java循环HashMap两种方法的效率比较

一、循环HashMap的两种方式

方式1：

Iterator<Entry<String, String>> entryKeyIterator = entrySetMap.entrySet().iterator();
       while (entryKeyIterator.hasNext()) {
            Entry<String, String> e = entryKeyIterator.next();
            String value = e.getValue();
}

方式2：

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

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

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

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

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.