LinkedHashMap是HashMap的字類(lèi)，但它是有序的，那它是怎么實(shí)現(xiàn)的呢，看源碼

@Override 
void addNewEntry(K key, V value, int hash, int index) {
     LinkedEntry<K, V> header = this.header;

     // Remove eldest entry if instructed to do so.
     LinkedEntry<K, V> eldest = header.nxt;
     if (eldest != header && removeEldestEntry(eldest)) {
         remove(eldest.key);
     }

     // Create new entry, link it on to list, and put it into table
     LinkedEntry<K, V> oldTail = header.prv;
     LinkedEntry<K, V> newTail = new LinkedEntry<K,V>(
             key, value, hash, table[index], header, oldTail);
     table[index] = oldTail.nxt = header.prv = newTail;
}

主要就是這個(gè)this.header的值，在每次put數(shù)據(jù)時(shí)都會(huì)更新結(jié)構(gòu)，最終形成如下圖所示的結(jié)構(gòu)

linkedHashMap.jpg

當(dāng)遍歷數(shù)據(jù)時(shí)，先看下源碼

private abstract class LinkedHashIterator<T> implements Iterator<T> {
    LinkedEntry<K, V> next = header.nxt;
    LinkedEntry<K, V> lastReturned = null;
    int expectedModCount = modCount;

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

    final LinkedEntry<K, V> nextEntry() {
        if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
         LinkedEntry<K, V> e = next;
         if (e == header)
             throw new NoSuchElementException();
         next = e.nxt;
         return lastReturned = e;
     }

     public final void remove() {
         if (modCount != expectedModCount)
             throw new ConcurrentModificationException();
         if (lastReturned == null)
             throw new IllegalStateException();
         LinkedHashMap.this.remove(lastReturned.key);
         lastReturned = null;
         expectedModCount = modCount;
     }
}

就是循環(huán)獲取this.header的nxt參數(shù)值，直至獲取的到next值與header值相等，則結(jié)束，就如上圖中的紅色箭頭方向一樣。