HashMap.java

import java.util.ArrayList;

class HashNode<K,V> {
    K key;
    V value;
    HashNode<K, V> next;
    public HashNode(K key, V value) {
        this.key = key;
        this.value = value;
    }
}

public class HashMap<K,V>{
    private ArrayList<HashNode<K,V>> bucketArray;
    private int numBuckets;
    private int size;
    public HashMap() {
        bucketArray  = new ArrayList<>();
        numBuckets = 10;
        size = 0;
        for ( int i = 0; i < numBuckets; i ++)
            bucketArray.add(null);
    }
    public int size() { return size;}
    public boolean isEmpty() { return size == 0;}
    public int  getBucketIndex( K key){
        int hashCode = key.hashCode();
        int index = hashCode % numBuckets;
        return index;
    }
    public V remove(K key){
        int bucketIndex = getBucketIndex(key);
        HashNode<K,V> head = bucketArray.get(bucketIndex);
        HashNode<K,V> prev = null;
        while(head != null){
            if(head.key.equals(key))
                break;
            prev = head;
            head = head.next;
        }
        // key not found
        if(head == null)
            return null;

        size--;
        if(prev != null)
            prev.next = head.next;
        else
            bucketArray.set(bucketIndex, head.next);
        return head.value;
    }

    public V get(K key){
        int bucketIndex = getBucketIndex(key);
        HashNode<K,V> head = bucketArray.get(bucketIndex);
        while(head!= null){
            if(head.key.equals(key))
                return head.value;
            head = head.next;
        }
        return null;
    }

    public void add(K key, V value){
        int bucketIndex = getBucketIndex(key);
        HashNode<K,V> head = bucketArray.get(bucketIndex);

        while(head!= null){
            if(head.key.equals(key)){
                head.value= value;
                return;
            }
            head = head.next;
        }

        size++;
        head = bucketArray.get(bucketIndex);
        HashNode<K,V> newNode = new HashNode<K,V>(key, value);
        newNode.next = head;
        bucketArray.set(bucketIndex, newNode);

        if((1.0*size)/numBuckets >= 0.7){
            ArrayList<HashNode<K,V>>temp = bucketArray;
            bucketArray = new ArrayList<>();
            numBuckets = 2 * numBuckets;
            size = 0;
            for (int i = 0; i < numBuckets; i ++)
                bucketArray.add(null);
            for(HashNode<K,V> headNode : temp){
                while(headNode != null){
                    add(headNode.key, headNode.value);
                    headNode = headNode.next;
                }
            }


        }
    }

    public static void main(String[] args) {
        HashMap<String,Integer> map = new HashMap<>();
        map.add("this",1);
        map.add("coder",2);
        map.add("this",4);
        map.add("hi",5);
        System.out.println(map.size());
        System.out.println(map.remove("this"));
        System.out.println(map.size());
        System.out.println(map.isEmpty());

    }
}