Strings

A String's character contents never change after construction; any method that seems to modify it returns a brand-new String instead. This immutability is what makes pool sharing safe, since no holder can mutate a shared instance. It also lets strings be reliable hash keys, because their hashCode stays stable, and makes them inherently thread-safe to share without synchronization.

Both are mutable string builders that append into a resizable buffer, avoiding the costly reallocation of building a string by repeatedly concatenating immutable String objects. StringBuffer synchronizes every method, so it is thread-safe but slower. StringBuilder is the unsynchronized, faster equivalent. Prefer StringBuilder for single-threaded building and reach for StringBuffer only when one instance is shared across threads.

The string pool is a JVM-managed cache of interned String objects. Identical string literals share one single pooled object, so a reference check with == on two equal literals is true. Writing new String("x") forces a distinct heap object outside the pool, so == against the literal is false even though equals is true. Calling intern() returns the pooled copy, adding it if absent.

Mark the class final so it cannot be subclassed and weakened. Make every field private final and set them only in the constructor, exposing no setters or other mutators. For any field of a mutable type, store a defensive copy in the constructor and return a defensive copy from its getter, so callers can never reach the internal state. Then every instance is safely shareable and a reliable hash key, like String.

It prints qwe. String is immutable, so concat does not modify a in place — it returns a brand-new String "qwerty". That return value is discarded because it is never assigned, leaving a still pointing at the original "qwe". To keep the result you must assign it: a = a.concat("rty");.