Why is String immutable in Java?

String is final and exposes no mutators. Internally it stores the characters in a final byte array with a compact-string encoding flag. Every apparent change, such as concatenation or toUpperCase, returns a new String rather than modifying the original.

String original = "java";
String upper = original.toUpperCase();

System.out.println(original);
System.out.println(upper);

Why the design pays off

• String literals can be shared safely in the string pool
• The hash code can be computed once and cached for use as a HashMap key
• Immutable values are inherently thread-safe and can be shared freely
• Security-sensitive values such as class names and paths cannot be changed underneath a check

Immutability also has costs. Concatenating in a loop creates many intermediate objects, and sensitive values remain in the heap until collected. For large data, a char array or stream may be more appropriate. For repeated concatenation, use StringBuilder.