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Add Null placeholder singleton 

git-svn-id: https://svn.apache.org/repos/asf/jakarta/commons/proper/lang/trunk@137044 13f79535-47bb-0310-9956-ffa450edef68
This commit is contained in:
Stephen Colebourne 2002-09-18 20:11:53 +00:00
parent 55ca32bd2e
commit 727c9b0fa4
1 changed files with 42 additions and 3 deletions
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45
src/java/org/apache/commons/lang/ObjectUtils.java
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@ -1,5 +1,3 @@
package org.apache.commons.lang;
/* ====================================================================
* The Apache Software License, Version 1.1
*
@ -53,7 +51,9 @@ package org.apache.commons.lang;
* information on the Apache Software Foundation, please see
* <http://www.apache.org/>.
*/
package org.apache.commons.lang;
import java.io.Serializable;
/**
* Common <code>Object</code> manipulation routines.
*
@ -61,10 +61,24 @@ package org.apache.commons.lang;
* @author <a href="mailto:janekdb@yahoo.co.uk">Janek Bogucki</a>
* @author <a href="mailto:dlr@finemaltcoding.com">Daniel Rall</a>
* @author <a href="mailto:scolebourne@joda.org">Stephen Colebourne</a>
* @version $Id: ObjectUtils.java,v 1.2 2002/09/18 19:49:08 scolebourne Exp $
* @version $Id: ObjectUtils.java,v 1.3 2002/09/18 20:11:53 scolebourne Exp $
*/
public class ObjectUtils {
/**
* Singleton used as a null placeholder where null has another meaning.
* <p>
* For example, in a <code>HashMap</code> the get(key) method returns null
* if the Map contains null or if there is no matching key. The Null
* placeholder can be used to distinguish between these two cases.
* <p>
* Another example is <code>HashTable</code>, where <code>null</code>
* cannot be stored.
* <p>
* This instance is Serializable.
*/
public static final Null NULL = new Null();
/**
* ObjectUtils instances should NOT be constructed in standard programming.
* Instead, the class should be used as <code>ObjectUtils.defaultIfNull("a","b");</code>.
@ -123,4 +137,29 @@ public class ObjectUtils {
.toString();
}
/**
* Class used as a null placeholder where null has another meaning.
* <p>
* For example, in a <code>HashMap</code> the get(key) method returns null
* if the Map contains null or if there is no matching key. The Null
* placeholder can be used to distinguish between these two cases.
* <p>
* Another example is <code>HashTable</code>, where <code>null</code>
* cannot be stored.
*/
public static class Null implements Serializable {
/**
* Restricted constructor - singleton
*/
private Null() {
}
/**
* Ensure singleton
*/
private Object readResolve() {
return ObjectUtils.NULL;
}
}
}