HashMap遍历时的性能对比

使用KeySet和EntrySet遍历的差别

public static void main(String[] args) {

        HashMap<Integer, Integer> hasMap = new HashMap();
        for (int i = 0; i < 10000; i++) {
            hasMap.put(i, i);
        }

        Long j = System.currentTimeMillis();
        for (int i = 1; i < 100000; i++) {
            for (Map.Entry<Integer, Integer> integerIntegerEntry : hasMap.entrySet()) {
                integerIntegerEntry.getKey();
                integerIntegerEntry.getValue();
            }
        }
        System.out.println("使用EntrySet:" + (System.currentTimeMillis() - j));

        Long k = System.currentTimeMillis();
        for (int i = 1; i < 100000; i++) {
            for (Integer key : hasMap.keySet()) {
                hasMap.get(key);
            }
        }
        System.out.println("使用KeySet:" + (System.currentTimeMillis() - k));

    }

运行多次后，两者差别有2秒左右

结论：使用EntrySet遍历时性能更高。《阿里开发手册》中也推荐使用EntrySet

原因分析：。。。

JDK8中Map interface有个default实现方法forEach, 遍历的也是EntrySet

default void forEach(BiConsumer<? super K, ? super V> action) {
        Objects.requireNonNull(action);
        for (Map.Entry<K, V> entry : entrySet()) {
            K k;
            V v;
            try {
                k = entry.getKey();
                v = entry.getValue();
            } catch(IllegalStateException ise) {
                // this usually means the entry is no longer in the map.
                throw new ConcurrentModificationException(ise);
            }
            action.accept(k, v);
        }
    }