A Map is an object that maps keys to values. A map cannot contain duplicate keys, each key can map to at most one value. This interface takes the place of the Dictionary class, which was a totally abstract class rather than an interface.
The Java platform contains three general-purpose Map implementations: Java HashMap Implementation, TreeMap In Java - java.util.TreeMap, and LinkedHashMap In Java with Code Example.
Map Methods :
import java.util.HashMap; import java.util.Map;
class MapTest { public static void main(String arg[]) { Map<String, String> m = new HashMap<String, String>(); Map<String, String> m1 = new HashMap<String, String>(); System.out.println("Map objects equal : " + m.equals(m1)); // Equals method m.put("8", "Prakash"); // Put method m.put("31", "Shabaz"); m.put("12", "Raj"); m.put("14", "Praveen"); m.put("5", "Gopi"); System.out.println("Map Elements : " + m); // Display map elements System.out.println("Size of Map : " + m.size()); System.out.println("Key contains : " + m.containsKey("31")); // LINE A System.out.println("Value contains : " + m.containsValue("Raj")); // LINE B System.out.println("Gets the value of key : " + m.get("14")); // LINE C System.out.println("Hash code for map : " + m.hashCode()); System.out.println("Is map empty : " + m.isEmpty()); System.out.println("Removes the key value : " + m.remove("12")); // LINE D System.out.println("Key set : " + m.keySet()); // LINE E System.out.println("Collection values : " + m.values()); // LINE F m1.putAll(m); // LINE G m.clear(); System.out.println("Map after clear : " + m); System.out.println("All data of m is put into m1 map : " + m1); System.out.println("Entry set : " + m1.entrySet()); } } OUTPUTMap objects equal : true Map Elements : {5=Gopi, 31=Shabaz, 8=Prakash, 14=Praveen, 12=Raj} Size of Map : 5 Key contains : true Value contains : true Gets the value of key : Praveen Hash code for map : 867799845 Is map empty : false Removes the key value : Raj Key set : [5, 31, 8, 14] Collection values : [Gopi, Shabaz, Prakash, Praveen] Map after clear : {} All data of m is put into m1 map : {5=Gopi, 31=Shabaz, 8=Prakash, 14=Praveen} Entry set : [5=Gopi, 31=Shabaz, 8=Prakash, 14=Praveen] DESCRIPTIONIn this program, all the Map methods are applied on map object. The map is of type HashMap . It contains five elements. THINGS TO TRY - At
LINE A replace 31 by 5 to see that output is true . Which means key 5 is present in map. Enter any other key which is not in the map to return false . The same thing repeat for value at LINE B .
- At
LINE C give any key to return the value associated with that key. It returns null if the key does not exist in map.
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LINE D , remove is used to delete the element from map. Try with key 5 .
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