What is the difference between String, StringBuilder, and StringBuffer?

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All three represent strings but differ in mutability and thread safety. String — immutable ; every modification creates a new object. Stored in the String pool for literal reuse. Thread-safe by virtue of immutability. StringBuilder — mutable , not thread-safe , but very fast. Use for string building inside a single thread (loops, builders). StringBuffer — mutable and thread-safe (synchronized methods), so slower. Use only when multiple threads concatenate the same buffer. The loop performance comparison is the key example. Know that the compiler optimizes a + b + c but NOT concatenation in loops. StringBuffer is rarely needed in modern Java — prefer StringBuilder with explicit synchronization if needed.