Java: What is the difference between String, StringBuilder, and StringBuffer in Java?

String — immutable. StringBuilder — mutable, not thread-safe. StringBuffer — mutable, thread-safe.

String:

• Immutable.
• Each modification creates new object.
• Thread-safe.

1String s = "Hello";
2s += " World";  // Creates NEW string
3s += "!";       // Creates another NEW string
4// 3 string objects created!

StringBuilder:

• Mutable.
• Not thread-safe.
• Fast.

1StringBuilder sb = new StringBuilder();
2sb.append("Hello");
3sb.append(" World");
4sb.append("!");
5String result = sb.toString();
6// Only 1 final string created

StringBuffer:

• Mutable.
• Thread-safe (synchronized).
• Slower than StringBuilder.

1StringBuffer sb = new StringBuffer();
2sb.append("Hello");

When to use:

• String — few operations.
• StringBuilder — single-threaded, many operations.
• StringBuffer — multi-threaded, many operations.