LinkHashMap源码详解

private transient Entry<K,V> header;//循环双向链表的头

    @Override
    void init() {
        header = new Entry<>(-1, null, null, null);
        //将元素的前驱和后续都指向自己,图解
        header.before = header.after = header;
    }

    void addEntry(int hash, K key, V value, int bucketIndex) {
        //调用父类的addEntry方法，但是重写了createEntry方法
        super.addEntry(hash, key, value, bucketIndex);

        // Remove eldest entry if instructed
        Entry<K,V> eldest = header.after;
        if (removeEldestEntry(eldest)) {
            removeEntryForKey(eldest.key);
        }
    }


    void createEntry(int hash, K key, V value, int bucketIndex) {
        HashMap.Entry<K,V> old = table[bucketIndex];
        Entry<K,V> e = new Entry<>(hash, key, value, old);
        table[bucketIndex] = e;
        //主要看这个方法,把链表头传进去
        e.addBefore(header);
        size++;
    }

 private void addBefore(Entry<K,V> existingEntry) {
            //图解
            after  = existingEntry;
            before = existingEntry.before;
            before.after = this;
            after.before = this;
        }

        private void remove() {
            //下面的代码相当于这样的，加个this比较好理解
            //this.before.after = this.after;
            //this.after.before = this.before;

            //源码
            before.after = after;
            after.before = before;
        }

    private abstract class LinkedHashIterator<T> implements Iterator<T> {
        Entry<K,V> nextEntry    = header.after;
        Entry<K,V> lastReturned = null;
        int expectedModCount = modCount;

        //如果结点不等于链表头，那么说明还有元素
        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<K,V> nextEntry() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            if (nextEntry == header)
                throw new NoSuchElementException();

            Entry<K,V> e = lastReturned = nextEntry;
            nextEntry = e.after;
            return e;
        }
    }