I'm practicing writing my own simple version of a Java HashMap. I don't have much experience with generics in Java.
public class HashMap < private class Entry < private K key; private V value; private Entry next; Entry(K key, V value) < this.key = key; this.value = value; >public K getKey() < return key; >public V getValue() < return value; >> private final static int TABLE_SIZE = 32; private Entry[] table; public HashMap() < table = new Entry[TABLE_SIZE]; >private int hash(K key) < return Math.abs(key.hashCode() % TABLE_SIZE); >public V get(K key) < int index = hash(key); Entry curr = table[index]; while (curr != null) < if (curr.getKey().equals(key)) < return (V) curr.getValue(); >curr = curr.next; > return null; > public V remove(K key) < int hash = hash(key); Entry curr = table[hash]; if (curr == null) < return null; >if (curr.getKey().equals(key)) < V temp = (V) curr.getValue(); table[hash] = curr.next; return temp; >while (curr.next != null) < if (curr.next.getKey().equals(key)) < V temp = (V) curr.next.getValue(); curr.next = curr.next.next; return temp; >> return null; > public void put(K key, V value) < int hash = hash(key); Entry curr = table[hash]; if (curr == null) < table[hash(key)] = new Entry(key, value); >else < while (curr.next != null) < curr = curr.next; >curr.next = new Entry(key, value); > > > Choose a better value for TABLE_SIZE :
You should specially avoid powers of 2 for this value in the division method. Why?
Because for TABLE_SIZE equal to the hash will simply be the p lowest-order bits. Is better to design a hash function that depends on all the bits of the key, so a prime number not too close to a power of 2 is generally a better choice.
Put method:
Your put method can be greatly simplified and also you could implement it so it doesn't depend on the load factor. By adding a constructor in Entry that also receives the next element you will have something like this:
public void put(K key, V value)
But you probably want to update the value if the key is already there :
public void put(K key, V value) < Entry n = get(key); if(n != null) < n.value = value; >else < int hash = hash(key); table[hash] = new Entry(key, value, table[hash]); >>