in java, why do closured variables need to be declared final?

Question

final Object o;

List l = new ArrayList(){{
    // closure over o, in lexical scope
    this.add(o);
}};

why must o be declared final? why don't other JVM languages with mutable vars have this requirement?

Answer 1

This is not JVM-deep, it all happens at syntactic-sugar level. The reason is that exporting a non-final var via a closure makes it vulnerable to datarace issues and, since Java was designed to be a "blue-collar" language, such a surprising change in the behavior of an otherwise tame and safe local var was deemed way too "advanced".