Why are Strings immutable? What is the string pool?

A String object never changes; methods like toUpperCase() or concat() return new strings. Reasons:

• Pooling — identical literals can share one object safely.
• Security and correctness — strings used as file names, class names or URLs cannot be changed after they are checked.
• Thread safety — immutable objects can be shared without locks.
• Hashing — the hash code can be cached, which makes strings fast HashMap keys.

String literals are interned in the string pool (part of the heap): every "hi" in the program refers to the same object. new String("hi") always creates a separate object.

String a = "hi", b = "hi", c = new String("hi");
System.out.println(a == b);          // true  - same pooled object
System.out.println(a == c);          // false - c is a different object
System.out.println(a.equals(c));     // true  - same characters
System.out.println(a == c.intern()); // true  - intern() returns the pooled copy

Compile-time constants are folded too: "h" + "i" == "hi" is true, but concatenation of a variable at runtime produces a new object.