Copying from Collections to Lang

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/lang/trunk@1154827 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/lang3/compare/ComparableComparator.java

@@ -0,0 +1,129 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.apache.commons.collections.comparators;
+
+import java.io.Serializable;
+import java.util.Comparator;
+
+/**
+ * A {@link Comparator Comparator} that compares 
+ * {@link Comparable Comparable} objects.
+ * <p />
+ * This Comparator is useful, for example,
+ * for enforcing the natural order in custom implementations
+ * of SortedSet and SortedMap.
+ * <p />
+ * Note: In the 2.0 and 2.1 releases of Commons Collections, 
+ * this class would throw a {@link ClassCastException} if
+ * either of the arguments to {@link #compare(Object, Object) compare}
+ * were <code>null</code>, not {@link Comparable Comparable},
+ * or for which {@link Comparable#compareTo(Object) compareTo} gave
+ * inconsistent results.  This is no longer the case.  See
+ * {@link #compare(Object, Object) compare} for details.
+ *
+ * @since Commons Collections 2.0
+ * @version $Revision$ $Date$
+ *
+ * @see java.util.Collections#reverseOrder()
+ */
+public class ComparableComparator<E extends Comparable<? super E>> implements Comparator<E>, Serializable {
+
+    /** Serialization version. */
+    private static final long serialVersionUID=-291439688585137865L;
+
+    /** The singleton instance. */
+    @SuppressWarnings("rawtypes")
+    public static final ComparableComparator<?> INSTANCE = new ComparableComparator();
+
+    //-----------------------------------------------------------------------
+    /**
+     * Gets the singleton instance of a ComparableComparator.
+     * <p>
+     * Developers are encouraged to use the comparator returned from this method
+     * instead of constructing a new instance to reduce allocation and GC overhead
+     * when multiple comparable comparators may be used in the same VM.
+     * 
+     * @return the singleton ComparableComparator
+     */
+    @SuppressWarnings("unchecked")
+    public static <E extends Comparable<? super E>> ComparableComparator<E> comparableComparator() {
+        return (ComparableComparator<E>) INSTANCE;
+    }
+
+    //-----------------------------------------------------------------------
+    /**
+     * Constructor whose use should be avoided.
+     * <p>
+     * Please use the {@link #comparableComparator()} method whenever possible.
+     */
+    public ComparableComparator() {
+        super();
+    }
+
+    //-----------------------------------------------------------------------
+    /**
+     * Compare the two {@link Comparable Comparable} arguments.
+     * This method is equivalent to:
+     * <pre>((Comparable)obj1).compareTo(obj2)</pre>
+     * 
+     * @param obj1  the first object to compare
+     * @param obj2  the second object to compare
+     * @return negative if obj1 is less, positive if greater, zero if equal
+     * @throws NullPointerException when <i>obj1</i> is <code>null</code>, 
+     *         or when <code>((Comparable)obj1).compareTo(obj2)</code> does
+     * @throws ClassCastException when <i>obj1</i> is not a <code>Comparable</code>,
+     *         or when <code>((Comparable)obj1).compareTo(obj2)</code> does
+     */
+    public int compare(E obj1, E obj2) {
+        return obj1.compareTo(obj2);
+    }
+
+    //-----------------------------------------------------------------------
+    /**
+     * Implement a hash code for this comparator that is consistent with
+     * {@link #equals(Object) equals}.
+     *
+     * @return a hash code for this comparator.
+     * @since Commons Collections 3.0
+     */
+    @Override
+    public int hashCode() {
+        return "ComparableComparator".hashCode();
+    }
+
+    /**
+     * Returns <code>true</code> iff <i>that</i> Object is 
+     * is a {@link Comparator Comparator} whose ordering is 
+     * known to be equivalent to mine.
+     * <p>
+     * This implementation returns <code>true</code>
+     * iff <code><i>object</i>.{@link Object#getClass() getClass()}</code>
+     * equals <code>this.getClass()</code>.
+     * Subclasses may want to override this behavior to remain consistent
+     * with the {@link Comparator#equals(Object)} contract.
+     * 
+     * @param object  the object to compare with
+     * @return true if equal
+     * @since Commons Collections 3.0
+     */
+    @Override
+    public boolean equals(Object object) {
+        return (this == object) || 
+               ((null != object) && (object.getClass().equals(this.getClass())));
+    }
+
+}