Java Glossary : anonymous classes

Java definitions: 0-9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

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A type of inner class that has no name. You define it, and create an object of that type as a parameter all in one line. Used for creating simple delegate callback objects. The syntax is strange. It does not use the keywords class, implements or extends. You can refer to this of the outer class via MyOuterClass.this. You can refer to the outer class's methods by myOuterInstanceMethod() or MyOuterClass.this.myOuterInstanceMethod(); Anonymous inner classes are often used in setting up listeners.

These anonymous inner classes can access the static and instance variables of the enclosing outer class. They can also, believe it or not, access a snapshot of the local variables in the method that created the inner class object. To remind you this you are accessing an immutable snapshot, Java insists that the local variables you reference be final. When the class is created, a copy of all the local variables is bundled along with the object for all future references. So in a sense these classes act like closures, taking a little of the environment in which they were created along with them.

Instead of passing arguments to a constructor, your inner class methods can reach out and grab what they need directly from local variables in the enclosing method. The other technique is to use an instance intialialiser block. You are only allowed one per anonymous inner class.

The requirement that the enclosing local variables that you access in your anonymous inner class be final is not quite as onerous as it first seems. Just create a new block and create new final local variables in it that copy the non-final ones of the outer block. You can also declare a local variable final inside a loop so long as the value does not change for the rest of the current iteration, even though it will change on the next iteration.

inner class ¤ sample source code

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