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Iterating Map in Java
Map doesn’t implement the Iterable interface. This means that we cannot cycle through a map using a for-each style for loop and we can’t obtain an iterator to a map. However we can obtain a collection-view of a map, which does allow the use of either the for loop or an iterator.
The Map.Entry interface enables you to work with a map entry. The entrySet( ) method declared by the Map interface returns a Set containing the map entries. Each of these set elements is a Map.Entry object.
Map.Entry is generic and is declared like this:
interface Map.Entry<K, V>
Here, K specifies the type of keys, and V specifies the type of values.
Methods present in Map interface which are used to obtain Collection view of a Map | |
Method | Description |
Set<Map.Entry<K, V>> entrySet( )
| Returns a Set that contains the entries in the map. The set contains objects of type Map.Entry. Thus, this method provides a set-view of the invoking map. |
Set<K> keySet( )
| Returns a Set that contains the keys in the invoking map. This method provides a set-view of the keys in the invoking map. |
Collection<V> values( )
| Returns a collection containing the values in the map. This method provides a collection-view of the values in the map. |
Methods present in Map.Entry interface which are used to iterate Map | |
Method | Description |
K getKey( ) | Returns the key for this map entry. |
V | getValue( ) Returns the value for this map entry. |
Example to iterate Map
MapIterationDemo.java
import java.util.Collection; import java.util.Iterator; import java.util.Map; import java.util.Set; import java.util.TreeMap;
public class MapIterationDemo {
public static void main(String[] args) { TreeMap map = new TreeMap(); map.put(new Integer(103), "CCC"); map.put(new Integer(105), "EEE"); map.put(new Integer(102), "BBB"); map.put(new Integer(101), "AAA"); map.put(new Integer(104), "DDD");
// Iterate using key Set s = map.keySet(); Iterator itr = s.iterator(); while (itr.hasNext()) { Integer k = (Integer) itr.next(); System.out.println(k); // using key can get value String v = (String) map.get(k); System.out.print(k + "\t" + v + "\n"); }
// Iterate using values Collection c = map.values(); Iterator itr1 = c.iterator(); while (itr1.hasNext()) { String name = (String) itr1.next(); System.out.println(name); }
// iterate using both key and value Set s1 = map.entrySet(); Iterator itr2 = s1.iterator(); while (itr2.hasNext()) { Map.Entry me = (Map.Entry) itr2.next(); Integer k = (Integer) me.getKey(); String v = me.getValue().toString(); System.out.print(k + "\t" + v + "\n"); } } } |
Output:
101 101 AAA 102 102 BBB 103 103 CCC 104 104 DDD 105 105 EEE AAA BBB CCC DDD EEE 101 AAA 102 BBB 103 CCC 104 DDD 105 EEE |
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