JavaObject[] is used to denote an expression that refers to an object residing in Java.

JavaObjectQ[expr] gives True if expr is a reference to a Java object or Null, and gives False otherwise.

JavaObjectToExpression[javaobject] converts the specified Java object reference into its value as a "native" Mathematica expression.

JavaShow[window] causes the specified Java window to be brought to the foreground, so that it appears in front of notebook windows.

JavaThrow["exception"] causes an exception of the specified class to be thrown in the Java thread that called the Mathematica program in which JavaThrow occurred. ...

JavaUILink[] returns the MathLink LinkObject used by calls to Mathematica that originate from Java user-interface actions, or Null if no such link is present.

JVM is the head of an expression that identifies a particular Java runtime installed into the current Mathematica session via InstallJava.

KeepJavaObject[object] causes the specified object or objects not to be released when the current JavaBlock ends. KeepJavaObject[object, Manual] causes the specified object ...

LoadedJavaClasses[] returns a list of the classes currently loaded into Java by Mathematica.

LoadedJavaObjects[] returns a list of the Java objects that have been sent to Mathematica (and not yet released with ReleaseJavaObject).