jdk1.7之HashSet

public class HashSet<E> extends AbstractSet<E> implements Set<E>, Cloneable, java.io.Serializable {
static final long serialVersionUID = -5024744406713321676L;


private transient HashMap<E, Object> map;


private static final Object PRESENT = new Object();


public HashSet() {
map = new HashMap<>();
}


public HashSet(Collection<? extends E> c) {
map = new HashMap<>(Math.max((int) (c.size() / .75f) + 1, 16));
addAll(c);
}


public HashSet(int initialCapacity, float loadFactor) {
map = new HashMap<>(initialCapacity, loadFactor);
}


public HashSet(int initialCapacity) {
map = new HashMap<>(initialCapacity);
}


HashSet(int initialCapacity, float loadFactor, boolean dummy) {
map = new LinkedHashMap<>(initialCapacity, loadFactor);
}


public Iterator<E> iterator() {
return map.keySet().iterator();
}


public int size() {
return map.size();
}


public boolean isEmpty() {
return map.isEmpty();
}


public boolean contains(Object o) {
return map.containsKey(o);
}


public boolean add(E e) {
return map.put(e, PRESENT) == null;
}


public boolean remove(Object o) {
return map.remove(o) == PRESENT;
}


public void clear() {
map.clear();
}


public Object clone() {
try {
HashSet<E> newSet = (HashSet<E>) super.clone();
newSet.map = (HashMap<E, Object>) map.clone();
return newSet;
} catch (CloneNotSupportedException e) {
throw new InternalError();
}
}


private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException {
// Write out any hidden serialization magic
s.defaultWriteObject();


// Write out HashMap capacity and load factor
s.writeInt(map.capacity());
s.writeFloat(map.loadFactor());


// Write out size
s.writeInt(map.size());


// Write out all elements in the proper order.
for (E e : map.keySet())
s.writeObject(e);
}


private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {
// Read in any hidden serialization magic
s.defaultReadObject();


// Read in HashMap capacity and load factor and create backing HashMap
int capacity = s.readInt();
float loadFactor = s.readFloat();
map = (((HashSet) this) instanceof LinkedHashSet ? new LinkedHashMap<E, Object>(capacity, loadFactor)
: new HashMap<E, Object>(capacity, loadFactor));


// Read in size
int size = s.readInt();


// Read in all elements in the proper order.
for (int i = 0; i < size; i++) {
E e = (E) s.readObject();
map.put(e, PRESENT);
}
}
}