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

The Java Map interface, java.util.Map, represents a mapping between a key and a value. More specifically, a Java Map can store pairs of keys and values. Each key is linked to a specific value. Once stored in a Map, you can later look up the value using just the key.

The Java Map interface is not a subtype of the Collection interface. Therefore it behaves a bit different from the rest of the collection types.

Since Map is an interface you need to instantiate a concrete implementation of the Map interface in order to use it. The Java Collections API contains the following Map implementations:

• java.util.HashMap
• java.util.Hashtable
• java.util.EnumMap
• java.util.IdentityHashMap
• java.util.LinkedHashMap
• java.util.Properties
• java.util.TreeMap
• java.util.WeakHashMap

Most commonly used Map implementations are HashMap and TreeMap.

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;
    });