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Groovy: A powerful, dynamic language for the JVM
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/*
 * Copyright 2003-2007 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.codehaus.groovy.runtime;

import java.util.Collections;
import java.util.Iterator;

import groovy.lang.GroovyObjectSupport;

public class NullObject extends GroovyObjectSupport {
    private static final NullObject INSTANCE = new NullObject();

    /**
     * private constructor
     */
    private NullObject() {
    }

    /**
     * get the NullObject reference
     *
     * @return the null object
     */
    public static NullObject getNullObject() {
        return INSTANCE;
    }

    /**
     * Since this is implemented as a singleton, we should avoid the
     * use of the clone method
     */
    public Object clone() {
        throw new NullPointerException("Cannot invoke method clone() on null object");
    }

    /**
     * Tries to get a property on null, which will always fail
     *
     * @param property - the property to get
     * @return a NPE
     */
    public Object getProperty(String property) {
        throw new NullPointerException("Cannot get property '" + property + "' on null object");
    }

    /**
     * Tries to set a property on null, which will always fail
     *
     * @param property - the proprty to set
     * @param newValue - the new value of the property
     */
    public void setProperty(String property, Object newValue) {
        throw new NullPointerException("Cannot set property '" + property + "' on null object");
    }

    /**
     * Tries to invoke a method on null, which will always fail
     *
     * @param name the name of the method to invoke
     * @param args - arguments to the method
     * @return a NPE
     */
    public Object invokeMethod(String name, Object args) {
        throw new NullPointerException("Cannot invoke method " + name + "() on null object");
    }

    /**
     * null is only equal to null
     *
     * @param to - the reference object with which to compare
     * @return - true if this object is the same as the to argument
     */
    public boolean equals(Object to) {
        return to == null;
    }

    /**
     * iterator() method to be able to iterate on null.
     * Note: this part is from Invoker
     *
     * @return an iterator for an empty list
     */
    public Iterator iterator() {
        return Collections.EMPTY_LIST.iterator();
    }

    /**
     * Allows to add a String to null.
     * The result is concatenated String of the result of calling
     * toString() on this object and the String in the parameter.
     *
     * @param s - the String to concatenate
     * @return the concatenated string
     */
    public Object plus(String s) {
        return getMetaClass().invokeMethod(this, "toString", new Object[]{}) + s;
    }

    /**
     * The method "is" is used to test for equal references.
     * This method will return true only if the given parameter
     * is null
     *
     * @param other - the object to test
     * @return true if other is null
     */
    public boolean is(Object other) {
        return other == null;
    }

    /**
     * Type conversion method for null.
     *
     * @param c - the class to convert to
     * @return always null
     */
    public Object asType(Class c) {
        return null;
    }

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

    public int hashCode() {
        throw new NullPointerException("Cannot invoke method hashCode() on null object");
    }
}