JDK 1.8 源码解析 HashSet

Posted 2020-10-20 wjq2017

　

package java.util;

 

// HashSet继承了AbstractSet，实现了Set、Cloneable和java.io.Serializable接口
public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable

 

// 序列化版本号
static final long serialVersionUID = -5024744406713321676L;

 

// 不可被默认序列化的HashMap引用
private transient HashMap<E,Object> map;

 

// Dummy value to associate with an Object in the backing Map
// 与后台映射中的对象关联的虚拟值
private static final Object PRESENT = new Object();

 

1 /**
2  * Constructs a new, empty set; the backing <tt>HashMap</tt> instance has
3  * default initial capacity (16) and load factor (0.75).
4  */
5 // 无参构造方法
6 public HashSet() {
7     map = new HashMap<>();
8 }

 

// 初始容量和负载因子作为参数的构造方法
public HashSet(int initialCapacity, float loadFactor) {
    map = new HashMap<>(initialCapacity, loadFactor);
}

 

// 初始容量作为参数的构造方法
public HashSet(int initialCapacity) {
    map = new HashMap<>(initialCapacity);
}

 

// 初始容量、负载因子和布尔类型值作为参数的构造方法，与前面不同的是创建了LinkedHashMap对象
HashSet(int initialCapacity, float loadFactor, boolean dummy) {
    map = new LinkedHashMap<>(initialCapacity, loadFactor);
}

 

// 获取迭代器
public Iterator<E> iterator() {
    return map.keySet().iterator();
}

 

// 获取元素数量
public int size() {
    return map.size();
}

 

// 判断是否为空
public boolean isEmpty() {
    return map.isEmpty();
}

 

// 判断是否包含某个元素
public boolean contains(Object o) {
    return map.containsKey(o);
}

 

1 // 添加元素 
2 // 元素重复时会覆盖但内容相同 
3 // 如果put方法调用的putVal方法返回null,说明原来没有该key
4 public boolean add(E e) {
5     return map.put(e, PRESENT)==null;
6 }

 

// 删除元素
public boolean remove(Object o) {
    return map.remove(o)==PRESENT;
}

 

// 清空集合
public void clear() {
    map.clear();
}

 

 1 /**
 2  * Returns a shallow copy of this <tt>HashSet</tt> instance: the elements
 3  * themselves are not cloned.
 4  *
 5  * @return a shallow copy of this set
 6  */
 7 // 克隆，浅复制
 8 @SuppressWarnings("unchecked")
 9 public Object clone() {
10     try {
11         HashSet<E> newSet = (HashSet<E>) super.clone();
12         newSet.map = (HashMap<E, Object>) map.clone();
13         return newSet;
14     } catch (CloneNotSupportedException e) {
15         throw new InternalError(e);
16     }
17 }

 

 1 /**
 2  * Save the state of this <tt>HashSet</tt> instance to a stream (that is,
 3  * serialize it).
 4  *
 5  * @serialData The capacity of the backing <tt>HashMap</tt> instance
 6  *             (int), and its load factor (float) are emitted, followed by
 7  *             the size of the set (the number of elements it contains)
 8  *             (int), followed by all of its elements (each an Object) in
 9  *             no particular order.
10  */
11 // 序列化
12 private void writeObject(java.io.ObjectOutputStream s)
13     throws java.io.IOException {
14     // Write out any hidden serialization magic
15     s.defaultWriteObject();
16 
17     // Write out HashMap capacity and load factor
18     s.writeInt(map.capacity());
19     s.writeFloat(map.loadFactor());
20 
21     // Write out size
22     s.writeInt(map.size());
23 
24     // Write out all elements in the proper order.
25     for (E e : map.keySet())
26         s.writeObject(e);
27 }

 

 1 /**
 2  * Reconstitute the <tt>HashSet</tt> instance from a stream (that is,
 3  * deserialize it).
 4  */
 5 // 反序列化
 6 private void readObject(java.io.ObjectInputStream s)
 7     throws java.io.IOException, ClassNotFoundException {
 8     // Read in any hidden serialization magic
 9     s.defaultReadObject();
10 
11     // Read capacity and verify non-negative.
12     int capacity = s.readInt();
13     if (capacity < 0) {
14         throw new InvalidObjectException("Illegal capacity: " +
15                                          capacity);
16     }
17 
18     // Read load factor and verify positive and non NaN.
19     float loadFactor = s.readFloat();
20     if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
21         throw new InvalidObjectException("Illegal load factor: " +
22                                          loadFactor);
23     }
24 
25     // Read size and verify non-negative.
26     int size = s.readInt();
27     if (size < 0) {
28         throw new InvalidObjectException("Illegal size: " +
29                                          size);
30     }
31 
32     // Set the capacity according to the size and load factor ensuring that
33     // the HashMap is at least 25% full but clamping to maximum capacity.
34     capacity = (int) Math.min(size * Math.min(1 / loadFactor, 4.0f),
35             HashMap.MAXIMUM_CAPACITY);
36 
37     // Create backing HashMap
38     map = (((HashSet<?>)this) instanceof LinkedHashSet ?
39            new LinkedHashMap<E,Object>(capacity, loadFactor) :
40            new HashMap<E,Object>(capacity, loadFactor));
41 
42     // Read in all elements in the proper order.
43     for (int i=0; i<size; i++) {
44         @SuppressWarnings("unchecked")
45             E e = (E) s.readObject();
46         map.put(e, PRESENT);
47     }
48 }