Immutability of Strings

Immutability of Strings

1. What Immutable Means (beginner)

Immutable = once built, a String object never changes. Every "edit" (+, replace, trim) secretly builds a brand-new String, old one becomes garbage. Like printed book: can't edit page, must reprint!

2. Proof Code (see new objects!)

String s = "hi";
System.out.println(System.identityHashCode(s)); // e.g. 111 (address!)
s = s + "!"; // NOT edited — NEW object "hi!" created, s now points there!
System.out.println(System.identityHashCode(s)); // e.g. 222 (DIFFERENT!)
System.out.println(s); // hi! (s moved, old "hi" garbage!)

String t = "hello";
t.toUpperCase(); // returns "HELLO" but THROWN AWAY!
System.out.println(t); // still hello (unchanged — trap!)
t = t.toUpperCase(); // MUST assign back!
System.out.println(t); // HELLO (correct!)

3. Why Java Did This (exam reasons!)

• Security: "password" can't be secretly altered (safe keys, URLs).
• Pool sharing: literals share one copy (saves memory — safe because no one can change it!).
• Thread-safe: many threads read same String, no locks needed.

4. Cost + Fix

Loop s += i 10,000× = 10,000 garbage objects (slow!). Fix: StringBuilder/StringBuffer (mutable, next topics!).

5. Exam Points

• Define + proof (hashCode changes, t unchanged) — 5-7 marks, very frequent.
• Why immutable (security/pool/threads) + loop cost — 5 marks, frequent.