3.2 String vs StringBuilder vs StringBuffer

← Back to Chapter 3: Strings in Java

When manipulating text in performance-critical loops, choosing the correct String abstraction prevents massive Heap memory churn and GC pressure.

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StringBuilder

For repeated String modification, use StringBuilder.

StringBuilder sb = new StringBuilder();

sb.append("Java");
sb.append(" ");
sb.append("Developer");

System.out.println(sb.toString());

Output:

Java Developer

Unlike String, StringBuilder is mutable.

That means its existing object can be modified.

StringBuffer

StringBuffer is also mutable.

StringBuffer sb = new StringBuffer();

sb.append("Java");
sb.append(" Developer");

System.out.println(sb);

Major Differences

Feature	String	StringBuilder	StringBuffer
Mutability	Immutable (new instance on change)	Mutable (modifies internal buffer)	Mutable (modifies internal buffer)
Thread Safety	Yes (Inherently thread-safe)	No (Unsynchronized)	Yes (synchronized methods)
Performance	Slow for repeated concatenation	Fastest (Zero lock overhead)	Slower due to synchronization locks
Introduced	Java 1.0	Java 5	Java 1.0
Primary Use Case	Constants, DTOs, Maps, APIs	Local text assembly, loops	Multithreaded shared string building

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String vs StringBuilder vs StringBuffer

Remember this simple rule:

Need an unchanging String?
        ↓
      String

Need to modify String frequently?
        ↓
   StringBuilder

Need mutable String + synchronized operations?
        ↓
   StringBuffer

when thread safety isn't required.

StringBuilder > StringBuffer

when thread safety is required.

StringBuffer > StringBuilder