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      1 /*
      2  * Copyright (C) 2007 Google Inc.
      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.util.Collection;
     22 import java.util.Set;
     23 
     24 import javax.annotation.Nullable;
     25 
     26 /**
     27  * An empty immutable set.
     28  *
     29  * @author Kevin Bourrillion
     30  */
     31 @GwtCompatible(serializable = true)
     32 final class EmptyImmutableSet extends ImmutableSet<Object> {
     33   static final EmptyImmutableSet INSTANCE = new EmptyImmutableSet();
     34 
     35   private EmptyImmutableSet() {}
     36 
     37   public int size() {
     38     return 0;
     39   }
     40 
     41   @Override public boolean isEmpty() {
     42     return true;
     43   }
     44 
     45   @Override public boolean contains(Object target) {
     46     return false;
     47   }
     48 
     49   @Override public UnmodifiableIterator<Object> iterator() {
     50     return Iterators.emptyIterator();
     51   }
     52 
     53   private static final Object[] EMPTY_ARRAY = new Object[0];
     54 
     55   @Override public Object[] toArray() {
     56     return EMPTY_ARRAY;
     57   }
     58 
     59   @Override public <T> T[] toArray(T[] a) {
     60     if (a.length > 0) {
     61       a[0] = null;
     62     }
     63     return a;
     64   }
     65 
     66   @Override public boolean containsAll(Collection<?> targets) {
     67     return targets.isEmpty();
     68   }
     69 
     70   @Override public boolean equals(@Nullable Object object) {
     71     if (object instanceof Set) {
     72       Set<?> that = (Set<?>) object;
     73       return that.isEmpty();
     74     }
     75     return false;
     76   }
     77 
     78   @Override public final int hashCode() {
     79     return 0;
     80   }
     81 
     82   @Override boolean isHashCodeFast() {
     83     return true;
     84   }
     85 
     86   @Override public String toString() {
     87     return "[]";
     88   }
     89 
     90   Object readResolve() {
     91     return INSTANCE; // preserve singleton property
     92   }
     93 
     94   private static final long serialVersionUID = 0;
     95 }
     96