src/main/java/org/luaj/vm3/LuaNil.java

/*******************************************************************************
* Copyright (c) 2009-2011 Luaj.org. All rights reserved.
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
* 
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
******************************************************************************/
package org.luaj.vm3;

/**
 * Class to encapsulate behavior of the singleton instance {@code nil} 
 * <p>
 * There will be one instance of this class, {@link LuaValue#NIL}, 
 * per Java virtual machine.  
 * However, the {@link Varargs} instance {@link LuaValue#NONE}
 * which is the empty list, 
 * is also considered treated as a nil value by default.  
 * <p>
 * Although it is possible to test for nil using Java == operator, 
 * the recommended approach is to use the method {@link LuaValue#isnil()} 
 * instead.  By using that any ambiguities between 
 * {@link LuaValue#NIL} and {@link LuaValue#NONE} are avoided.
 * @see LuaValue
 * @see LuaValue#NIL
 */
public class LuaNil extends LuaValue {

	static final LuaNil _NIL = new LuaNil();

	public static LuaValue s_metatable;

	LuaNil() {}

	public int type() {
		return LuaValue.TNIL;
	}

	public String toString() {
		return "nil";
	}

	public String typename() {
		return "nil";
	}

	public String tojstring() {
		return "nil";
	}

	public LuaValue not() {
		return LuaValue.TRUE;
	}

	public boolean toboolean() {
		return false;
	}

	public boolean isnil() {
		return true;
	}

	public LuaValue getmetatable() {
		return s_metatable;
	}

	public boolean equals(Object o) {
		return o instanceof LuaNil;
	}

	public LuaValue checknotnil() {
		return argerror("value");
	}

	public boolean isvalidkey() {
		return false;
	}

	// optional argument conversions - nil alwas falls badk to default value
	public boolean optboolean(boolean defval) {
		return defval;
	}

	public LuaClosure optclosure(LuaClosure defval) {
		return defval;
	}

	public double optdouble(double defval) {
		return defval;
	}

	public LuaFunction optfunction(LuaFunction defval) {
		return defval;
	}

	public int optint(int defval) {
		return defval;
	}

	public LuaInteger optinteger(LuaInteger defval) {
		return defval;
	}

	public long optlong(long defval) {
		return defval;
	}

	public LuaNumber optnumber(LuaNumber defval) {
		return defval;
	}

	public LuaTable opttable(LuaTable defval) {
		return defval;
	}

	public LuaThread optthread(LuaThread defval) {
		return defval;
	}

	public String optjstring(String defval) {
		return defval;
	}

	public LuaString optstring(LuaString defval) {
		return defval;
	}

	public Object optuserdata(Object defval) {
		return defval;
	}

	public Object optuserdata(Class c, Object defval) {
		return defval;
	}

	public LuaValue optvalue(LuaValue defval) {
		return defval;
	}
}