Difference between final and effectively final

... 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.

For example, suppose that the variable numberLength is not declared final, and you add the marked assignment statement in the PhoneNumber constructor:

public class OutterClass {    int numberLength; // <== not *final*  class PhoneNumber {    PhoneNumber(String phoneNumber) {        numberLength = 7;   // <== assignment to numberLength        String currentNumber = phoneNumber.replaceAll(            regularExpression, "");        if (currentNumber.length() == numberLength)            formattedPhoneNumber = currentNumber;        else            formattedPhoneNumber = null;     }  ...  }...}

Because of this assignment statement, the variable numberLength is not effectively final anymore. As a result, the Java compiler generates an error message similar to "local variables referenced from an inner class must be final or effectively final" where the inner class PhoneNumber tries to access the numberLength variable:

http://codeinventions.blogspot.in/2014/07/difference-between-final-and.html

http://docs.oracle.com/javase/tutorial/java/javaOO/localclasses.html

I find the simplest way to explain "effectively final" is to imagine adding the final modifier to a variable declaration. If, with this change, the program continues to behave in the same way, both at compile time and at run time, then that variable is effectively final.

According to the docs:

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

Basically, if the compiler finds a variable does not appear in assignments outside of its initialization, then the variable is considered effectively final.

For example, consider some class:

public class Foo {    public void baz(int bar) {        // While the next line is commented, bar is effectively final        // and while it is uncommented, the assignment means it is not        // effectively final.        // bar = 2;    }}