Question

So I have a HashTable implementation here that I wrote using only Arrays and had a little bit of help with the code. Unfortunately, I don’t quite understand one of the lines someone added while running the “get” or “put” method. What exactly is happening in the while loop below? It is a method for linear probing correct? Also why is the loop checking the conditions it’s checking?

Specifically,

int hash = hashThis(key);

    while(data[hash] != AVAILABLE && data[hash].key() != key) {

        hash = (hash + 1) % capacity;
    }

Here’s the whole Java class below for full reference.

public class Hashtable2 {

private Node[] data;
private int capacity;
private static final Node AVAILABLE = new Node("Available", null);

public Hashtable2(int capacity) {

    this.capacity = capacity; 
    data = new Node[capacity];
    for(int i = 0; i < data.length; i++) {

        data[i] = AVAILABLE;
    }
}

public int hashThis(String key) {

    return key.hashCode() % capacity; 
}

public Object get(String key) {

    int hash = hashThis(key);

    while(data[hash] != AVAILABLE && data[hash].key() != key) {

        hash = (hash + 1) % capacity;
    }
    return data[hash].element();
}

public void put(String key, Object element) {

    if(key != null) {
        int hash = hashThis(key);
        while(data[hash] != AVAILABLE && data[hash].key() != key) {

            hash = (hash + 1) % capacity;
        }

        data[hash] = new Node(key, element);

    }

}



public String toString(){

    String s="<";

    for (int i=0;i<this.capacity;i++)
    {
        s+=data[i]+", ";    

    }

    s+=">";

    return s;
    }

Thank you.

Answer 1

I just rewrote some part of the code and added the findHash-method – try to avoid code-duplication!

private int findHash(String key) {
    int hash = hashThis(key);

    // search for the next available element or for the next matching key
    while(data[hash] != AVAILABLE && data[hash].key() != key) {

        hash = (hash + 1) % capacity;
    }
    return hash;
}

public Object get(String key) {

    return data[findHash(key)].element();
}

public void put(String key, Object element) {

    data[findHash(key)] = new Node(key, element); 
}

What you asked for is – what exactly does this findHash-loop? The data was initialized with AVAILABLE – meaning: the data does not (yet) contain any actual data. Now – when we add an element with put – first a hashValue is calculated, that is just an index in the data array where to put the data. Now – if we encounter that the position has already been taken by another element with the same hash value but a different key, we try to find the next AVAILABLE position. And the get method essentially works the same – if a data element with a different key is detected, the next element is probed and so on.
The data itself is a so called ring-buffer. That is, it is searched until the end of the array and is next search again at the beginning, starting with index 0. This is done with the modulo % operator.

Alright?