implement hash table

Implement hash table 

 

https://www.youtube.com/watch?v=shs0KM3wKv8&t=8s

 

https://www.youtube.com/watch?v=KyUTuwz_b7Q

 

https://www.geeksforgeeks.org/implementing-our-own-hash-table-with-separate-chaining-in-java/

 

 

 

 

Key -> hashcode - > index 

 

Infinite number of keys, finite number of hash code .

Since we are remapping the hash code into even smaller  index 

How to  handle hash collision 

 

// Java program to demonstrate implementation of our 
// own hash table with chaining for collision detection 
import java.util.ArrayList; 

// A node of chains 
class HashNode<K, V> 
{ 
    K key; 
    V value; 

    // Reference to next node 
    HashNode<K, V> next; 

    // Constructor 
    public HashNode(K key, V value) 
    { 
        this.key = key; 
        this.value = value; 
    } 
} 

// Class to represent entire hash table 
class Map<K, V> 
{ 
    // bucketArray is used to store array of chains 
    private ArrayList<HashNode<K, V>> bucketArray; 

    // Current capacity of array list 
    private int numBuckets; 

    // Current size of array list 
    private int size; 

    // Constructor (Initializes capacity, size and 
    // empty chains. 
    public Map() 
    { 
        bucketArray = new ArrayList<>(); 
        numBuckets = 10; 
        size = 0; 

        // Create empty chains 
        for (int i = 0; i < numBuckets; i++) 
            bucketArray.add(null); 
    } 

    public int size() { return size; } 
    public boolean isEmpty() { return size() == 0; } 

    // This implements hash function to find index 
    // for a key 
    private int getBucketIndex(K key) 
    { 
        int hashCode = key.hashCode(); 
        int index = hashCode % numBuckets; 
        return index; 
    } 

    // Method to remove a given key 
    public V remove(K key) 
    { 
        // Apply hash function to find index for given key 
        int bucketIndex = getBucketIndex(key); 

        // Get head of chain 
        HashNode<K, V> head = bucketArray.get(bucketIndex); 

        // Search for key in its chain 
        HashNode<K, V> prev = null; 
        while (head != null) 
        { 
            // If Key found 
            if (head.key.equals(key)) 
                break; 

            // Else keep moving in chain 
            prev = head; 
            head = head.next; 
        } 

        // If key was not there 
        if (head == null) 
            return null; 

        // Reduce size 
        size--; 

        // Remove key 
        if (prev != null) 
            prev.next = head.next; 
        else
            bucketArray.set(bucketIndex, head.next); 

        return head.value; 
    } 

    // Returns value for a key 
    public V get(K key) 
    { 
        // Find head of chain for given key 
        int bucketIndex = getBucketIndex(key); 
        HashNode<K, V> head = bucketArray.get(bucketIndex); 

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

        // If key not found 
        return null; 
    } 

    // Adds a key value pair to hash 
    public void add(K key, V value) 
    { 
        // Find head of chain for given key 
        int bucketIndex = getBucketIndex(key); 
        HashNode<K, V> head = bucketArray.get(bucketIndex); 

        // Check if key is already present 
        while (head != null) 
        { 
            if (head.key.equals(key)) 
            { 
                head.value = value; 
                return; 
            } 
            head = head.next; 
        } 

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

        // If load factor goes beyond threshold, then 
        // double hash table size 
        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; 
                } 
            } 
        } 
    } 

    // Driver method to test Map class 
    public static void main(String[] args) 
    { 
        Map<String, Integer>map = new Map<>(); 
        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.remove("this")); 
        System.out.println(map.size()); 
        System.out.println(map.isEmpty()); 
    } 
} 

 

每一個hashnode 存的 是 key, value pair