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

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());}

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  Unitstest3_UsingForEachAndJava8         avgt  10   0.308 ±  0.021  µs/optest10_UsingEclipseMap             avgt  10   0.309 ±  0.009  µs/optest1_UsingWhileAndMapEntry        avgt  10   0.380 ±  0.014  µs/optest6_UsingForAndIterator          avgt  10   0.387 ±  0.016  µs/optest2_UsingForEachAndMapEntry      avgt  10   0.391 ±  0.023  µs/optest7_UsingJava8StreamApi          avgt  10   0.510 ±  0.014  µs/optest9_UsingApacheIterableMap       avgt  10   0.524 ±  0.008  µs/optest4_UsingKeySetAndForEach        avgt  10   0.816 ±  0.026  µs/optest5_UsingKeySetAndIterator       avgt  10   0.863 ±  0.025  µs/optest8_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   Unitstest10_UsingEclipseMap              avgt   10    37.606 ±   0.790  µs/optest3_UsingForEachAndJava8          avgt   10    50.368 ±   0.887  µs/optest6_UsingForAndIterator           avgt   10    50.332 ±   0.507  µs/optest2_UsingForEachAndMapEntry       avgt   10    51.406 ±   1.032  µs/optest1_UsingWhileAndMapEntry         avgt   10    52.538 ±   2.431  µs/optest7_UsingJava8StreamApi           avgt   10    54.464 ±   0.712  µs/optest4_UsingKeySetAndForEach         avgt   10    79.016 ±  25.345  µs/optest5_UsingKeySetAndIterator        avgt   10    91.105 ±  10.220  µs/optest8_UsingJava8StreamApiParallel   avgt   10   112.511 ±   0.365  µs/optest9_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    Unitstest1_UsingWhileAndMapEntry        avgt   10   1184.767 ±   332.968  µs/optest10_UsingEclipseMap             avgt   10   1191.735 ±   304.273  µs/optest2_UsingForEachAndMapEntry      avgt   10   1205.815 ±   366.043  µs/optest6_UsingForAndIterator          avgt   10   1206.873 ±   367.272  µs/optest8_UsingJava8StreamApiParallel  avgt   10   1485.895 ±   233.143  µs/optest5_UsingKeySetAndIterator       avgt   10   1540.281 ±   357.497  µs/optest4_UsingKeySetAndForEach        avgt   10   1593.342 ±   294.417  µs/optest3_UsingForEachAndJava8         avgt   10   1666.296 ±   126.443  µs/optest7_UsingJava8StreamApi          avgt   10   1706.676 ±   436.867  µs/optest9_UsingApacheIterableMap       avgt   10   3289.866 ±  1445.564  µs/op

Graphs (performance tests depending on map size)

Table (perfomance tests depending on map size)

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

All tests are on GitHub.

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!