3770

What's the best way to iterate over the items in a HashMap?

2

7 Answers 7

5381

If you're only interested in the keys, you can iterate through the keySet() of the map:

Map<String, Object> map = ...;

for (String key : map.keySet()) {
    // ...
}

If you only need the values, use values():

for (Object value : map.values()) {
    // ...
}

Finally, if you want both the key and value, use entrySet():

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

One caveat: if you want to remove items mid-iteration, you'll need to do so via an Iterator (see karim79's answer). However, changing item values is OK (see Map.Entry).

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  • 3
    So how to do loop through 2 maps simultaneously? using the entrySet method? I tried using && but it ddnt work
    – DaMainBoss
    Jul 26, 2011 at 18:18
  • 2
    Use two iterators. See the accepted answer for example usage of an iterator.
    – harto
    Jul 27, 2011 at 4:08
  • 21
    It's only more efficient to use entrySet when you need both keys and values. If you only need one or the other then just use that one: stackoverflow.com/questions/3870064/…
    – rogerdpack
    Oct 5, 2011 at 23:14
  • 4
    One more important point, the Set returned by keySet() and Collection returned by values() are both backed by the original Map. That is, if you make any modification in them they will be reflected back in the Map, however, both of them don't support add() and addAll() methods i.e. you can't add new key to the Set or new value in the Collection.
    – sactiw
    Jan 24, 2014 at 8:27
  • About getting both values and keys, it's not just simpler to use the first foreach example and get the value inside the loop, with value = map.get(key)? Is the performance of entrySet more high? Apr 18, 2017 at 8:54
3452

Iterate through the entrySet() like so:

public static void printMap(Map mp) {
    Iterator it = mp.entrySet().iterator();
    while (it.hasNext()) {
        Map.Entry pair = (Map.Entry)it.next();
        System.out.println(pair.getKey() + " = " + pair.getValue());
        it.remove(); // avoids a ConcurrentModificationException
    }
}

Read more about Map.

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  • 41
    Though old style, this will help avoid ConcurrentModificationExceptions over the new foreach style in the answers below. You can for instance remove via the seperate iterator. Dec 16, 2010 at 9:22
  • 471
    @karim79 what do you think about the following way: 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()); }
    – fresh_dev
    Oct 25, 2011 at 8:56
  • 15
    by calling 'it.remove(); ' you are emptying the map making it not reusable if this map was a class variable. Do you have any solution to that?
    – vim
    Jan 10, 2012 at 9:47
  • 28
    @vimukthi what do you mean a solution to that? Just remove the it.remove(); line.
    – Danny
    Jan 26, 2012 at 19:07
  • 114
    The for (Map.Entry<String, Object> cursor : map.entrySet()) {...} syntax is much better.
    – Chad Okere
    Jan 28, 2012 at 17:29
874

Extracted from the reference How to Iterate Over a Map in Java:

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 (HashMap, TreeMap, LinkedHashMap, Hashtable, 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 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 older version of Java (less than 5) or planning to remove entries during iteration, you have to use method #3. Otherwise use method #1.

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  • 1
    Lets add the small caevet, that in case of ConcurrentMaps, iteration on keySet() will crash in general (there is no guarantee the values exist for earlier-gathered keys). On the other hand using iterators or entries is safe (they always refer to existing objects).
    – P Marecki
    Feb 29, 2016 at 12:34
  • 3
    @arvind How would method #4 ever be inefficient? By definition, calling get() is always O(1) for a HashMap. That is the definition of a HashMap and the user asked for a HashMap. I do not get why this is so highly upvoted. If you are going to reference someone else's link, make sure it actually makes sense for the question asked.
    – ohbrobig
    Jun 3, 2018 at 13:09
  • 1
    @ohbrobig yet it's O(1) but that's the runtime, that's how it scale. It doesn't mean it will necessarily get the value in the first cycle.Method#4 will definitely be slower then Method#1
    – user961954
    Apr 13, 2019 at 8:45
184
for (Map.Entry<String, String> item : hashMap.entrySet()) {
    String key = item.getKey();
    String value = item.getValue();
}
0
107

You can iterate through the entries in a Map in several ways. Get each key and value like this:

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

Or you can get the list of keys with

Collection<?> keys = map.keySet();
for(Object key: keys){
    System.out.println("Key " + key);
    System.out.println("Value " + map.get(key));
}

If you just want to get all of the values and aren't concerned with the keys, you can use:

Collection<?> values = map.values();
76

Smarter:

for (String key : hashMap.keySet()) {
    System.out.println("Key: " + key + ", Value: " + map.get(key));
}
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  • 10
    this actually depends on whether or not you need the keys. if not, it's more efficient to use entrySet() as hashCode() does not get called.
    – icfantv
    Oct 5, 2011 at 22:33
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    map.get(key) for every iteration is not smarter -- its way slower
    – CompEng88
    Apr 18, 2013 at 21:30
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    map.entrySet() which returns entries which already contains both the key and the value. This way you don't have to call hashCode() and search the hash during the iteration.
    – CompEng88
    Apr 16, 2016 at 13:11
  • Java 8 syntax. May still not work for Android development. "Android is not intended to be 100% compatible with any Java SE API version, not 6 nor 8 nor any. ... The JRE is the Java Runtime Environment while the JDK is the Java Development Kit. It is the JDK that you need for Android application development along with the existing Android SDK.Dec 9, 2013" source Aug 5, 2017 at 22:40
56

It depends. If you know you're going to need both the key and the value of every entry, then go through the entrySet. If you just need the values, then there's the values() method. And if you just need the keys, then use keyset().

A bad practice would be to iterate through all of the keys, and then within the loop, always do map.get(key) to get the value. If you're doing that, then the first option I wrote is for you.

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  • 1
    One more important point, the Set returned by keySet() and Collection returned by values() are both backed by the original Map. That is, if you make any modification in them they will be reflected back in the Map, however, both of them don't support add() and addAll() methods i.e. you can't add new key to the Set or new value in the Collection.
    – sactiw
    Jan 24, 2014 at 8:25

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