Why are strings often immutable? What problems does mutability cause?

Why strings are immutable

Immutable strings cannot change after creation—operations return new strings. Benefits: thread safety, safe sharing as map keys, string interning; JVM can optimize literals.

• Security: Password strings not cleared if mutable.
• Hashing: hashCode cached safely for HashMap keys.
• Pitfall: Concatenating in a loop—use StringBuilder.

String a = "hi";
a = a + "!"; // new object; original "hi" unchanged