Before deep dive in to Map interface concept & its implimentation, lets try to put some basic & fundamental rule of Map interface in java.
Here are hierarchey of Map Interface & its child classes & interfaces. There are other classes & interface which is not mentioned in the hierarchy tree. You can explore more on oracle official website here
Key Point About Map Interface
- Map is not a child interface of Collection
- Map is used to represent a group of object as a Key Value pair
- Each Key is unique and maps to a single value
- Map is considered as collection of Entry objects
- Each key-value pair is called an entry in the map.
- Maps are specially designed for efficient retrieval and modification of key-value pairs object in java
- The time complexity for put(), get(), and remove() is generally O(1) or O(log n) but it depends on the implementation
HashMap put() Method & return value
HashMap is a direct implementation class of the Map. Lets try to understand the return value of put() method in HasMap class.
package javadsa;
import java.util.HashMap;
import java.util.Map;
public class Main {
public static void main(String[] args) {
Map<Integer,String> hm=new HashMap();
String apple = hm.put(1, "Apple");
String banana = hm.put(2, "Banana");
String avocado = hm.put(3, "Avocado");
System.out.println(apple);
System.out.println(banana);
System.out.println(avocado);
}
}
As we can see in the above example, the return value of all three put method is null.
Lets try to replace an entry with the same key and see the output.
package javadsa;
import java.util.HashMap;
import java.util.Map;
public class Main {
public static void main(String[] args) {
Map<Integer,String> hm=new HashMap();
String apple = hm.put(1, "Apple");
String banana = hm.put(2, "Banana");
String avocado = hm.put(3, "Avocado");
String replacedFruit = hm.put(3, "Mango");
System.out.println(apple);
System.out.println(banana);
System.out.println(avocado);
System.out.println(replacedFruit);
}
}
As we can see in the above example, when we try to insert any new entry(key,value pair) , if key is already present , then new entry replaced the old value and return the old value .
Here Avocado is replaced by Mango & return Avacado.
