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One of the common interview question, is to build own CustomHashMap. Following code sample demonstrate generic map with put(), get() and remove() operations.

/**
 * The CustomHashMap uses an array of KeyValuePair.
 * KeyValuePair class  where K is the key and V value and  next is the element appended to it. The KeyValuePair acts 
 * as a list
 * 
 *  MapList is used to store elements. the getHash() method is used to find the index of the array. 
 * 
 * @param <K>
 * @param <V>
 */
public class CustomHashMap<K, V> {

	KeyValuePair<K, V> mapList[] = new KeyValuePair[100];

	public V get(K key) {
		int index = getHash(key);
		KeyValuePair<K,V> list = mapList[index];
		return getMatchValue(list, key);
	}

	public void put(K key, V value) {
		int index = getHash(key);
		storeValue(index, key, value);
	}
	
	public void remove(K key) {
		int index = getHash(key);
		KeyValuePair<K,V> list = mapList[index];
		if (list == null)
			return;
		// if only one element is present in the list ,set the index to null
		if(list.getKey().equals(key)){
			if (list.next == null){
				mapList[index] = null;
				return;
			}
		}
		KeyValuePair<K,V> prev = null;
		do{
			if(list.key.equals(key)){
				if (prev == null){
					list = list.getNext();
				}else{
					prev.next = list.getNext();
				}
				break;
			}
			list = list.next;
		}while(list != null);
		
		mapList[index] = list;
	}

	/*
	 * find the match value and return , if not found either throw exception or return null.
	 */
	private V getMatchValue(KeyValuePair<K, V> list, K key) {
		while (list != null) {
			if (list.getKey().equals(key))
				return list.getValue();
			list = list.next;
		}
		return null;
	}

	private void storeValue(int index, K key, V value) {
		KeyValuePair<K, V> list = mapList[index];
		
		// if list is empty , enter as first element
		if (list == null) {
			mapList[index] = new KeyValuePair<K, V>(key, value);
		} else {
			boolean done = false;
			// traverse through list , if a key is found ,replace the value or add it at the end of the list
			while(list.next != null) {
				if (list.getKey().equals(key)) {
					list.setValue(value);
					done = true;
					break;
				} 
				list = list.next;
			}
			// add at the end of the list
			if (!done)
				list.next = new KeyValuePair<K, V>(key, value);
		}

	}
	
	private int getHash(K key) {
		int hash = key.hashCode();
		return hash % 100;
	}
	
	public static void main(String args[]) {
		CustomHashMap<Integer, Integer> map = new CustomHashMap<Integer, Integer>();
		map.put(1, 1);
		map.put(2, 2);
		map.put(201,201);
		System.out.println("get value is " + map.get(1));
		System.out.println("get value is " + map.get(201));
		System.out.println("get value is " + map.get(2));
		map.remove(1);
		System.out.println("After deletion " + map.get(1));
		System.out.println("get value is " + map.get(201));
	}

}

class KeyValuePair<K, V> {
	K key;
	V value;
	KeyValuePair<K, V> next = null;

	public KeyValuePair<K, V> getNext() {
		return next;
	}

	public void setNext(KeyValuePair<K, V> next) {
		this.next = next;
	}

	public KeyValuePair(K key, V value) {
		super();
		this.key = key;
		this.value = value;
	}

	public K getKey() {
		return key;
	}

	public void setKey(K key) {
		this.key = key;
	}

	public V getValue() {
		return value;
	}

	public void setValue(V value) {
		this.value = value;
	}

}
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