Can inner class refer non finalvariable defined in outer class in Java 8

Question

I have always thought that an inner class CANNOT refer non final variable defined in outer class.

But, when I write the following code, it compiles correctly and can be run in Intellij Idea.

Why does the following code work?

public class Outer {

    /**
     * non final variable greeting is used in inner class, non final is OK?
     */
    private String greeting = "Greeting1";

    /**
     * non final variable s is used in inner class, non final is OK?
     */
    public String doWork(String s) {
        class Inner {
            public String speak(String ss) {
                return greeting + "--- " + s + "---" + ss;
            }
        }
        Inner obj = new Inner();
        return obj.speak("Inner");
    }

    public static void main(String[] args) {
        Outer obj = new Outer();
        //Can assign here.
        obj.greeting="Greeting2";
        System.out.println(obj.doWork("Outer"));
    }
}

Answer 1

The Java Language Specification says:

Any local variable, formal parameter, or exception parameter used but not declared in an inner class must either be declared final or be effectively final (§4.12.4), or a compile-time error occurs where the use is attempted.

You most likely recalled/remembered an incorrect version of that. You might have mistook it as:

Any instance variable, local variable, formal parameter, or exception parameter used but not declared in an inner class must either be declared final or be effectively final (§4.12.4), or a compile-time error occurs where the use is attempted.

greeting is a field (aka instance variable), so can be used in inner classes whether or not it is final or not.

ss, though it is a formal parameter, is effectively final, so it can be used in inner classes too. Note that the phrase "effectively final" has a formal definition in §4.12.4, but informally, it just means "you did not reassign it another value in this method" in this context.

See this post for why this restriction exists.