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Rserve is a TCP/IP server which allows other programs to use facilities of R (see www.r-project.org) from various languages without the need to initialize R or link against R library. Every connection has a separate workspace and working directory. Client-side implementations are available for popular languages such as C/C++, PHP and Java. Rserve supports remote connection, authentication and file transfer. Typical use is to integrate R backend for computation of statstical models, plots etc. in other applications.

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package org.rosuda.REngine;

/** represents a NULL object in R.
 
 Note: there is a slight asymmetry - in R NULL is represented by a zero-length pairlist. For this reason REXPNull returns true for {@link #isList()} and {@link #asList()} will return an empty list. Nonetheless REXPList of the length 0 will NOT return true in {@link #isNull()} (currently), becasue it is considered a different object in Java. These nuances are still subject to change, because it's not clear how it should be treated. At any rate use REXPNull instead of empty REXPList if NULL is the intended value.
 */
public class REXPNull extends REXP {
	public REXPNull() { super(); }
	public REXPNull(REXPList attr) { super(attr); }
	
	public boolean isNull() { return true; }
	public boolean isList() { return true; } // NULL is a list
	public RList asList() { return new RList(); }
	public Object asNativeJavaObject() { return null; }
}