String is immutable so it can be safely shared and cached in the String Pool without defensive copying, safely used as a HashMap key (its hashcode can be cached), safely shared across threads without synchronization, and protected from tampering when used in security-sensitive contexts like class loading and network connections.

Intermediate5 min

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#String#Immutability#String Pool

Why Immutability Matters

Once a String object is created, its internal character array cannot be changed — any operation that appears to modify a String (like concat or replace) actually returns a brand-new String object. This enables safe caching (String Pool), thread-safety without locks, and secure use as identifiers.

How It's Enforced

The String class is declared `final` (can't be subclassed to break the contract) and its backing character array is private and never exposed or mutated after construction.

String Pool (Heap): "hello" literal → shared reference · new String("hello") → separate object outside the pool

Immutability in Action

java

String s = "hello";
s.concat(" world");
System.out.println(s);           // still "hello" — original unchanged

s = s.concat(" world");
System.out.println(s);           // "hello world" — new object assigned

Interview Answer

String is immutable by design so instances can be safely reused via the String Pool, safely shared across threads without synchronization, and safely used as hash keys since the hashcode never changes. Every 'modifying' method on String actually returns a new object rather than mutating the original.