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Groovy example source code file (NullObject.java)

This example Groovy source code file (NullObject.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Groovy tags/keywords

cannot, cannot, groovyobjectsupport, instance, iterator, nullobject, nullobject, nullpointerexception, nullpointerexception, object, object, string, util

The Groovy NullObject.java source code

/*
 * 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;
    }

    /**
     * A null object always coerces to false.
     * 
     * @return false
     */
    public boolean asBoolean() {
        return false;
    }

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

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

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