Iterate through a HashMap

Question

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

Solution 1

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.

Solution 2

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).

Solution 3

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.

Solution 4

for (Map.Entry<String, String> item : hashMap.entrySet()) {
    String key = item.getKey();
    String value = item.getValue();
}

Solution 5

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();

View more solutions

Comments

[burntsugar]

I need to get the keys and values and add them to a multidimensional array

[Nitin Mahesh]

In Java 8 using Lambda Expression : stackoverflow.com/a/25616206/1503859

[Benjamin Wootton]

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.

[DaMainBoss]

So how to do loop through 2 maps simultaneously? using the entrySet method? I tried using && but it ddnt work

[harto]

Use two iterators. See the accepted answer for example usage of an iterator.

[icfantv]

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.

[rogerdpack]

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/…

[fresh_dev]

@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()); }

[vim]

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?

[Danny]

@vimukthi what do you mean a solution to that? Just remove the it.remove(); line.

[Chad Okere]

The for (Map.Entry<String, Object> cursor : map.entrySet()) {...} syntax is much better.

[Whimusical]

Is there any difference on chances of ConcurrentModificationExceptions between both syntaxs in terms of calling remove() or not? Some answers imply contradictory assertions (first vs fourth comments)

[etlds]

Iterator interface is generic since 1.2. It need to be Iterator<E>

[CompEng88]

map.get(key) for every iteration is not smarter -- its way slower

[sactiw]

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]

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.

[Victor]

I consider that the answer of arvnid, despite is just a copy/paste or not. (I agree that is ethical to give the proper credits to the one who belongs)

[Puru--]

I landed up here accidentally, why did we do Iterator.remove() here? OP just asked for a best way for iterating over a Map

[J E Carter II]

Note that it.remove will not work with unmodifiable Maps, like System.getenv() return String Map.

[Moebius]

The full type of iterator is : import java.util.Iterator;

[Hele]

@harto's answer is a better one as it tells the reader how to actually use this in a variety of cases. Your solution's application range is tiny. Very tiny.

[Jürgen K.]

Don't you jump over the first entry using the "linewhile (it.hasNext())"?

[Jwan622]

So the big picture steps seem to be 1. convert to a set using entrySet 2. turn it into an iterator and 3. call hasNext and turn each next Object into a Map.Entry Object?

[P Marecki]

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).

[CompEng88]

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.

[ashishduh]

Everyone saying that for (Map.Entry<String, Object> cursor : map.entrySet()) is better than this is ignoring the fact that you will still receive a ConcurrentModificationException using that method. The selected answer remains the best method to iterate through a Map.

[Marco Sulla]

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?

[apraetor]

@JürgenK. No, it.hasNext() returns a boolean based on whether the iterator has another element beyond the current one but does not advance the iterator's state. The iterator isn't advanced to the next element until it.next() is called.

[NimChimpsky]

@Alex yeah me too

[nemetroid]

@ashishduh The selected answer is only the best in the special case where you need to modify the map during iteration. For all other uses, using an enhanced for loop is preferable.

[phil]

This API is not intuitive, so I google this answer repeatedly and was at least 20 times on this answer!!

[jasonleonhard]

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

[doug65536]

@MarcoSulla Performance questions like that depend, you should measure and see in cases that matter. It is likely that using entrySet is faster because it can avoid doing a lookup for every entry. The implementation of the particular Map determines which way is better and by how much. The GC pressure at that point could also be a factor.

[Paŭlo Ebermann]

@etlds Actually, Java 1.2 introduced Iterator, but didn't have generics back then. Generics were only added in 1.5.

[Impulse The Fox]

Please do not use Iterator anymore.

[Androidcoder]

i get unchecked cast and unchecked assignment warnings for '(Map.Entry)it.next()'. I did add a check to Map,Entry: 'Map.Entry<Integer,MyObject> entry' to avoid other issues.

[Rishabh Agarwal]

Since map doesn't come under Iterator class how in the first solution the person has used iterator?

[Rishabh Agarwal]

Since Map is not under Iterator class, how can you use iterator with map ? Only List, Queue and Set are iterable.

[Peter Mortensen]

Perhaps update this for Java 8? (E.g. providing two versions, one for before Java 8 and one that only works on Java 8.)

[technazi]

ConcurrentHashMap is an implementation of ConcurrentMap interface. ConcurrentMap Interface extends Map Interface. The confusion is because the question specifically asked for HashMap. HashMap is an implementation of Map interface. It is always better to declare a ConcurrentHashMap like: Map<String,String> myMap = new ConcurrentHashMap<String,String>(); instead of : ConcurrentHashMap<String,String> myMap = new ConcurrentHashMap<String,String>(); This first practice allows extensibility. That is the reason this is a mature answer.

[ohbrobig]

@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.

[Androidcoder]

I get and incompatible types error for 'Map.Entry entry' and 'Map.Entry<Integer,HashMap> entry: "required Object, found Entry". I'm mapping integers to hashmaps. Having to use the keyset for loop because of this and pull the value through 'value = (HashMap) integertomap.get(key)', though less efficient then if I could use entrySet without iterater as per your example.

[c-an]

Is param the name of the HashMap?

[heman123]

@ChanjungKim yes , its the Name of the HashMap

[Mader Levap]

I consider it bad answer. OP was asking just for iterating over map, not how to modify map during iteration.

[user961954]

@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

[CAMD_3441]

@MaderLevap I disagree. He's showing the safest way of iterating over a map. The remove(); is an example of something that could cause exceptions on other types of lists/collections that would be avoided using this method.

[Rohit Chaurasiya]

import java.util.*; class MapToList { public static void main(String args[]){ Map<Integer,String> map = new HashMap<Integer,String>(); map.put(1,"Rohit"); map.put(2,"Rahul"); map.put(3,"Mohit"); Iterator itr = map.entrySet().iterator(); while(itr.hasNext()) { Map.Entry it = (Map.Entry)itr.next(); System.out.println("Printing map" + it.getKey() + it.getValue()) ; } } }

[EM-Creations]

Best explanation thank you.

[Heath Mitchell]

What about adding values?

[ONE]

Wow, if there ever is a place for the word "succinct," this is it. Merci.