Proxies are a common technique to virtualize objects in object-oriented languages. A proxy is a placeholder object that emulates or wraps another target object. Both the proxy's representation and behavior may differ substantially from that of its target object.
In many object-oriented languages, objects may have language-enforced invariants associated with them. For instance, an object may declare immutable fields, which are guaranteed to point to the same value throughout the execution of the program. Clients of an object can blindly rely on these invariants, as they are enforced by the language.