How to Iterate Over a Map in Java

There are several ways of iterating over a  Map  in Java. Lets go over the most common methods and review their advantages and disadvantages. Since all maps in Java implement  Map interface, following techniques will work for any map implementation ( HashMap ,  TreeMap , LinkedHashMap ,  Hashtable , etc.)
Method #1: Iterating over entries using For-Each loop.
This is the most common method and is preferable in most cases. 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 For-Each loop was introduced in Java 5 so this method is working only in newer versions of the language. Also 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 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 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 performance point of view this method is equal to 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.
Method 1: / 출력 맵 의 키 쌍
Map map = new HashMap();
for(Map.Entry entry : map.entrySet()){
System.out.println("Key=" + entry.getKey() + ", Value = " + entry.getValue());
}
Method 2: / / 맵 의 키 나 값 만 있 으 면 이 방법 을 사용 합 니 다.
Map map = new Map();
//Iterating over keys only
for(String key:map.keySet()){
System.out.println("Key="+key);
}
//Iterating over values only
for(String value:map.values()){
System.out.println("Value="+value);
}