How to implement linked list using HashMap in java

Question

How can I create linked list using HashMap in java? I searched online, there are implementations using LinkedList data structure. Interviewer asked me to implement it without using LinkedList data structure, I tried to do with HashTable but at the end he said I should have done it using HashMap.

Thank you so much for answers.

I have done this after reading your comments:

public class hashMap {
    static Map<Integer, Integer> mMap = new HashMap<Integer, Integer>();

        public static void main(String[] args) {
        int a;

        mMap.put(1, 2);
        mMap.put(2, 3);
        mMap.put(3, 4);
        mMap.put(4, 7);
        mMap.put(7, 8);

        Scanner in = new Scanner(System.in);
        System.out.println("Enter: ");
        a = in.nextInt();

            itera();

                    if(mMap.containsKey(a)) {
                add.insert(a);

            }
                else if (!mMap.containsKey(a)) {
                add.remove(a);

            }

        itera();
    }

    public static void itera() {
        for (Iterator<Integer> iter = (Iterator<Integer>) mMap.keySet().iterator(); iter.hasNext();) {

            int key = iter.next();
            int sa = mMap.get(key);
            System.out.println(key + " : " + sa);
        }

    }
    static class add {
    public static void insert(int a) {
        int s = a-1;
        int newKey = s; 
        int sa = mMap.get(a);
        mMap.put(newKey, sa);
        mMap.remove(a);
    }

    public static void remove(int a) {
        int newa = a;
        while(!mMap.containsKey(a)) {
            a--;
            System.out.println("while a: " + a);

        }

        mMap.put(newa, mMap.get(a));

        mMap.put(a, newa);

    }
}


}

It just inserts and deletes nodes into Linked List. But there is problem if some keys are missing, like 5 & 6 is not there in keys. So if I try to insert 6, it doesn't work. Could anyone explain what am I doing wrong?

Answer 1

I tried to do with HashTable but at the end he said I should have done it using HashMap.

HashTable is an old Java 1.0 class that has a couple of undesirable properties:

• All operations are synchronized ... which is typically unnecessary
• The class doesn't allow null keys or values ... which HashMap does.

Apart from that (and the fact that HashTable supports the old Enumeration API) HashMap and HashTable behave pretty much the same. It is common knowledge that it is a bad idea to use HashTable ... unless you are working on a codebase / platform that actually requires it.

So the interviewer is pointing out (correctly) that you used an out-of-date class for (probably) no good reason. Ooops!

---

But unless he specifically told you to do this, implementing a linked list using a hash table of any kind is a pretty bad idea.

• It is obscure
• It is unnecessarily complicated
• It is inefficient
• It strongly suggests that your understanding of data structures and algorithms is weak.

A simple linked list is easy to implement from scratch in plain Java. That's what you should have done.

Much bigger Ooops!!!

Answer 2

Stupid interview questions.

Easiest way I can think of is to store the "index" for each value in the key and the value as the, well, value.

Have to externally maintain the head and tail index but that is not too surprising.

Add will then look like:

public void add(V value) {
    _map.put(_tail, value);
    _tail += 1;
}

Remove:

public boolean remove(V value) {
    Iterator<Map.Entry<Integer,V>> _map.entrySet().iterator();
    while( iter.hasNext() ) {
        Map.Entry<Integer,V> entry = iter.next();
        if( value.equals(entry.getValue()) ) {
            iter.remove();
            return true;
        }
    }
    return false;
}

Leave the rest as an exercise for the reader.