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I'm playing with lambdas in Java 8 and I came across warning local variables referenced from a lambda expression must be final or effectively final. I know that when I use variables inside anonymous class they must be final in outer class, but still - what is the difference between final and effectively final?
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Variables used in lambda should be final or effectively final. . On further research using SO forums and blog I learned that Java 8 has new concept called “Effectively final” variable. It means that a non-final local variable whose value never changes after initialization is called “Effectively Final”.
An effectively final variable is a variable whose value is never changed, but it isn't declared with the final keyword.
The static keyword means the value is the same for every instance of the class. The final keyword means once the variable is assigned a value it can never be changed.
You use the final keyword in a method declaration to indicate that the method cannot be overridden by subclasses. The Object class does this—a number of its methods are 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
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