, String, StringBuffer, and StringBuilder are all classes used to handle strings, but they have different characteristics and use cases. Here’s a breakdown of their differences and when to use each:

1. String

  • Immutability: String objects are immutable, meaning once a String object is created, its value cannot be changed. Any operation that seems to modify a String actually creates a new String object.
  • Performance: Because of immutability, using String in scenarios where many modifications are needed (like concatenation in loops) can lead to performance issues due to the creation of multiple String objects.
  • Usage Scenario: Use String when you have a fixed set of characters that do not change, or when you are working with a small number of string manipulations.

2. StringBuffer

  • Mutability: StringBuffer is mutable, meaning you can modify the contents of a StringBuffer object without creating a new object.
  • Thread Safety: StringBuffer is synchronized, which means it is thread-safe. Multiple threads can safely use a StringBuffer instance without causing data corruption.
  • Performance: While StringBuffer is more efficient than String for concatenation and modifications, the synchronization overhead can make it slower than StringBuilder in single-threaded scenarios.
  • Usage Scenario: Use StringBuffer when you need a mutable string in a multi-threaded environment where thread safety is a concern.

3. StringBuilder

  • Mutability: Like StringBuffer, StringBuilder is also mutable and allows modifications to the string without creating new objects.
  • Thread Safety: StringBuilder is not synchronized, which means it is not thread-safe. This makes it faster than StringBuffer in single-threaded scenarios.
  • Performance: StringBuilder is generally preferred over StringBuffer when thread safety is not a concern, as it provides better performance for string manipulations.
  • Usage Scenario: Use StringBuilder when you need to perform many modifications to a string in a single-threaded environment, such as in loops or when building strings dynamically.

Summary

  • Use String for fixed strings or when you have a small number of modifications.
  • Use StringBuffer when you need a mutable string in a multi-threaded environment.
  • Use StringBuilder for mutable strings in a single-threaded environment where performance is a priority.

Example Code

// Using String
String str = "Hello";
str += " World"; // Creates a new String object

// Using StringBuffer
StringBuffer stringBuffer = new StringBuffer("Hello");
stringBuffer.append(" World"); // Modifies the existing object

// Using StringBuilder
StringBuilder stringBuilder = new StringBuilder("Hello");
stringBuilder.append(" World"); // Modifies the existing object

In summary, choose the appropriate class based on your specific needs regarding mutability, performance, and thread safety.