LinkedHashMap是HashMap的字类，但它是有序的，那它是怎么实现的呢，看源码
@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;
}
主要就是这个this.header的值，在每次put数据时都会更新结构，最终形成如下图所示的结构
linkedHashMap.jpg
当遍历数据时，先看下源码
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;
}
}
就是循环获取this.header的nxt参数值，直至获取的到next值与header值相等，则结束，就如上图中的红色箭头方向一样。
