A closure is a block of code that can be referenced (and passed around) with access to the variables of the enclosing scope.
Since Java 1.1, anonymous inner class have provided this facility in a highly verbose manner. They also have a restriction of only being able to use
final (and definitely assigned) local variables. (Note, even non-
final local variables are in scope, but cannot be used.)
Java SE 8 is intended to have a more concise version of this for single-method interfaces*, called "lambdas". Lambdas have much the same restrictions as anonymous inner classes, although some details vary randomly.
Lambdas are being developed under Project Lambda and JSR 335.
*Originally the design was more flexible allowing Single Abstract Methods (SAM) types. Unfortunately the new design is less flexible, but does attempt to justify allowing implementation within interfaces.
~ Answered on 2011-03-26 19:02:24