Original article , Reprint please mark the source ：https://www.cnblogs.com/V1haoge/p/10755431.html
One 、 summary
HashSet It's based on hash set aggregate , In fact, it is a value fixed HashMap.
HashMap It's out of order , therefore HashSet It's also disordered , and HashSet allow null value , But you can only have one null value , That is, it is not allowed to store the same elements .
Two 、 constant variables
public class HashSet<E> extends AbstractSet<E>
implements Set<E>, Cloneable, java.io.Serializable
{
//...
private transient HashMap<E,Object> map;
private static final Object PRESENT = new Object();
//...
}
above map That is to say HashSet At the bottom HashMap, in the light of HashSet The operation of , All to this map To complete .
above PRESENT The bottom is HashMap The fixed value of the value of the key value pair . Because in HashSet Only the key .
3、 ... and 、 Constructors
public class HashSet<E> extends AbstractSet<E>
implements Set<E>, Cloneable, java.io.Serializable{
//...
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);
}
//...
}
Obviously , be-all HashSet In the end, the constructors are creating the underlying HashMap.
The last constructor creates a LinkedHashMap example , In fact, it is also a HashMap, Because it inherits from HashMap, It's right HashMap A set of functional extensions for , It supports traversal in multiple orders （ Insert order and access order ）.
Four 、 operation
public class HashSet<E> extends AbstractSet<E>
implements Set<E>, Cloneable, java.io.Serializable{
//...
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();
}
//...
}
All the basic operations above , It's all open HashMap To complete .
5、 ... and 、 Serialization operation
5.1 serialize
HashSet When the serialization of an instance is executed , It doesn't serialize map attribute , Because it was transient Keyword decorated . Refer to the source code ：
// This is done when the serialization operation is performed writeObject Method
public class HashSet<E> extends AbstractSet<E>
implements Set<E>, Cloneable, java.io.Serializable
{
//...
private transient HashMap<E,Object> map;
private static final Object PRESENT = new Object();
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
// Write out any hidden serialization magic
// It is used to change the non in an object static He Fei transient The field value of is written to the stream
s.defaultWriteObject();
// Write out HashMap capacity and load factor
// Put the bottom layer HashMap The current capacity and load factor of are written to the stream
s.writeInt(map.capacity());
s.writeFloat(map.loadFactor());
// Write out size
// Put the bottom layer HashMap The current number of elements of size Write to the stream
s.writeInt(map.size());
// Write out all elements in the proper order.
// Finally, all the elements are written to the stream
for (E e : map.keySet())
s.writeObject(e);
}
//...
}
5.2 Deserialization
// This is done when the deserialization operation is performed readObject Method
public class HashSet<E> extends AbstractSet<E>
implements Set<E>, Cloneable, java.io.Serializable
{
//...
private transient HashMap<E,Object> map;
private static final Object PRESENT = new Object();
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
// Read in any hidden serialization magic
// Read the non of the corresponding current class in the stream static He Fei transient Value
s.defaultReadObject();
// Read capacity and verify non-negative.
// Read the capacity value in the stream
int capacity = s.readInt();
if (capacity < 0) {
throw new InvalidObjectException("Illegal capacity: " +
capacity);
}
// Read load factor and verify positive and non NaN.
// Read the load factor value in the stream
float loadFactor = s.readFloat();
if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
throw new InvalidObjectException("Illegal load factor: " +
loadFactor);
}
// Read size and verify non-negative.
// Read the value of the number of elements in the stream
int size = s.readInt();
if (size < 0) {
throw new InvalidObjectException("Illegal size: " +
size);
}
// Set the capacity according to the size and load factor ensuring that
// the HashMap is at least 25% full but clamping to maximum capacity.
capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f),
HashMap.MAXIMUM_CAPACITY);
// Constructing the backing map will lazily create an array when the first element is
// added, so check it before construction. Call HashMap.tableSizeFor to compute the
// actual allocation size. Check Map.Entry[].class since it's the nearest public type to
// what is actually created.
SharedSecrets.getJavaOISAccess()
.checkArray(s, Map.Entry[].class, HashMap.tableSizeFor(capacity));
// Create backing HashMap
// Create the bottom layer HashMap example
map = (((HashSet<?>)this) instanceof LinkedHashSet ?
new LinkedHashMap<E,Object>(capacity, loadFactor) :
new HashMap<E,Object>(capacity, loadFactor));
// Read in all elements in the proper order.
// Read the elements saved in the stream , And add them one by one to the newly created HashMap In the example
for (int i=0; i<size; i++) {
@SuppressWarnings("unchecked")
E e = (E) s.readObject();
map.put(e, PRESENT);
}
}
//...
}
6、 ... and 、 summary
HashSet Is to rely on HashMap Realized .