Java – HashMap with Null Key and Null Value

collectionshashcodehashmapjavanull

Consider the following Code :

import java.util.*;

class Employee {
    
    String name;
    
    public Employee(String nm) {
        this.name=nm;
    }
}

public class HashMapKeyNullValue {
    
    Employee e1;
    
    public void display(){

        Employee e2=null;
        Map map=new HashMap();

        map.put(e2, "25");
        System.out.println("Getting the Value When e2 is set as KEY");
        System.out.println("e2 : "+map.get(e2));
        System.out.println("e1 : "+map.get(e1));
        System.out.println("null : "+map.get(null));

        map.put(e1, "");
        System.out.println("Getting the Value when e1 is set as KEY");
        System.out.println("e2 : "+map.get(e2));
        System.out.println("e1 : "+map.get(e1));
        System.out.println("null : "+map.get(null));

        map.put(null, null);   // null as key and null as value
        System.out.println("Getting the Value when setting null as KEY and null as value");
        System.out.println("e2 : "+map.get(e2));
        System.out.println("e1 : "+map.get(e1));
        System.out.println("null : "+map.get(null));

        map.put(null, "30");
        System.out.println("Getting the Value when setting only null as KEY");
        System.out.println("e2 : "+map.get(e2));
        System.out.println("e1 : "+map.get(e1));
        System.out.println("null : "+map.get(null));
    }
    
    public static void main(String[] args) {
        
        new HashMapKeyNullValue().display();
        
    }
}

The Output of program is :

Getting the Value When e2 is set as KEY
e2 : 25
e1 : 25
null : 25
Getting the Value when e1 is set as KEY
e2 : 
e1 : 
null : 
Getting the Value when setting null as KEY and null as value
e2 : null
e1 : null
null : null
Getting the Value when setting only null as KEY
e2 : 30
e1 : 30
null : 30

Here how e1, e2, and null as keys are related to each other. Are all three assigned to same hashcode? If yes, WHY?

Since all three look different, the change in one value changes the other. Does it mean, only one entry for key is being made into HashMap either e1, e2 or null? Because all are treated to be same key.

Best Answer

HashMap does not call hashcode when null is passed as key and null Key is handled as special case.

Put Method

HashMap puts null key in bucket 0 and maps null as key to passed value. HashMap does it by linked list data structure. HashMap uses linked list data structure internally.

Linked list data structure used by HashMap (a static class in HashMap.java)

static class Entry<K,V> implements Map.Entry<K,V> {
        final K key;
        V value;
        Entry<K,V> next;
        final int hash;
}

In Entry class the K is set to null and value mapped to value passed in put method.

Get Method

While in Hashmap get method the checks if key is passed as null. Search Value for null key in bucket 0.

Hence there can only be one null key in one hashmap object.