8 efficient ways to iterate over each entry in a Java Map

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8 effective ways

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);
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2 foreach + Map.Entry

long i = 0;
for (Map.Entry<Integer, Integer> pair : map.entrySet()) {
    i += pair.getKey() + pair.getValue();
}
System.out.println(i);
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3 foreach Java8

final long[] i = {0};
map.forEach((k, v) -> i[0] += k + v);
System.out.println(i[0]);
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4 keySet + foreach

long i = 0;
for (Integer key : map.keySet()) {
    i += key + map.get(key);
}
System.out.println(i);
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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);
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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);
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7 Java8 Stream Api

System.out.println(map.entrySet().stream().mapToLong(e -> e.getKey() + e.getValue()).sum());
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8 Java8 Stream Api parallel

System.out.println(map.entrySet().parallelStream().mapToLong(e -> e.getKey() + e.getValue()).sum());
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Efficiency comparison

# 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;
}
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SIZE = 10000

SIZE = 1000000

SIZE = 10000000

Summarize

Through data comparison, we know that:

1 Method 6 takes the longest time, and method 8 takes longer when the number is small, but takes the shortest time when the number is large, because method 8 is executed concurrently.

2 An interesting phenomenon, the execution order of Test is always for -> while -> foreach/stream, and the author did not figure out why.

refer to

How do I efficiently iterate over each entry in a Java Map?