How to add elements to a LinkedList that is contained within a HashMap?

Question

I create an adjacency list of vertices using a hashmap where 'V' is the total amount of vertices to be created and 'v' is the individual vertex. 'v' represents both a key for hashmap adj and the name of that vertex. Each key 'v' points to an initially empty LinkedList. I need to access that LinkedList to: 1. add elements to the LinkedList 2. Iterate through the LinkedList 1 element at a time to be implemented within a for-each loop.

HashMap<Integer, List<Integer>>adj = new HashMap<Integer, List<Integer>>();
    for(int v = 0; v < V; v++)
        adj.put(v, new LinkedList<Integer>());

1. Intended function: add int w to end of LinkedList at key 'v'

((LinkedList<Integer>)adj.get(v)).addLast(w);

2. Intended function: for each Node in LinkedList at key 'v', add that Node to key'w' and perform the specified function at each iteration.

for(int w : ((LinkedList<Integer>)adj.get(v).listIterator(0)))
    //DO THIS

Any suggestions for better implementation or possible improvements greatly appreciated.

Answer 1

You can do it like this:

for(Map.Entry<Integer, List<Integer>> entry : adj.entrySet()) {
    for (int w : entry.getValue()) {
        // Do something here
    }
}

Answer 2

Though your question depicts lack of research. But i believe in distributing knowledge, follow the below code snippet that will answer your question.

import java.util.HashMap;
import java.util.LinkedList;
import java.util.Map;

/**
 *
 * @author vaibhav.kashyap
 */
public class LinkedListInHashMap {

    private Map<String,LinkedList<String>> myMap = null;
    private LinkedList<String> myLinkedList;
    public static void main(String... wq){
        LinkedListInHashMap lObj = new LinkedListInHashMap();
        lObj.fillMap();
        lObj.displayLinkedListItem();
        lObj.addElement("e");
        lObj.addElement("f");
        lObj.displayLinkedListItem();  
    }

    public void addElement(String str){
        myLinkedList.add(new String(str));
    }
    public void fillMap(){
        myMap = new HashMap<String,LinkedList<String>>();
        myLinkedList  = new LinkedList<String>();
        myLinkedList.add(new String("a"));
        myLinkedList.add(new String("b"));
        myLinkedList.add(new String("c"));
        myLinkedList.add(new String("d"));
        myMap.put("myKey", myLinkedList);
    }
    public void displayLinkedListItem(){
        LinkedList<String> tempList = null;
        for(String key: myMap.keySet()){
            tempList = (LinkedList<String>)myMap.get(key);
            for(String str : tempList){
             System.out.println(str);
            }
        }
        System.out.println("**** End of Result****");
    }
}

output :

a
b
c
d
**** End of Result****
a
b
c
d
e
f
**** End of Result****

& Next time better refer a standard book first & then question. I'll be really happy you run this code and comment dry run of this code. If you'll do that my intent is done. Hope this helps.