编程知识 cdmana.com

Java collection series - HashSet

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 .

版权声明
本文为[Only brother Hao]所创,转载请带上原文链接,感谢

Scroll to Top