set.contains()分析

先看一段代码

Set s = new HashSet();
List list = new ArrayList<>();
list.add("a");
Set> set = new HashSet>();
set.add(list);
System.out.println(set.contains(list));  ==>true
list.add("b");
System.out.println(set.contains(list));  ==>false

作为set存储内容的list在向list中增加元素好像后被set"清除"了
实际上list还在set中,只是通过contains找不到了

分析

为了理解问题我们需要通读hashset的源代码:

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

    private transient HashMap map;

    // Dummy value to associate with an Object in the backing Map
    private static final Object PRESENT = new Object();

    /**
     * Constructs a new, empty set; the backing HashMap instance has
     * default initial capacity (16) and load factor (0.75).
     */
    public HashSet() {
        map = new HashMap<>();
    }

    /**
     * Constructs a new set containing the elements in the specified
     * collection.  The HashMap is created with default load factor
     * (0.75) and an initial capacity sufficient to contain the elements in
     * the specified collection.
     *
     * @param c the collection whose elements are to be placed into this set
     * @throws NullPointerException if the specified collection is null
     */
    public HashSet(Collection c) {
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
    }

    /**
     * Constructs a new, empty set; the backing HashMap instance has
     * the specified initial capacity and the specified load factor.
     *
     * @param      initialCapacity   the initial capacity of the hash map
     * @param      loadFactor        the load factor of the hash map
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero, or if the load factor is nonpositive
     */
    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }

    /**
     * Constructs a new, empty set; the backing HashMap instance has
     * the specified initial capacity and default load factor (0.75).
     *
     * @param      initialCapacity   the initial capacity of the hash table
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero
     */
    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }

    /**
     * Constructs a new, empty linked hash set.  (This package private
     * constructor is only used by LinkedHashSet.) The backing
     * HashMap instance is a LinkedHashMap with the specified initial
     * capacity and the specified load factor.
     *
     * @param      initialCapacity   the initial capacity of the hash map
     * @param      loadFactor        the load factor of the hash map
     * @param      dummy             ignored (distinguishes this
     *             constructor from other int, float constructor.)
     * @throws     IllegalArgumentException if the initial capacity is less
     *             than zero, or if the load factor is nonpositive
     */
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

    /**
     * Returns an iterator over the elements in this set.  The elements
     * are returned in no particular order.
     *
     * @return an Iterator over the elements in this set
     * @see ConcurrentModificationException
     */
    public Iterator iterator() {
        return map.keySet().iterator();
    }

    /**
     * Returns the number of elements in this set (its cardinality).
     *
     * @return the number of elements in this set (its cardinality)
     */
    public int size() {
        return map.size();
    }

    /**
     * Returns true if this set contains no elements.
     *
     * @return true if this set contains no elements
     */
    public boolean isEmpty() {
        return map.isEmpty();
    }

    /**
     * Returns true if this set contains the specified element.
     * More formally, returns true if and only if this set
     * contains an element e such that
     * (o==null ? e==null : o.equals(e)).
     *
     * @param o element whose presence in this set is to be tested
     * @return true if this set contains the specified element
     */
    public boolean contains(Object o) {
        return map.containsKey(o);
    }

    /**
     * Adds the specified element to this set if it is not already present.
     * More formally, adds the specified element e to this set if
     * this set contains no element e2 such that
     * (e==null ? e2==null : e.equals(e2)).
     * If this set already contains the element, the call leaves the set
     * unchanged and returns false.
     *
     * @param e element to be added to this set
     * @return true if this set did not already contain the specified
     * element
     */
    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

    /**
     * Removes the specified element from this set if it is present.
     * More formally, removes an element e such that
     * (o==null ? e==null : o.equals(e)),
     * if this set contains such an element.  Returns true if
     * this set contained the element (or equivalently, if this set
     * changed as a result of the call).  (This set will not contain the
     * element once the call returns.)
     *
     * @param o object to be removed from this set, if present
     * @return true if the set contained the specified element
     */
    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }

    /**
     * Removes all of the elements from this set.
     * The set will be empty after this call returns.
     */
    public void clear() {
        map.clear();
    }

    /**
     * Returns a shallow copy of this HashSet instance: the elements
     * themselves are not cloned.
     *
     * @return a shallow copy of this set
     */
    @SuppressWarnings("unchecked")
    public Object clone() {
        try {
            HashSet newSet = (HashSet) super.clone();
            newSet.map = (HashMap) map.clone();
            return newSet;
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }
    }

    /**
     * Save the state of this HashSet instance to a stream (that is,
     * serialize it).
     *
     * @serialData The capacity of the backing HashMap instance
     *             (int), and its load factor (float) are emitted, followed by
     *             the size of the set (the number of elements it contains)
     *             (int), followed by all of its elements (each an Object) in
     *             no particular order.
     */
    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);
    }

    /**
     * Reconstitute the HashSet instance from a stream (that is,
     * deserialize it).
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();

        int capacity = s.readInt();
        if (capacity < 0) {
            throw new InvalidObjectException("Illegal capacity: " +
                                             capacity);
        }
        float loadFactor = s.readFloat();
        if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
            throw new InvalidObjectException("Illegal load factor: " +
                                             loadFactor);
        }

        // Read size and verify non-negative.
        int size = s.readInt();
        if (size < 0) {
            throw new InvalidObjectException("Illegal size: " +
                                             size);
        }
      
        SharedSecrets.getJavaOISAccess()
                     .checkArray(s, Map.Entry[].class, HashMap.tableSizeFor(capacity));
        map = (((HashSet)this) instanceof LinkedHashSet ?
               new LinkedHashMap(capacity, loadFactor) :
               new HashMap(capacity, loadFactor));
        for (int i=0; i spliterator() {
        return new HashMap.KeySpliterator(map, 0, -1, 0, 0);
    }
}

我们知道hashmap不允许key重复出现,set正是利用了这一点来复用hashmap;
观察set的add contains clear remove方法就能明白这一点
对private static final Object PRESENT = new Object();做出一点解释,它只是一个为了符合map使用规范而创建的占位类,并没有任何意义。
现在我们知道了问题的原因:map的contains方法不能识别出更改后的list。

由set指向map

HashMap底层利用拉链法存储散列表并利用类的hashcode方法确定哈希值。
存储:底层维护一个数组,数组的每一位链向一个链表,若数组大小为n,则会将对象o添加到o.hashcode()%n数组位指向的链表中,它会依次使用key的equals方法比较key和链表已有内容,若有重复则添加key失败。
寻找:寻找key也是调用hashcode,去底层数组的o.hashcode()%n位指向的链表中寻找key,在链表中寻找时使用key的equals方法。
我们知道List的hashcode被重写为和其内部存储的数据有关,也就是说任何对list所存储内容的hashcode的改变也会改变list的hashcode。
在将list存入map时是利用原有hashcode确定存储地址,现在寻址也是利用hashcode,但是hashcode改变了,因而当再次寻找list时会进入错误的链表中寻找,当然不会有符合的,自然就找不到目标key了。

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