How to iterate over a Map in Java?

Query explained:

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?

How to iterate over a Map in Java?

The following code will help:

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  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

How to iterate over each entry in a Java Map?Answer #3:

In Java 8 you can do it cleanly 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!

Answer #4:

Typical code for iterating over a map is:

Map<String,Thing> map = ...;
for (Map.Entry<String,Thing> entry : map.entrySet()) {
    String key = entry.getKey();
    Thing thing = entry.getValue();
    ...
}

HashMap is the canonical map implementation and doesn’t make guarantees (or though it should not change the order if no mutating operations are performed on it). SortedMap will return entries based on the natural ordering of the keys, or a Comparator, if provided. LinkedHashMap will either return entries in insertion-order or access-order depending upon how it has been constructed. EnumMap returns entries in the natural order of keys.

(Update: I think this is no longer true.) Note, IdentityHashMap entrySet iterator currently has a peculiar implementation that returns the same Map.Entry instance for every item in the entrySet! However, every time a new iterator advances the Map.Entry is updated.

How to iterate over a HashMap in Java? Answer #5:

There are several ways of iterating over a Map in Java. Let’s go over the most common methods and review their advantages and disadvantages. Since all maps in Java implement the Map interface, the following techniques will work for any map implementation (HashMapTreeMapLinkedHashMapHashtable, etc.)

Method #1: Iterating over entries using a For-Each loop.

This is the most common method and is preferable in most cases. It should be used if you need both map keys and values in the loop.

Map<Integer, Integer> map = new HashMap<Integer, Integer>();
for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
    System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
}

Note that the For-Each loop was introduced in Java 5, so this method is working only in newer versions of the language. Also a For-Each loop will throw NullPointerException if you try to iterate over a map that is null, so before iterating you should always check for null references.

Method #2: Iterating over keys or values using a For-Each loop.

If you need only keys or values from the map, you can iterate over keySet or values instead of entrySet.

Map<Integer, Integer> map = new HashMap<Integer, Integer>();

// Iterating over keys only
for (Integer key : map.keySet()) {
    System.out.println("Key = " + key);
}

// Iterating over values only
for (Integer value : map.values()) {
    System.out.println("Value = " + value);
}

This method gives a slight performance advantage over entrySet iteration (about 10% faster) and is more clean.

Method #3: Iterating using Iterator.

Using Generics:

Map<Integer, Integer> map = new HashMap<Integer, Integer>();
Iterator<Map.Entry<Integer, Integer>> entries = map.entrySet().iterator();
while (entries.hasNext()) {
    Map.Entry<Integer, Integer> entry = entries.next();
    System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
}

Without Generics:

Map map = new HashMap();
Iterator entries = map.entrySet().iterator();
while (entries.hasNext()) {
    Map.Entry entry = (Map.Entry) entries.next();
    Integer key = (Integer)entry.getKey();
    Integer value = (Integer)entry.getValue();
    System.out.println("Key = " + key + ", Value = " + value);
}

You can also use the same technique to iterate over keySet or values.

This method might look redundant, but it has its own advantages. First of all, it is the only way to iterate over a map in older versions of Java. The other important feature is that it is the only method that allows you to remove entries from the map during iteration by calling iterator.remove(). If you try to do this during For-Each iteration you will get “unpredictable results” according to Javadoc.

From a performance point of view, this method is equal to a For-Each iteration.

Method #4: Iterating over keys and searching for values (inefficient).

Map<Integer, Integer> map = new HashMap<Integer, Integer>();
for (Integer key : map.keySet()) {
    Integer value = map.get(key);
    System.out.println("Key = " + key + ", Value = " + value);
}

This might look like a cleaner alternative for method #1, but in practice, it is pretty slow and inefficient as getting values by a key might be time-consuming (this method in different Map implementations is 20%-200% slower than method #1). If you have FindBugs installed, it will detect this and warn you about inefficient iteration. This method should be avoided.

Conclusion:

If you need only keys or values from the map, use method #2. If you are stuck with an older version of Java (less than 5) or planning to remove entries during iteration, you have to use method #3. Otherwise use method #1.

Answer #6:

Example of using iterator and generics:

Iterator<Map.Entry<String, String>> entries = myMap.entrySet().iterator();
while (entries.hasNext()) {
  Map.Entry<String, String> entry = entries.next();
  String key = entry.getKey();
  String value = entry.getValue();
  // ...
}

Answer #7:

This is a two part question:

How to iterate over the entries of a Map – This has been answered that perfectly in above answers.

What is the order of iteration – if you are just using Map, then strictly speaking, there are no ordering guarantees. So you shouldn’t really rely on the ordering given by any implementation. However, the SortedMap interface extends Map and provides exactly what you are looking for – implementations will aways give a consistent sort order.

NavigableMap is another useful extension – this is a SortedMap with additional methods for finding entries by their ordered position in the key set. So potentially this can remove the need for iterating in the first place – you might be able to find the specific entry you are after using the higherEntrylowerEntryceilingEntry, or floorEntry methods. The descendingMap method even gives you an explicit method of reversing the traversal order.

To summarize:

There are several ways to iterate over map.

Here is a comparison of their performances for a common data set stored in map by storing a million key-value pairs in map and will iterate over map.

1) Using entrySet() in for each loop

for (Map.Entry<String,Integer> entry : testMap.entrySet()) {
    entry.getKey();
    entry.getValue();
}

50 milliseconds

2) Using keySet() in for each loop

for (String key : testMap.keySet()) {
    testMap.get(key);
}

76 milliseconds

3) Using entrySet() and iterator

Iterator<Map.Entry<String,Integer>> itr1 = testMap.entrySet().iterator();
while(itr1.hasNext()) {
    Map.Entry<String,Integer> entry = itr1.next();
    entry.getKey();
    entry.getValue();
}

50 milliseconds

4) Using keySet() and iterator

Iterator itr2 = testMap.keySet().iterator();
while(itr2.hasNext()) {
    String key = itr2.next();
    testMap.get(key);
}

Hope you learned something from this post.

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