In Java, the Map is an interface that doesn’t extend the Collection interface. The map interface allows storing elements as key-value pairs. The Map doesn’t allow duplicates keys and values may have a duplicate. We cannot traverse the elements in the Map, so we need to convert it into Set using keySet() or entrySet() method.
The Map is an interface, so we cannot provide a direct implementation of it. We can use the following classes to implement the functionalities of Map.
- HashMap
- LinkedHashMap
- TreeMap
- WeakHashMap
- EnumMap
The Map is present in Java.util package, we must import Java .util.Map before using it.
Map map= HashMap<>(); Map map= LinkedHashMap<>(); Map map= TreeMap<>();
HashMap implementation of Map retriving values using keySet:
import java.util.Set;
import java.util.Map;
import java.util.HashMap;
import java.util.Iterator;
public class HashMapImpl {
public static void main(String[] args) {
//Create Map with HashMap class
Map<Integer, String> map = new HashMap<>();
//Adding elements to HashMap
map.put(1, "One");
map.put(2, "Two");
map.put(3, "Three");
map.put(4, "Four");
map.put(2, "Two");
//Converting Map to Set
Set set=map.entrySet();
Iterator itr=set.iterator();
while(itr.hasNext()){
//Converting to Map.Entry so that we can access key and values separately
Map.Entry entry=(Map.Entry)itr.next();
System.out.println(entry.getKey()+" "+entry.getValue());
}
}
}
output:
1 One
2 Two
3 Three
4 Four
Form the above program and output; You can see we tried to add a new value with a duplicate key (2) HashMap removed the duplicate element. We converted the Map into Set and retrieved the elements using keySet through Iterator.
HashMap implementation of Map retriving values using entrySet:
import java.util.Set;
import java.util.Map;
import java.util.HashMap;
public class HashMapImpl {
public static void main(String[] args) {
//Create Map with HashMap class
Map<Integer, String> map = new HashMap<>();
//Adding elements to HashMap
map.put(1, "One");
map.put(2, "Two");
map.put(3, "Three");
map.put(4, "Four");
// Converting Map to Set using entrySet
Set< Map.Entry<Integer, String> > set = map.entrySet();
//Iterating using foreach loop
for (Map.Entry<Integer, String> obj:set)
{
System.out.println(obj.getKey()+":"+(obj.getValue()));
}
}
}
output:
1:One
2:Two
3:Three
4:Four
Form the above program and output; We converted the Map into Set and retrieved the elements using entrySet through the foreach loop.