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://输出Map中的键值对

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

for(Map.Entry<String,String> entry : map.entrySet()){

System.out.println("Key=" + entry.getKey() + ", Value = " + entry.getValue());

}

Method 2://只需要Map中的键 或者 值 则使用此方法

Map<String,String> map = new Map<String,String>();

//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);

}