Differences Between String, StringBuffer, and StringBuilder in Java

回答：

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