Effectively final variables in Java

TIL what effectively final variables in Java are.

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

—The Java™ Tutorials: Local Classes

Example

Suppose that the variable postcodeLength is not declared final, and you add the assignment statement in the Postcode constructor to change the length of a valid postcode to 7 characters:

Postcode(String postalCode) {
    postcodeLength = 7;

    String currentPostalCode = postalCode.replaceAll(
            regularExpression, "");

    if (currentPostcode.length() == postcodeLength) {
        formatterPostcode = currentPostalCode;
    } else {
        formattedPostcode = null;
    }
}

Because of this assignment statement, the variable postcodeLength 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 PostCode tries to access the postcodeLength variable:

if (currentPostcode.length() == postcodeLength)
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