DS_Blank.java

/* --------------------------------------------------------- *
 *    __________    D E L T A S C R I P T                    *
 *   (_________()                                            *
 *    /  ===   /   - A fast, dynamic scripting language      *
 *   |  ==    |    - Version 4.13.11.0                       *
 *   /  ===   /    - Developed by Adam R. Nelson             *
 *  |  =  =  |     - 2011-2013                               *
 *  /  ===   /     - Distributed under GNU LGPL v3           *
 * (________()     - http://github.com/ar-nelson/deltascript *
 *                                                           *
 * --------------------------------------------------------- */

package com.sector91.delta.script.objects;

import java.io.ObjectStreamException;
import java.io.Serializable;
import java.util.HashSet;
import java.util.Set;

import com.sector91.delta.script.DScriptErr;
import com.sector91.delta.script.DeltaScript;
import com.sector91.delta.script.Operator;
import com.sector91.delta.script.annotations.DSType;

/**
 * <p>DeltaScript's equivalent of a "null" value, represented with the keyword
 * {@code blank} in code.</p>
 * 
 * <p>Actual Java {@code null} values should never exist in a DeltaScript scope;
 * any nulls should be replaced with this object.</p>
 * 
 * <p>{@code DS_Blank} is a singleton. New instances cannot be created; instead,
 * access the single instance with {@link #BLANK}.</p>
 * 
 * @author Adam R. Nelson
 * @version 4.13.11.0
 */
@DSType("Blank")
public class DS_Blank implements DS_Object,
                                 Serializable
{
    private static final long serialVersionUID =
            DeltaScript.VERSION.majorVersion();
    
    /** <p>The value returned by {@link #getTypeName()}.</p> */
    public static final String TYPE_NAME = "Blank";
    
    /** 
     * <p>The single, immutable instance of {@code blank}.</p>
     * @see {@link DS_Blank}
     */
    public static final DS_Blank
        BLANK = new DS_Blank();
    
    private DS_Blank() {}

    public boolean booleanValue()
    {return false;}
    
    public String getTypeName()
    {return TYPE_NAME;}

    public Object unbox() 
    {return null;}

    public int compare(DS_Object other)
    {
        if (other instanceof DS_Blank)
            return 0;
        else
            return -1;
    }
    
    @Override public String toString()
    {return "blank";}

    public boolean equals(DS_Object other)
    {return other == this;}

    @Override public DS_Object clone() throws CloneNotSupportedException
    {return this;}

    public DS_Object dotGet(DS_Tag prop) throws DScriptErr
    {
        throw new DScriptErr("Attempted to get property \"" +prop.str + 
                "\" of blank.", DScriptErr.T_UNDEFINED, DScriptErr.T_BLANK);
    }

    public void dotSet(DS_Tag prop, DS_Object value) throws DScriptErr
    {
        throw new DScriptErr("Attempted to set property \"" + prop.str +
                "\" of blank.", DScriptErr.T_UNDEFINED, DScriptErr.T_BLANK);
    }

    public Set<DS_Tag> getMembers()
    {return new HashSet<DS_Tag>();}

    public DS_Object operator(Operator op, DS_Object other)
            throws DScriptErr
    {throw DScriptErr.invalidOperation(op.str, this, other);}

    public boolean is(DS_Object typeObj)
    {return typeObj == this;}
    
    private Object readResolve() throws ObjectStreamException
    {
        assert(BLANK != null);
        return BLANK;
    }
}