Effectively final variable is a variable that has not been explicitly marked as final, but it doesn't change its value (basically, an implicit constant).
Effectively final variables can be referenced in lambdas, without the need to explicitly mark them as "final":
// s is effectively final (not changed anywhere)
String s = "foo";
// s can be referenced in the lambda
Callable<String> callable = () -> s;
callable.call(); // "foo"
