How to efficiently iterate over each entry in a 'Map'?
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?
Map<String, String> map = ...
for (Map.Entry<String, String> 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:
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();
}
Using foreach and Map.Entry
long i = 0;
for (Map.Entry<Integer, Integer> pair : map.entrySet()) {
i += pair.getKey() + pair.getValue();
}
Using forEach from Java 8
final long[] i = {0};
map.forEach((k, v) -> i[0] += k + v);
Using keySet and foreach
long i = 0;
for (Integer key : map.keySet()) {
i += key + map.get(key);
}
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);
}
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();
}
Using the Java 8 Stream API
final long[] i = {0};
map.entrySet().stream().forEach(e -> i[0] += e.getKey() + e.getValue());
Using the Java 8 Stream API parallel
final long[] i = {0};
map.entrySet().stream().parallel().forEach(e -> i[0] += e.getKey() + e.getValue());
Using IterableMap of Apache Collections
long i = 0;
MapIterator<Integer, Integer> it = iterableMap.mapIterator();
while (it.hasNext()) {
i += it.next() + it.getValue();
}
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)
For 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
For 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
For 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 (perfomance tests depending on map size)
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.
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!
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