前言:有收获的话请加颗小星星,没有收获的话可以 反对 没有帮助 举报三连
8种有效方式
1 iterator + Map.Entry
long i = 0;
Iterator<Map.Entry<Integer, Integer>> it = map.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<Integer, Integer> pair = it.next();
i += pair.getKey() + pair.getValue();
}
System.out.println(i);
2 foreach + Map.Entry
long i = 0;
for (Map.Entry<Integer, Integer> pair : map.entrySet()) {
i += pair.getKey() + pair.getValue();
}
System.out.println(i);
3 foreach Java8
final long[] i = {0};
map.forEach((k, v) -> i[0] += k + v);
System.out.println(i[0]);
4 keySet + foreach
long i = 0;
for (Integer key : map.keySet()) {
i += key + map.get(key);
}
System.out.println(i);
5 keySet + iterator
long i = 0;
Iterator<Integer> it = map.keySet().iterator();
while (it.hasNext()) {
Integer key = it.next();
i += key + map.get(key);
}
System.out.println(i);
6 for + Map.Entry
long i = 0;
for (Iterator<Map.Entry<Integer, Integer>> entries = map.entrySet().iterator(); entries.hasNext(); ) {
Map.Entry<Integer, Integer> entry = entries.next();
i += entry.getKey() + entry.getValue();
}
System.out.println(i);
7 Java8 Stream Api
System.out.println(map.entrySet().stream().mapToLong(e -> e.getKey() + e.getValue()).sum());
8 Java8 Stream Api parallel
System.out.println(map.entrySet().parallelStream().mapToLong(e -> e.getKey() + e.getValue()).sum());
效率对比
# 初始化Map
public final static Integer SIZE = 10000;
public Map<Integer, Integer> map = toMap();
public Map<Integer, Integer> toMap(){
map = new HashMap<>(SIZE);
for (int i = 0; i < SIZE; i++) {
map.put(i, i);
}
return map;
}
SIZE = 10000
SIZE = 1000000
SIZE = 10000000
总结
通过数据对比,我们得知:
1 方式6是耗时最长的,方式8在数量少的时候耗时较长,但是数量大的时候耗时最短,因为方式8是并发执行。
2 一个有趣的现象,Test执行顺序总是for -> while -> foreach/stream,作者也没搞清楚为什么。