Lambdas and variable scope

Sooner or later when programming with lambdas, we hit upon the following question: which fields or variables can we access from a lambda expression? The simple answer is that accessibility is similar to that of anonymous inner classes. Lambdas generally share or "capture" the scope of the code that calls it. This means that:

• any fields that were accessible to the code calling the lambda expression are accessible within the lambda expression;
• any local variables accessible to the caller are accessible from within the lambda expression provided that they are em>effectively final (i.e. assigned to exactly once before the lambda expression is called);
• local variables therefore cannot be modified from within a lambda expression;
• data accessed via final local references can be modified (either accidentally or deliberately!).

However, they differ from anonymous inner classes in their naming scope. Specifically:

• this refers to the same object inside the lambda as it did outside of the lambda. (The same therefore applies to super.)

Apart from the last rule, most rules about variable name scope and field and variable access from within lambdas will seem intuitive if you are used to programming with anonymous inner classes. In addition, if you are programming with a good IDE, it will generally help you when you go wrong!

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