What changed in Java 9

4 changes across 3 questions — each with the behaviour it replaced, because “and how did it work before?” is the follow-up.

• When would you use LinkedList instead of ArrayList?

  • List.of() returns a genuinely immutable list that rejects nulls and refuses every mutator.

    Before: Arrays.asList() gave you a fixed-size list that still wrote through to the backing array, and Collections.unmodifiableList() gave you a view whose source could still change underneath you.

• Why is String immutable, and what is the string pool?

  • Compact Strings: the backing array became a byte[] plus a one-byte coder, storing Latin-1 text at one byte per character.

    Before (now gone): The backing array was always char[] — two bytes per character, so ASCII text cost double for nothing. Strings are typically the largest single category of live objects in a heap, so this was among the largest free wins in JVM history.

  • String concatenation with + compiles to invokedynamic makeConcatWithConstants, letting the JVM build the result in one pass at runtime (JEP 280).

    Before (now gone): javac emitted an explicit StringBuilder chain: new StringBuilder().append(..).append(..).toString(). That is why 'the compiler rewrites + into StringBuilder' is now a decade out of date.

• What is the difference between checked and unchecked exceptions?

  • try-with-resources accepts an existing effectively-final variable, so you can write try (conn) { }.

    Before: You had to redeclare the resource inside the parentheses, even when you already held it.