/*
    * Copyright (C) 2007 The Guava 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 com.google.common.collect;
  
  import static com.google.common.base.Preconditions.checkNotNull;
  
  import com.google.common.annotations.GwtCompatible;
  
  import java.io.Serializable;
  
  /** An ordering that uses the natural order of the values. */
  @GwtCompatible(serializable = true)
  @SuppressWarnings("unchecked") // TODO(kevinb): the right way to explain this??
  final class NaturalOrdering
      extends Ordering<Comparable> implements Serializable {
    static final NaturalOrdering INSTANCE = new NaturalOrdering();
  
    @Override public int compare(Comparable left, Comparable right) {
      checkNotNull(left); // for GWT
      checkNotNull(right);
      return left.compareTo(right);
    }
  
    @Override public <S extends Comparable> Ordering<S> reverse() {
      return (Ordering<S>) ReverseNaturalOrdering.INSTANCE;
    }
  
    // preserving singleton-ness gives equals()/hashCode() for free
    private Object readResolve() {
      return INSTANCE;
    }
  
    @Override public String toString() {
      return "Ordering.natural()";
    }
  
    private NaturalOrdering() {}
  
    private static final long serialVersionUID = 0;
  }