Referencing non-final variable: why does this code compile?

Question

First off, I apologise if this is a duplicate question. I found many similar ones, but none that directly address my question.

In preparation for an upcoming exam, I am

Answer 1

It may work in Java8 as the stress is on Effectively Final which means once the value is assigned to jtf it should not be changed after wards. As per Java doc:

A variable or parameter whose value is never changed after it is initialized is effectively final.

Answer 2

I guess you are compiling with Java 8. Here your jtf variable is effectively final, so it compiles fine. A variable is effectively final if its value is never changed after you initialized it.

See also Local Classes:

However, starting in Java SE 8, a local class can access local variables and parameters of the enclosing block that are final or effectively final. A variable or parameter whose value is never changed after it is initialized is effectively final.

and

Accessing Local Variables of the Enclosing Scope, and Declaring and Accessing Members of the Anonymous Class

Like local classes, anonymous classes can capture variables; they have the same access to local variables of the enclosing scope:

• An anonymous class has access to the members of its enclosing class.

• An anonymous class cannot access local variables in its enclosing scope that are not declared as final or effectively final.

[...]

If you tried with:

javac -source 1.7 MyFile.java

you'll have your expected error.

.java:13: error: local variable jtf is accessed from within inner class; needs to be declared final
                jtf.setText(evt.getLocationOnScreen().toString());
                ^
1 error

So the answer of the exam question is: it compiles only if you're using Java 8+.

Answer 3

Seems that your Eclipse IDE uses Java 7 compiler. To change this to Java 8 use Project->Properties->Java Compiler->Compiler compliance level.

Answer 4

Java 8 added the ability to access "effectively final" variables. The final keyword is no longer required as long as a variable is never changed after it is initialized.