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      1 /*
      2  * Copyright (C) 2007 The Guava Authors
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  * http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 package com.google.common.collect;
     18 
     19 import com.google.common.annotations.GwtCompatible;
     20 
     21 import java.io.Serializable;
     22 
     23 import javax.annotation.Nullable;
     24 
     25 /** An ordering that treats {@code null} as less than all other values. */
     26 @GwtCompatible(serializable = true)
     27 final class NullsFirstOrdering<T> extends Ordering<T> implements Serializable {
     28   final Ordering<? super T> ordering;
     29 
     30   NullsFirstOrdering(Ordering<? super T> ordering) {
     31     this.ordering = ordering;
     32   }
     33 
     34   @Override public int compare(@Nullable T left, @Nullable T right) {
     35     if (left == right) {
     36       return 0;
     37     }
     38     if (left == null) {
     39       return RIGHT_IS_GREATER;
     40     }
     41     if (right == null) {
     42       return LEFT_IS_GREATER;
     43     }
     44     return ordering.compare(left, right);
     45   }
     46 
     47   @Override public <S extends T> Ordering<S> reverse() {
     48     // ordering.reverse() might be optimized, so let it do its thing
     49     return ordering.reverse().nullsLast();
     50   }
     51 
     52   @SuppressWarnings("unchecked") // still need the right way to explain this
     53   @Override public <S extends T> Ordering<S> nullsFirst() {
     54     return (Ordering<S>) this;
     55   }
     56 
     57   @Override public <S extends T> Ordering<S> nullsLast() {
     58     return ordering.nullsLast();
     59   }
     60 
     61   @Override public boolean equals(@Nullable Object object) {
     62     if (object == this) {
     63       return true;
     64     }
     65     if (object instanceof NullsFirstOrdering) {
     66       NullsFirstOrdering<?> that = (NullsFirstOrdering<?>) object;
     67       return this.ordering.equals(that.ordering);
     68     }
     69     return false;
     70   }
     71 
     72   @Override public int hashCode() {
     73     return ordering.hashCode() ^ 957692532; // meaningless
     74   }
     75 
     76   @Override public String toString() {
     77     return ordering + ".nullsFirst()";
     78   }
     79 
     80   private static final long serialVersionUID = 0;
     81 }
     82