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Raster
A two-dimensional rectangular grid of pixels.

Realm
See security policy domain. Also, a string, passed as part of an HTTP request during basic authentication, that defines a protection space. The protected resources on a server can be partitioned into a set of protection spaces, each with its own authentication scheme and/or authorization database.

Reference
A variable data type in which the variable's value is an address.

Return
A Java keyword used to finish the execution of a method. It can be followed by a value required by the method definition.

RMI
See Java Remote Method Invocation.

Rollback
The point in a transaction when all updates to any databases involved in the transaction are reversed.

Root
In a hierarchy of items, the one item from which all other items are descended. The root item has nothing above it in the hierarchy. See also hierarchy, class, package.

RPC
Remote Procedure Call. Executing what looks like a normal procedure call (or method invocation) by sending network packets to some remote host.

Runtime system
The software environment in which programs compiled for the Java virtual machine can run. The runtime system includes all the code necessary to load programs written in the Java programming language, dynamically link native methods, manage memory, handle exceptions, and an implementation of the Java virtual machine, which may be a Java interpreter.