content: /* 
   * Kodkod -- Copyright (c) 2005-present, Emina Torlak
   *
   * Permission is hereby granted, free of charge, to any person obtaining a copy
   * of this software and associated documentation files (the "Software"), to deal
   * in the Software without restriction, including without limitation the rights
   * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
   * copies of the Software, and to permit persons to whom the Software is
   * furnished to do so, subject to the following conditions:
   *
   * The above copyright notice and this permission notice shall be included in
   * all copies or substantial portions of the Software.
   *
   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
   * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
   * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
   * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
   * THE SOFTWARE.
   */
  package kodkod.util.collections;
  
  import java.util.AbstractSet;
  import java.util.Collection;
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  import java.util.Set;
  
  /**
   * <p>This is an immutable singleton set implementation that tests for 
   * membership using reference-equality in place of object-equality when comparing elements.  </p>
   * @specfield element: V
   * @author Emina Torlak
   */
  public final class SingletonIdentitySet<V> extends AbstractSet<V> {
  	private final V element;
  	
  	/**
  	 * Constructs a SingletonIdentitySet that will hold the given element.
  	 * @ensures this.element' = element
  	 */
  	public SingletonIdentitySet(V element) {
  		this.element = element;
  	}
  	
  	/**
  	 * Constructs a SingletonIdentitySet that will hold the first element
  	 * returned by the given collection's iterator.
  	 * @ensures this.element' = collection.iterator().next()
  	 * @throws NoSuchElementException  collection.isEmpty()
  	 */
  	public SingletonIdentitySet(Collection<? extends V> collection) {
  		this.element = collection.iterator().next();
  	}
  	
  
  	/**
  	 * Returns true iff this.element and obj are the same object.
  	 * @return this.element == obj
  	 */
  	public boolean contains(Object obj) {
  		return element == obj;
  	}
  
  	/**
  	 * @see java.util.Set#containsAll(java.util.Collection)
  	 */
  	public boolean containsAll(Collection<?> collection) {
  		if (collection.isEmpty()) return true;
  		else if (collection.size()==1) return collection.iterator().next()==this.element;
  		else return false;
  	}
  
  	/**
  	 * Returns false.
  	 * @return false.
  	 */
  	public boolean isEmpty() {
  		return false;
  	}
  
  	/**
  	 * @see java.util.Set#iterator()
  	 */
  	public Iterator<V> iterator() {
  		return Containers.iterate(this.element);
  	}
  
  	/**
  	 * Returns 1.
  	 * @return 1
  	 */
  	public int size() {
  		return 1;
  	}
  
  	/**
       * Compares the specified object with this set for equality.  Returns
       * <tt>true</tt> if the given object is also a set and the two sets
       * contain identical object-references.  
       *
       * <p><b>Owing to the reference-equality-based semantics of this set it is
       * possible that the symmetry and transitivity requirements of the
       * <tt>Object.equals</tt> contract may be violated if this set is compared
       * to a normal set.  However, the <tt>Object.equals</tt> contract is
       * guaranteed to hold among <tt>SingletonHashSet</tt> instances.</b>
       * 
       * @return <tt>true</tt> if the specified object is equal to this set.
       * @see Object#equals(Object)
       */
      public boolean equals(Object o) {
          if (o == this) {
              return true;
          } else if (o instanceof Set) {
              final Set<?> s = (Set<?>) o;
              return s.size()==1 && s.iterator().next()==this.element;
          } else {
              return false;  
          }
      }
      
      /**
       * Returns 0 if this.element is null; otherwise returns this.element.hashCode().
       * @return this.element = null ? 0 : this.element.hashCode()
       * @see java.util.AbstractSet#hashCode()
       */
      public int hashCode() {
      	return this.element==null ? 0 : element.hashCode();
      }
  }
  