在開發中經常會遇到對Map中的key和value值對的遍歷操作,一般有如下兩種方法:
方法一:
- Set keySet = map.keySet();
- for(Iterator it = keySet.iterator();it.hasNext();){
- Object key = it.next();
- Object value = map.get(key);
- }
Set keySet = map.keySet();
for(Iterator it = keySet.iterator();it.hasNext();){
Object key = it.next();
Object value = map.get(key);
}
方法二:
- Set entrySet = map.entrySet();
- for(Iterator it = entrySet.iterator();it.hasNext();){
- Entry entry = (Entry)it.next();
- Object key = entry.getKey();
- Object value = entry.getValue();
- }
Set entrySet = map.entrySet();
for(Iterator it = entrySet.iterator();it.hasNext();){
Entry entry = (Entry)it.next();
Object key = entry.getKey();
Object value = entry.getValue();
}
寫一個簡單的測試就會發現,方法一比方法二的效率高了接近1倍。
分析如下:
方法一的
- Set keySet = map.keySet();
Set keySet = map.keySet();
是先從Map中獲取keySet,代碼如下:
- public Set keySet() {
- Set ks = keySet;
- return (ks != null ? ks : (keySet = new KeySet()));
- }
- private class KeySet extends AbstractSet {
- public Iterator iterator() {
- return newKeyIterator();
- }
- public int size() {
- return size;
- }
- public boolean contains(Object o) {
- return containsKey(o);
- }
- public boolean remove(Object o) {
- return HashMap.this.removeEntryForKey(o) != null;
- }
- public void clear() {
- HashMap.this.clear();
- }
- }
public Set keySet() {
Set ks = keySet;
return (ks != null ? ks : (keySet = new KeySet()));
}
private class KeySet extends AbstractSet {
public Iterator iterator() {
return newKeyIterator();
}
public int size() {
return size;
}
public boolean contains(Object o) {
return containsKey(o);
}
public boolean remove(Object o) {
return HashMap.this.removeEntryForKey(o) != null;
}
public void clear() {
HashMap.this.clear();
}
}
返回一個私有類KeySet, 它是從AbstractSet繼承而來,實現了Set接口。
方法二的
- Set entrySet = map.entrySet();
Set entrySet = map.entrySet();
實現與keySet基本相同,可以認爲效率是一樣的,但是方法已還有一個
- map.get(key);
map.get(key);
這個方法是這樣的:
- public V get(Object key) {
- Object k = maskNull(key);
- int hash = hash(k);
- int i = indexFor(hash, table.length); //Entry[] table
- Entry e = table;
- while (true) {
- if (e == null)
- return null;
- if (e.hash == hash && eq(k, e.key))
- return e.value;
- e = e.next;
- }
- }
public V get(Object key) {
Object k = maskNull(key);
int hash = hash(k);
int i = indexFor(hash, table.length); //Entry[] table
Entry e = table;
while (true) {
if (e == null)
return null;
if (e.hash == hash && eq(k, e.key))
return e.value;
e = e.next;
}
}
其實就是再次利用Hash值取出相應的Entry做比較得到結果,所以方法一兩次進入HashMap的Entry中, 而方法二取得Entry的值之後直接取key和value,效率比方法一高。
其實看Map的概念,本來就是key和value的鍵值對,將key和value分開操作在這種場景下顯然不是個好選擇。