Displaying key-values in HashMap

Question

I have 2 questions regarding the code bellow,

1.I have the key "two" twice in my hashmap, while printing, "two" is displayed only once.Why its not displaying "two" twice?

2.How to selectively display the key "two"?

import java.util.HashMap;
import java.util.Iterator;
import java.util.Set;

public class main {
public static void main(String[] args){
HashMap<String,String> myMap = new HashMap<String,String>();

    myMap.put("one", "1");
    myMap.put("two", "2");
    myMap.put("three", "3");
    myMap.put("two", "4");

    Set <String> mySet =myMap.keySet();
    Iterator itr = mySet.iterator();

    while(itr.hasNext()){
        String key = (String) itr.next();
        System.out.println(key);
    }

}
}

Java Map is a One-To_one object, hence only the last value entered is being pulled from the map. — Yogev Caspi
– Yogev Caspi, Commented Aug 4, 2015 at 12:40
possible duplicate of Does adding a duplicate value to a HashSet/HashMap replace the previous value — Ankush soni
– Ankush soni, Commented Aug 4, 2015 at 12:42

GregH · Accepted Answer · 2015-08-04 14:18:44Z

3

Hashmaps may only have one key entry per key in their keyset. The second time you put a key-value pair in the map will override the first when you are using the same key for Maps (which include HashMap).

If you want a one-to-many mapping, you can use a Multimap or a HashMap that maps an object to a collection of objects (although Multimap will most likely make this easier for you)

To display a value for a given key, use:

System.out.println(myMap.get(myKey));
System.out.println(myMap.get("two"));

edited Aug 4, 2015 at 14:18

answered Aug 4, 2015 at 12:40

GregH

5,53112 gold badges62 silver badges115 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

hoijui Over a year ago

actually this is part of the Map interface, not something specific to HashMap, and it is not something specific to Java, but it is how the map idiom works in general (programming and math and probably more).

darijan · Accepted Answer · 2015-08-04 12:44:54Z

2

Hashtable and HashMap are one-to-one key value store. That means that for one key you can have only one element. You can still achieve what you want with:

HashMap<String, List<String>>

when you add an element to the map, you have to add it to the list for this key, i.e.

public void add(String key, String value) {
    List<String> list = map.get(key);
    if (list == null) { //if the list does not exist, create it, only once
        list = new ArrayList<String>();
        map.put(key, list);
    }
    list.add(value);
}

And now, when you want to get all elements with this key:

List<String> elements = map.get("two");

The list will contain all elements you added.

answered Aug 4, 2015 at 12:44

darijan

9,80327 silver badges38 bronze badges

Collectives™ on Stack Overflow

Displaying key-values in HashMap

2 Answers 2

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related