/******************************************************************************
* Compilation: javac HashST.java
* Execution: java HashST
* Dependencies: StdIn.java StdOut.java
*
* A symbol table implemented with a separate-chaining hash table.
*
******************************************************************************/
/**
* The {@code HashST} class represents a symbol table of generic
* key-value pairs.
* It supports the usual put, get, contains,
* remove, size, and is-empty methods.
* It also provides a keys method for iterating over all the keys.
* A symbol table implements the associative array abstraction:
* when associating a value with a key that is already in the symbol table,
* the convention is to replace the old value with the new value.
* Unlike {@link java.util.Map}, this class uses the convention that
* values cannot be {@code null}; setting the
* value associated with a key to {@code null} is equivalent to deleting the key
* from the symbol table.
*
* This implementation uses a separate-chaining hash table. It requires that
* the key type overrides the {@code equals()} and {@code hashCode()} methods.
* The expected time per put, contains, or remove
* operation is constant, subject to the uniform hashing assumption.
* The size, and is-empty operations take constant time.
* Construction takes constant time.
*
* For additional documentation,
* see Section 4.4 of
* Computer Science: An Interdisciplinary Approach
* by Robert Sedgewick and Kevin Wayne.
* For other implementations, see {@link ST} and {@link BST}.
*
* @author Robert Sedgewick
* @author Kevin Wayne
*/
public class HashST {
private static final int INIT_CAPACITY = 4;
private int n; // number of key-value pairs
private int m; // number of chains
private Node[] st; // array of linked-list symbol tables
// a helper linked list data type
private static class Node {
private final Object key;
private Object val;
private Node next;
public Node(Object key, Object val, Node next) {
this.key = key;
this.val = val;
this.next = next;
}
}
/**
* Initializes an empty symbol table.
*/
public HashST() {
this(INIT_CAPACITY);
}
/**
* Initializes an empty symbol table with {@code m} chains.
* @param m the initial number of chains
*/
public HashST(int m) {
this.m = m;
st = new Node[m];
}
// resize the hash table to have the given number of chains,
// rehashing all the keys
@SuppressWarnings("unchecked")
private void resize(int chains) {
HashST temp = new HashST(chains);
for (int i = 0; i < m; i++) {
for (Node x = st[i]; x != null; x = x.next) {
temp.put((Key) x.key, (Value) x.val);
}
}
this.m = temp.m;
this.n = temp.n;
this.st = temp.st;
}
// hash value between 0 and m-1
private int hash(Key key) {
return (key.hashCode() & 0x7fffffff) % m;
}
/**
* Returns the number of key-value pairs in this symbol table.
*
* @return the number of key-value pairs in this symbol table
*/
public int size() {
return n;
}
/**
* Returns true if this symbol table is empty.
*
* @return {@code true} if this symbol table is empty;
* {@code false} otherwise
*/
public boolean isEmpty() {
return size() == 0;
}
/**
* Returns true if this symbol table contains the specified key.
*
* @param key the key
* @return {@code true} if this symbol table contains {@code key};
* {@code false} otherwise
* @throws IllegalArgumentException if {@code key} is {@code null}
*/
public boolean contains(Key key) {
if (key == null) throw new IllegalArgumentException("argument to contains() is null");
return get(key) != null;
}
/**
* Returns the value associated with the specified key in this symbol table.
*
* @param key the key
* @return the value associated with {@code key} in the symbol table;
* {@code null} if no such value
* @throws IllegalArgumentException if {@code key} is {@code null}
*/
@SuppressWarnings("unchecked")
public Value get(Key key) {
if (key == null) throw new IllegalArgumentException("argument to get() is null");
int i = hash(key);
for (Node x = st[i]; x != null; x = x.next) {
if (key.equals(x.key)) return (Value) x.val;
}
return null;
}
/**
* Inserts the specified key-value pair into the symbol table, overwriting the old
* value with the new value if the symbol table already contains the specified key.
* Removes the specified key (and its associated value) from this symbol table
* if the specified value is {@code null}.
*
* @param key the key
* @param val the value
* @throws IllegalArgumentException if {@code key} is {@code null}
*/
public void put(Key key, Value val) {
if (key == null) throw new IllegalArgumentException("first argument to put() is null");
if (val == null) {
remove(key);
return;
}
// double table size if average length of list >= 10
if (n >= 10*m) resize(2*m);
int i = hash(key);
for (Node x = st[i]; x != null; x = x.next) {
if (key.equals(x.key)) {
x.val = val;
return;
}
}
n++;
st[i] = new Node(key, val, st[i]);
}
/**
* Removes the specified key and its associated value from this symbol table
* (if the key is in this symbol table).
*
* @param key the key
* @throws IllegalArgumentException if {@code key} is {@code null}
*/
public void remove(Key key) {
if (key == null) throw new IllegalArgumentException("argument to remove() is null");
int i = hash(key);
st[i] = remove(st[i], key);
// halve table size if average length of list <= 2
if (m > INIT_CAPACITY && n <= 2*m) resize(m/2);
}
// remove key in linked list beginning at Node x
// warning: function call stack too large if table is large
private Node remove(Node x, Key key) {
if (x == null) return null;
if (key.equals(x.key)) {
n--;
return x.next;
}
x.next = remove(x.next, key);
return x;
}
/**
* Returns all keys in the symbol table.
*
* @return all keys in the symbol table, as in iterable
*/
@SuppressWarnings("unchecked")
public Iterable keys() {
Queue queue = new Queue();
for (int i = 0; i < m; i++) {
for (Node x = st[i]; x != null; x = x.next) {
queue.enqueue((Key) x.key);
}
}
return queue;
}
/**
* Unit tests the {@code HashST} data type.
*/
public static void main(String[] args) {
HashST st = new HashST();
for (int i = 0; !StdIn.isEmpty(); i++) {
String key = StdIn.readString();
st.put(key, i);
}
// print keys
for (String s : st.keys())
StdOut.println(s + " " + st.get(s));
}
}