java hashset 序列化_Java集合系列-HashSet

一、概述

HashSet是基于哈希实现的set集合,其实它底层是一个value固定的HashMap。

HashMap是无序存储的,所以HashSet也一样是无序的,而且HashSet允许null值,但只能拥有一个null值,即不允许存储相同的元素。

二、常量变量

public class HashSet extends AbstractSet

implements Set, Cloneable, java.io.Serializable

{

//...

private transient HashMap map;

private static final Object PRESENT = new Object();

//...

}

上面的map即为HashSet底层的HashMap,针对HashSet的操作,全部转交给这个map来完成。

上面的PRESENT即为底层HashMap中键值对的值的固定值。因为在HashSet中只关注键。

三、构造器

public class HashSet extends AbstractSet

implements Set, 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);

}

//...

}

很明显,所有的HashSet的构造器最终都在创建底层的HashMap。

最后一个构造器创建了一个LinkedHashMap实例,其实它也是一个HashMap,因为它继承自HashMap,是对HashMap的一个功能扩展集合,它支持多种顺序的遍历(插入顺序和访问顺序)。

四、操作

public class HashSet extends AbstractSet

implements Set, Cloneable, java.io.Serializable{

//...

public Iterator 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();

}

//...

}

上面的所有基础操作,全部已开HashMap的对应方法来完成。

五、序列化操作

5.1 序列化

HashSet实例的序列化执行时,并不会序列化map属性,因为其被transient关键字所修饰。参照源码:

// 在执行序列化操作的时候会执行这个writeObject方法

public class HashSet extends AbstractSet

implements Set, Cloneable, java.io.Serializable

{

//...

private transient HashMap 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

// 用于将对象中的非static和非transient的字段值写入流中

s.defaultWriteObject();

// Write out HashMap capacity and load factor

// 将底层HashMap的当前容量和加载因子写入流中

s.writeInt(map.capacity());

s.writeFloat(map.loadFactor());

// Write out size

// 将底层HashMap的当前元素数量size写入流中

s.writeInt(map.size());

// Write out all elements in the proper order.

// 最后将所有的元素写入流中

for (E e : map.keySet())

s.writeObject(e);

}

//...

}

5.2 反序列化

// 在执行反序列化操作的时候会执行这个readObject方法

public class HashSet extends AbstractSet

implements Set, Cloneable, java.io.Serializable

{

//...

private transient HashMap 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

// 读取流中对应的当前类的非static和非transient的值

s.defaultReadObject();

// Read capacity and verify non-negative.

// 读取流中的容量值

int capacity = s.readInt();

if (capacity < 0) {

throw new InvalidObjectException("Illegal capacity: " +

capacity);

}

// Read load factor and verify positive and non NaN.

// 读取流中的加载因子值

float loadFactor = s.readFloat();

if (loadFactor <= 0 || Float.isNaN(loadFactor)) {

throw new InvalidObjectException("Illegal load factor: " +

loadFactor);

}

// Read size and verify non-negative.

// 读取流中的元素数量值

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

// 创建底层HashMap实例

map = (((HashSet>)this) instanceof LinkedHashSet ?

new LinkedHashMap(capacity, loadFactor) :

new HashMap(capacity, loadFactor));

// Read in all elements in the proper order.

// 读取流中保存的元素,并将其逐个添加到新创建的HashMap实例中

for (int i=0; i

@SuppressWarnings("unchecked")

E e = (E) s.readObject();

map.put(e, PRESENT);

}

}

//...

}

六、总结

HashSet就是依靠HashMap实现的。

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