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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 import com.google.common.base.Preconditions;
     21 
     22 import java.util.Set;
     23 
     24 import javax.annotation.Nullable;
     25 
     26 /**
     27  * Implementation of {@link ImmutableSet} with exactly one element.
     28  *
     29  * @author Kevin Bourrillion
     30  * @author Nick Kralevich
     31  */
     32 @GwtCompatible(serializable = true)
     33 @SuppressWarnings("serial") // uses writeReplace(), not default serialization
     34 final class SingletonImmutableSet<E> extends ImmutableSet<E> {
     35   final transient E element;
     36 
     37   // Non-volatile because:
     38   //   - Integer is immutable and thus thread-safe;
     39   //   - no problems if one thread overwrites the cachedHashCode from another.
     40   private transient Integer cachedHashCode;
     41 
     42   SingletonImmutableSet(E element) {
     43     this.element = Preconditions.checkNotNull(element);
     44   }
     45 
     46   SingletonImmutableSet(E element, int hashCode) {
     47     // Guaranteed to be non-null by the presence of the pre-computed hash code.
     48     this.element = element;
     49     cachedHashCode = hashCode;
     50   }
     51 
     52   public int size() {
     53     return 1;
     54   }
     55 
     56   @Override public boolean isEmpty() {
     57     return false;
     58   }
     59 
     60   @Override public boolean contains(Object target) {
     61     return element.equals(target);
     62   }
     63 
     64   @Override public UnmodifiableIterator<E> iterator() {
     65     return Iterators.singletonIterator(element);
     66   }
     67 
     68   @Override public Object[] toArray() {
     69     return new Object[] { element };
     70   }
     71 
     72   @SuppressWarnings({"unchecked"})
     73   @Override public <T> T[] toArray(T[] array) {
     74     if (array.length == 0) {
     75       array = ObjectArrays.newArray(array, 1);
     76     } else if (array.length > 1) {
     77       array[1] = null;
     78     }
     79     // Writes will produce ArrayStoreException when the toArray() doc requires.
     80     Object[] objectArray = array;
     81     objectArray[0] = element;
     82     return array;
     83   }
     84 
     85   @Override public boolean equals(@Nullable Object object) {
     86     if (object == this) {
     87       return true;
     88     }
     89     if (object instanceof Set) {
     90       Set<?> that = (Set<?>) object;
     91       return that.size() == 1 && element.equals(that.iterator().next());
     92     }
     93     return false;
     94   }
     95 
     96   @Override public final int hashCode() {
     97     Integer code = cachedHashCode;
     98     if (code == null) {
     99       return cachedHashCode = element.hashCode();
    100     }
    101     return code;
    102   }
    103 
    104   @Override boolean isHashCodeFast() {
    105     return false;
    106   }
    107 
    108   @Override public String toString() {
    109     String elementToString = element.toString();
    110     return new StringBuilder(elementToString.length() + 2)
    111         .append('[')
    112         .append(elementToString)
    113         .append(']')
    114         .toString();
    115   }
    116 }
    117