一、循环HashMap的两种方式
方式1：
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Iterator<Entry<String, String>> entryKeyIterator = entrySetMap.entrySet().iterator(); while
(entryKeyIterator.hasNext()) { Entry<String, String> e = entryKeyIterator.next(); String value = e.getValue();} |
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Iterator<String> keySetIterator = keySetMap.keySet().iterator(); while
(keySetIterator.hasNext()) { String key = keySetIterator.next(); String value = keySetMap.get(key);} |
二、性能比较
到底第一种方式的性能比第二种方式的性能高多少呢,通过一个简单的测试类可以看一下,测试代码如下
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public
class Test { public
static void main(String[] args) { HashMap<String, String> entrySetMap = new
HashMap<String, String>(); HashMap<String, String> keySetMap = new
HashMap<String, String>(); for
(int
i = 0; i < 500000; i++) { entrySetMap.put(""
+ i, "entrySet"); } for
(int
i = 0; i < 500000; i++) { keySetMap.put(""
+ i, "keySet"); } long
startTimeOne = System.currentTimeMillis(); Iterator<Entry<String, String>> entryKeyIterator = entrySetMap.entrySet().iterator(); while
(entryKeyIterator.hasNext()) { Entry<String, String> e = entryKeyIterator.next(); //System.out.println(e.getValue()); } StringBuffer result = new
StringBuffer("entrySet spent times:"); result.append((System.currentTimeMillis() - startTimeOne)); System.out.println(result.toString()); long
startTimeTwo = System.currentTimeMillis(); Iterator<String> keySetIterator = keySetMap.keySet().iterator(); while
(keySetIterator.hasNext()) { String key = keySetIterator.next(); String value = keySetMap.get(key); //System.out.println(value); } result.setLength(0); result.append("keyset spent times:"); result.append((System.currentTimeMillis() - startTimeTwo)); System.out.println(result.toString()); }} |
通过测试发现,第一种方式的性能通常要比第二种方式高一倍
三、原因分析
通过查看源代码发现,调用这个方法keySetMap.keySet()会生成KeyIterator迭代器,其next方法只返回其key值
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private
class KeyIterator extends HashIterator<K> { public
K next() { return
nextEntry().getKey(); }} private
class KeyIterator extends HashIterator<K> { public
K next() { return
nextEntry().getKey(); }} |
而调用entrySetMap.entrySet()方法会生成EntryIterator 迭代器,其next方法返回一个Entry对象的一个实例,其中包含key和value
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private
class EntryIterator extends HashIterator<Map.Entry<K,V>> { public
Map.Entry<K,V> next() { return
nextEntry(); }}private
class EntryIterator extends HashIterator<Map.Entry<K,V>> { public
Map.Entry<K,V> next() { return
nextEntry(); }} |
二者在此时的性能应该是相同的,但方式一再取得key所对应的value时,此时还要访问Map的这个方法,这时,方式一多遍历了一次table
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public
V get(Object key) { Object k = maskNull(key); int
hash = hash(k); int
i = indexFor(hash, table.length); Entry<K,V> e = table[i]; 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<K,V> e = table[i]; while
(true) { if
(e == null) return
null; if
(e.hash == hash && eq(k, e.key)) return
e.value; e = e.next; }} |
这个方法就是二者性能差别的主要原因.
java循环HashMap两种方法的效率比较,布布扣,bubuko.com
原文：http://www.cnblogs.com/brucetie/p/3569679.html