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collections dictionary iteration java

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

3798

If I have an object implementing the Map interface in Java and I wish to iterate over every pair contained within it, what is the most efficient way of going through the map?

Will the ordering of elements depend on the specific map implementation that I have for the interface?

2

5687

Map<String, String> map = ...
for (Map.Entry<String, String> entry : map.entrySet()) {
    System.out.println(entry.getKey() + "/" + entry.getValue());
}

On Java 10+:

for (var entry : map.entrySet()) {
    System.out.println(entry.getKey() + "/" + entry.getValue());
}

8

  • 95

    If you do that, then it won’t work as Entry is a nested Class in Map. java.sun.com/javase/6/docs/api/java/util/Map.html

    – ScArcher2

    Mar 22, 2010 at 13:30

  • 287

    you can write the import as “import java.util.Map.Entry;” and it will work.

    – jjujuma

    Apr 30, 2010 at 10:34


  • 61

    @Pureferret The only reason you might want to use an iterator is if you need to call its remove method. If that is the case, this other answer shows you how to do it. Otherwise, the enhanced loop as shown in the answer above is the way to go.

    – assylias

    Oct 8, 2012 at 10:34

  • 108

    I believe the form Map.Entry is clearer than importing the inner class into the current namespace.

    Dec 4, 2014 at 20:31

  • 38

    Note that you can use map.values() or map.keySet() if you want to loop through values or keys only.

    – dguay

    Oct 12, 2016 at 21:03


1508

To summarize the other answers and combine them with what I know, I found 10 main ways to do this (see below). Also, I wrote some performance tests (see results below). For example, if we want to find the sum of all of the keys and values of a map, we can write:

  1. Using iterator and 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();
    }
    
  2. Using foreach and Map.Entry

    long i = 0;
    for (Map.Entry<Integer, Integer> pair : map.entrySet()) {
        i += pair.getKey() + pair.getValue();
    }
    
  3. Using forEach from Java 8

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

    long i = 0;
    for (Integer key : map.keySet()) {
        i += key + map.get(key);
    }
    
  5. Using keySet and iterator

    long i = 0;
    Iterator<Integer> itr2 = map.keySet().iterator();
    while (itr2.hasNext()) {
        Integer key = itr2.next();
        i += key + map.get(key);
    }
    
  6. Using for and 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();
    }
    
  7. Using the Java 8 Stream API

    final long[] i = {0};
    map.entrySet().stream().forEach(e -> i[0] += e.getKey() + e.getValue());
    
  8. Using the Java 8 Stream API parallel

    final long[] i = {0};
    map.entrySet().stream().parallel().forEach(e -> i[0] += e.getKey() + e.getValue());
    
  9. Using IterableMap of Apache Collections

    long i = 0;
    MapIterator<Integer, Integer> it = iterableMap.mapIterator();
    while (it.hasNext()) {
        i += it.next() + it.getValue();
    }
    
  10. Using MutableMap of Eclipse (CS) collections

    final long[] i = {0};
    mutableMap.forEachKeyValue((key, value) -> {
        i[0] += key + value;
    });
    

Perfomance tests (mode = AverageTime, system = Windows 8.1 64-bit, Intel i7-4790 3.60 GHz, 16 GB)

  1. For a small map (100 elements), score 0.308 is the best

    Benchmark                          Mode  Cnt  Score    Error  Units
    test3_UsingForEachAndJava8         avgt  10   0.308 ±  0.021  µs/op
    test10_UsingEclipseMap             avgt  10   0.309 ±  0.009  µs/op
    test1_UsingWhileAndMapEntry        avgt  10   0.380 ±  0.014  µs/op
    test6_UsingForAndIterator          avgt  10   0.387 ±  0.016  µs/op
    test2_UsingForEachAndMapEntry      avgt  10   0.391 ±  0.023  µs/op
    test7_UsingJava8StreamApi          avgt  10   0.510 ±  0.014  µs/op
    test9_UsingApacheIterableMap       avgt  10   0.524 ±  0.008  µs/op
    test4_UsingKeySetAndForEach        avgt  10   0.816 ±  0.026  µs/op
    test5_UsingKeySetAndIterator       avgt  10   0.863 ±  0.025  µs/op
    test8_UsingJava8StreamApiParallel  avgt  10   5.552 ±  0.185  µs/op
    
  2. For a map with 10000 elements, score 37.606 is the best

    Benchmark                           Mode   Cnt  Score      Error   Units
    test10_UsingEclipseMap              avgt   10    37.606 ±   0.790  µs/op
    test3_UsingForEachAndJava8          avgt   10    50.368 ±   0.887  µs/op
    test6_UsingForAndIterator           avgt   10    50.332 ±   0.507  µs/op
    test2_UsingForEachAndMapEntry       avgt   10    51.406 ±   1.032  µs/op
    test1_UsingWhileAndMapEntry         avgt   10    52.538 ±   2.431  µs/op
    test7_UsingJava8StreamApi           avgt   10    54.464 ±   0.712  µs/op
    test4_UsingKeySetAndForEach         avgt   10    79.016 ±  25.345  µs/op
    test5_UsingKeySetAndIterator        avgt   10    91.105 ±  10.220  µs/op
    test8_UsingJava8StreamApiParallel   avgt   10   112.511 ±   0.365  µs/op
    test9_UsingApacheIterableMap        avgt   10   125.714 ±   1.935  µs/op
    
  3. For a map with 100000 elements, score 1184.767 is the best

    Benchmark                          Mode   Cnt  Score        Error    Units
    test1_UsingWhileAndMapEntry        avgt   10   1184.767 ±   332.968  µs/op
    test10_UsingEclipseMap             avgt   10   1191.735 ±   304.273  µs/op
    test2_UsingForEachAndMapEntry      avgt   10   1205.815 ±   366.043  µs/op
    test6_UsingForAndIterator          avgt   10   1206.873 ±   367.272  µs/op
    test8_UsingJava8StreamApiParallel  avgt   10   1485.895 ±   233.143  µs/op
    test5_UsingKeySetAndIterator       avgt   10   1540.281 ±   357.497  µs/op
    test4_UsingKeySetAndForEach        avgt   10   1593.342 ±   294.417  µs/op
    test3_UsingForEachAndJava8         avgt   10   1666.296 ±   126.443  µs/op
    test7_UsingJava8StreamApi          avgt   10   1706.676 ±   436.867  µs/op
    test9_UsingApacheIterableMap       avgt   10   3289.866 ±  1445.564  µs/op
    

Graphs (performance tests depending on map size)

Enter image description here

Table (perfomance tests depending on map size)

          100     600      1100     1600     2100
test10    0.333    1.631    2.752    5.937    8.024
test3     0.309    1.971    4.147    8.147   10.473
test6     0.372    2.190    4.470    8.322   10.531
test1     0.405    2.237    4.616    8.645   10.707
test2     0.376    2.267    4.809    8.403   10.910
test7     0.473    2.448    5.668    9.790   12.125
test9     0.565    2.830    5.952   13.220   16.965
test4     0.808    5.012    8.813   13.939   17.407
test5     0.810    5.104    8.533   14.064   17.422
test8     5.173   12.499   17.351   24.671   30.403

All tests are on GitHub.

9

  • 11

    @Viacheslav : very nice answer. Just wondering how Java8 apis are hindered, in your benchmark, by capturing lambdas… (e.g. long sum = 0; map.forEach( /* accumulate in variable sum*/); captures the sum long, which may be slower than say stream.mapToInt(/*whatever*/).sum for example. Of course you can not always avoid capturing state, but that may be a reasonnable addition to the bench.

    – GPI

    May 12, 2016 at 11:53

  • 59

    @ZhekaKozlov: look at the mindblowingly large error values. Consider that a test result of x±e implies that there were result within the interval from x-e to x+e, so the fastest result (1184.767±332.968) ranges from 852 to 1518, whereas the second slowest (1706.676±436.867) runs between 1270 and 2144, so the results still overlap significantly. Now look at the slowest result, 3289.866±1445.564, which implies diverging between 1844 and 4735 and you know that these test results are meaningless.

    – Holger

    Mar 17, 2017 at 18:28


  • 8

    What about comparing the 3 main implementations : HashMap, LinkedHashMap and TreeMap ?

    – Thierry

    Nov 7, 2017 at 23:37

  • 24

    #1 and #6 are exactly the same. Using while vs. a for loop is not a different technique for iterating. And I am surprised they have such variation between them in your tests—which suggests that the tests are not properly isolated from external factors unrelated to the things you intend to be testing.

    – ErikE

    Jul 14, 2018 at 19:04

  • 9

    #8 is a terrible example, because of the parallel there’s now a race condition when adding to i.

    Jan 2, 2019 at 13:00


326

In Java 8 you can do it clean and fast using the new lambdas features:

 Map<String,String> map = new HashMap<>();
 map.put("SomeKey", "SomeValue");
 map.forEach( (k,v) -> [do something with key and value] );

 // such as
 map.forEach( (k,v) -> System.out.println("Key: " + k + ": Value: " + v));

The type of k and v will be inferred by the compiler and there is no need to use Map.Entry anymore.

Easy-peasy!

3

  • 12

    Depending on what you want to do with a map, you can also use stream API on the entries returned by map.entrySet().stream() docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html

    Jun 28, 2014 at 12:46


  • 2

    This won’t work if you want to reference non-final variables declared outside your lambda expression from within the forEach()…

    – Chris

    Apr 20, 2017 at 20:29

  • 10

    @Chris Correct. It won’t work if you try to use effectively non-final variables from outside the lambda.

    Apr 21, 2017 at 21:44