linkedhashmap.java

java源代码 请看看啊 提点宝贵的意见

    /**
     * Returns <tt>true</tt> if this map maps one or more keys to the
     * specified value.
     *
     * @param value value whose presence in this map is to be tested.
     * @return <tt>true</tt> if this map maps one or more keys to the
     *         specified value.
     */
    public boolean containsValue(Object value) {
        // Overridden to take advantage of faster iterator
        if (value==null) {
            for (Entry e = header.after; e != header; e = e.after)
                if (e.value==null)
                    return true;
        } else {
            for (Entry e = header.after; e != header; e = e.after)
                if (value.equals(e.value))
                    return true;
        }
        return false;
    }

    /**
     * Returns the value to which this map maps the specified key.  Returns
     * <tt>null</tt> if the map contains no mapping for this key.  A return
     * value of <tt>null</tt> does not <i>necessarily</i> indicate that the
     * map contains no mapping for the key; it's also possible that the map
     * explicitly maps the key to <tt>null</tt>.  The <tt>containsKey</tt>
     * operation may be used to distinguish these two cases.
     *
     * @return the value to which this map maps the specified key.
     * @param key key whose associated value is to be returned.
     */
    public Object get(Object key) {
        Entry e = (Entry)getEntry(key);
        if (e == null)
            return null;
        e.recordAccess(this);
        return e.value;
    }

    /**
     * Removes all mappings from this map.
     */
    public void clear() {
        super.clear();
        header.before = header.after = header;
    }

    /**
     * LinkedHashMap entry.
     */
    private static class Entry extends HashMap.Entry {
        // These fields comprise the doubly linked list used for iteration.
        Entry before, after;

	Entry(int hash, Object key, Object value, HashMap.Entry next) {
            super(hash, key, value, next);
        }

        /**
         * Remove this entry from the linked list.
         */
        private void remove() {
            before.after = after;
            after.before = before;
        }

        /**                                             
         * Insert this entry before the specified existing entry in the list.
         */
        private void addBefore(Entry existingEntry) {
            after  = existingEntry;
            before = existingEntry.before;
            before.after = this;
            after.before = this;
        }

        /**
         * This method is invoked by the superclass whenever the value
         * of a pre-existing entry is read by Map.get or modified by Map.set.
         * If the enclosing Map is access-ordered, it moves the entry
         * to the end of the list; otherwise, it does nothing. 
         */
        void recordAccess(HashMap m) {
            LinkedHashMap lm = (LinkedHashMap)m;
            if (lm.accessOrder) {
                lm.modCount++;
                remove();
                addBefore(lm.header);
            }
        }

        void recordRemoval(HashMap m) {
            remove();
        }
    }

    private abstract class LinkedHashIterator implements Iterator {
	Entry nextEntry    = header.after;
	Entry lastReturned = null;

	/**
	 * The modCount value that the iterator believes that the backing
	 * List should have.  If this expectation is violated, the iterator
	 * has detected concurrent modification.
	 */
	int expectedModCount = modCount;

	public boolean hasNext() {
            return nextEntry != header;
	}

	public void remove() {
	    if (lastReturned == null)
		throw new IllegalStateException();
	    if (modCount != expectedModCount)
		throw new ConcurrentModificationException();

            LinkedHashMap.this.remove(lastReturned.key);
            lastReturned = null;
            expectedModCount = modCount;
	}

	Entry nextEntry() {
	    if (modCount != expectedModCount)
		throw new ConcurrentModificationException();
            if (nextEntry == header)
                throw new NoSuchElementException();

            Entry e = lastReturned = nextEntry;
            nextEntry = e.after;
            return e;
	}
    }

    private class KeyIterator extends LinkedHashIterator {
	public Object next() { return nextEntry().getKey(); }
    }

    private class ValueIterator extends LinkedHashIterator {
	public Object next() { return nextEntry().value; }
    }

    private class EntryIterator extends LinkedHashIterator {
	public Object next() { return nextEntry(); }
    }

    // These Overrides alter the behavior of superclass view iterator() methods
    Iterator newKeyIterator()   { return new KeyIterator();   }
    Iterator newValueIterator() { return new ValueIterator(); }
    Iterator newEntryIterator() { return new EntryIterator(); }

    /**
     * This override alters behavior of superclass put method. It causes newly
     * allocated entry to get inserted at the end of the linked list and
     * removes the eldest entry if appropriate.
     */
    void addEntry(int hash, Object key, Object value, int bucketIndex) {
        createEntry(hash, key, value, bucketIndex);

        // Remove eldest entry if instructed, else grow capacity if appropriate
        Entry eldest = header.after;
        if (removeEldestEntry(eldest)) {
            removeEntryForKey(eldest.key);
        } else {
            if (size >= threshold) 
                resize(2 * table.length);
        }
    }

    /**
     * This override differs from addEntry in that it doesn't resize the
     * table or remove the eldest entry.
     */
    void createEntry(int hash, Object key, Object value, int bucketIndex) {
        Entry e = new Entry(hash, key, value, table[bucketIndex]);
        table[bucketIndex] = e;
        e.addBefore(header);
        size++;
    }

    /**
     * Returns <tt>true</tt> if this map should remove its eldest entry.
     * This method is invoked by <tt>put</tt> and <tt>putAll</tt> after
     * inserting a new entry into the map.  It provides the implementer
     * with the opportunity to remove the eldest entry each time a new one
     * is added.  This is useful if the map represents a cache: it allows
     * the map to reduce memory consumption by deleting stale entries.
     *
     * <p>Sample use: this override will allow the map to grow up to 100
     * entries and then delete the eldest entry each time a new entry is
     * added, maintaining a steady state of 100 entries.
     * <pre>
     *     private static final int MAX_ENTRIES = 100;
     *
     *     protected boolean removeEldestEntry(Map.Entry eldest) {
     *        return size() > MAX_ENTRIES;
     *     }
     * </pre>
     *
     * <p>This method typically does not modify the map in any way,
     * instead allowing the map to modify itself as directed by its
     * return value.  It <i>is</i> permitted for this method to modify
     * the map directly, but if it does so, it <i>must</i> return
     * <tt>false</tt> (indicating that the map should not attempt any
     * further modification).  The effects of returning <tt>true</tt>
     * after modifying the map from within this method are unspecified.
     *
     * <p>This implementation merely returns <tt>false</tt> (so that this
     * map acts like a normal map - the eldest element is never removed).
     *
     * @param    eldest The least recently inserted entry in the map, or if 
     *           this is an access-ordered map, the least recently accessed
     *           entry.  This is the entry that will be removed it this
     *           method returns <tt>true</tt>.  If the map was empty prior
     *           to the <tt>put</tt> or <tt>putAll</tt> invocation resulting
     *           in this invocation, this will be the entry that was just
     *           inserted; in other words, if the map contains a single
     *           entry, the eldest entry is also the newest.
     * @return   <tt>true</tt> if the eldest entry should be removed
     *           from the map; <tt>false</t> if it should be retained.
     */
    protected boolean removeEldestEntry(Map.Entry eldest) {
        return false;
    }
}