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## Solution for Programming Exercise 10.3

This page contains a sample solution to one of the exercises from Introduction to Programming Using Java.

### Exercise 10.3:

The fact that Java has a HashMap class means that no Java programmer has to write an implementation of hash tables from scratch -- unless, of course, that programmer is a computer science student.

For this exercise, you should write a hash table in which both the keys and the values are of type String. (This is not an exercise in generic programming; do not try to write a generic class.) Write an implementation of hash tables from scratch. Define the following methods: get(key), put(key,value), remove(key), containsKey(key), and size(). Remember that every object, obj, has a method obj.hashCode() that can be used for computing a hash code for the object, so at least you don't have to define your own hash function. Do not use any of Java's built-in generic types; create your own linked lists using nodes as covered in Subsection 9.2.2. However, you do not have to worry about increasing the size of the table when it becomes too full.

You should also write a short program to test your solution.

Discussion

Hash tables are discussed in Subsection 10.3.3. A hash table is just an array of linked lists. Each linked list holds all the items in the table that share the same hash code. Initially, all the lists are empty (represented as null in the array). We need to be able to add and delete items in the list. Methods for inserting and deleting items can be found in Subsection 9.2.2. In a hash table, the order of items in a particular list doesn't matter, so I simply insert each new item at the beginning of the list. This makes the insert operation fairly simple. Deletion, however, is more complicated, since we need to be able to delete an item no matter where it occurs in a list.

A hash table really contains pairs of items, where each pair consists of a key and an associated value. To implement this, I use linked lists in which each node contains both a key and a value, as well as the pointer to the next node in the list. The end of a list is marked, as usual, by a null pointer. The nodes are defined by a static nested class:

    private static class ListNode {
       String key;
       String value;
       ListNode next;
    }

The array of linked lists that stores all the data is of type ListNode[]. Each item in the array is either null to indicate an empty list, or it is a pointer to the first node in a linked list.

Given any key, we can find the linked list that should contain that key by looking at the hash code of the key. The code computed by calling key.hashCode() is of type int. We need a value that is in the range of legal indices for the array. As noted in Subsection 10.3.3, the value can be computed as Math.abs(key.hashCode()) % table.length, where table is the array.

The hash code is used in all the methods that deal with keys, to decide which linked list to look at. Once a list has been selected, the operations on the list are pretty straightforward (given all the information and code in Subsection 9.2.2), so I will not discuss them further here. You can look at the solution, below.

Although it is not required by the exercise, I defined a resize() method that is used to increase the size of the table when the table becomes too full. I call this method in the put() method when the table becomes more than 3/4 full.

A major part of developing a class for general use is testing. It's important to design a testing procedure that will test all aspects of the class. For this problem, I wrote a testing program that would allow me to test each of the methods in the class. I also added a method dump() to the class; this method displays the entire hash table. This method does not really belong in the class, since users of a hash table shouldn't care how the data is stored. But I needed it to make sure that my resize() method is working properly and that I could delete items correctly from all positions in the lists (beginning, middle, and end).

By the way, you might be tempted to try to write a generic version of HashTable in which the types of the keys and values are given as type parameters in the definition of the class. Unfortunately, you would run into problems as soon as you tried to use an array in your class. Generic classes don't work well with arrays, because arrays of generic type can't be created at run time, as noted in Subsection 10.1.3.

The Solution

The HashTable Class

    /**
     * This file defines a HashTable class.  Keys and values in the table are of
     * type String.  Keys cannot be null.  The default constructor creates a table
     * that initially has 64 locations, but a different initial size can be specified
     * as a parameter to the constructor.  The table increases in size if it
     * becomes more than 3/4 full; to help implement this, a count of the number
     * of items in the hash table is kept in an instance variable.
     */
    public class HashTable {

       /**
        * Keys that have the same hash code are placed together in a linked list.
        * This private nested class is used internally to implement linked lists.
        * A ListNode holds a (key,value) pair.
        */
       private static class ListNode {
          String key;
          String value;
          ListNode next;  // Pointer to next node in the list;
                          // A null marks the end of the list.
       }

       private ListNode[] table;  // The hash table, represented as
                                  // an array of linked lists.

       private int count;  // The number of (key,value) pairs in the
                           // hash table.

       /**
        * Create a hash table with an initial size of 64.
        */
       public HashTable() {
          table = new ListNode[64];
       }

       /**
        * Create a hash table with a specified initial size.
        * Precondition: initalSize > 0.
        */
       public HashTable(int initialSize) {
          if (initialSize <= 0)
             throw new IllegalArgumentException("Illegal table size");
          table = new ListNode[initialSize];
       }

       /**
        * This method is NOT part of the usual interface for a hash table.
        * It is here only to be used for testing purposes, and should be
        * removed before the class is released for general use.  This
        * lists the (key,value) pairs in each location of the table.
        */
       void dump() {
          System.out.println();
          for (int i = 0; i < table.length; i++) {
                 // Print out the location number and the list of
                 // key/value pairs in this location.
             System.out.print(i + ":");
             ListNode list = table[i]; // For traversing linked list number i.
             while (list != null) {
                System.out.print("  (" + list.key + "," + list.value + ")");
                list = list.next;
             }
             System.out.println();
          }
       } // end dump()

       /**
        * Associate the specified value with the specified key.
        * Precondition:  The key is not null.
        */
       public void put(String key, String value) {

          assert key != null : "The key must be non-null";

          int bucket = hash(key); // Which location should this key be in?

          ListNode list = table[bucket]; // For traversing the linked list
                                         // at the appropriate location.
          while (list != null) {
                // Search the nodes in the list, to see if the key already exists.
             if (list.key.equals(key))
                break;
             list = list.next;
          }

          // At this point, either list is null, or list.key.equals(key).

          if (list != null) {
                // Since list is not null, we have found the key.
                // Just change the associated value.
             list.value = value;
          }
          else {
                 // Since list == null, the key is not already in the list.
                 // Add a new node at the head of the list to contain the
                 // new key and its associated value.
             if (count >= 0.75*table.length) {
                   // The table is becoming too full.  Increase its size
                   // before adding the new node.
                resize();
                bucket = hash(key);  // Recompute hash code, since it
                                     // depends on the table size.
             }
             ListNode newNode = new ListNode();
             newNode.key = key;
             newNode.value = value;
             newNode.next = table[bucket];
             table[bucket] = newNode;
             count++;  // Count the newly added key.
          }
       }

       /**
        * Retrieve the value associated with the specified key in the table,
        * if there is any.  If not, the value null will be returned.
        * @param key The key whose associated value we want to find
        * @return the associated value, or null if there is no associated value
        */
       public String get(String key) {

          int bucket = hash(key);  // At what location should the key be?

          ListNode list = table[bucket];  // For traversing the list.
          while (list != null) {
                // Check if the specified key is in the node that
                // list points to.  If so, return the associated value.
             if (list.key.equals(key))
                return list.value;
             list = list.next;  // Move on to next node in the list.
          }

          // If we get to this point, then we have looked at every
          // node in the list without finding the key.  Return
          // the value null to indicate that the key is not in the table.

          return null;
       }

       /**
        * Remove the key and its associated value from the table,
        * if the key occurs in the table.  If it does not occur,
        * then nothing is done.
        */
       public void remove(String key) {

          int bucket = hash(key);  // At what location should the key be?

          if (table[bucket] == null) {
                // There are no keys in that location, so key does not
                // occur in the table.  There is nothing to do, so return.
             return;
          }

          if (table[bucket].key.equals(key)) {
                // If the key is the first node on the list, then
                // table[bucket] must be changed to eliminate the
                // first node from the list.
             table[bucket] = table[bucket].next;
             count--; // Remove new number of items in the table.
             return;
          }

          // We have to remove a node from somewhere in the middle
          // of the list, or at the end.  Use a pointer to traverse
          // the list, looking for a node that contains the
          // specified key, and remove it if it is found.

          ListNode prev = table[bucket];  // The node that precedes
                                          // curr in the list.  Prev.next
                                          // is always equal to curr.
          ListNode curr = prev.next;  // For traversing the list,
                                      // starting from the second node.
          while (curr != null && ! curr.key.equals(key)) {
             curr = curr.next;
             prev = curr;
          }

          // If we get to this point, then either curr is null,
          // or curr.key is equal to key.  In the latter case,
          // we have to remove curr from the list.  Do this
          // by making prev.next point to the node after curr,
          // instead of to curr.  If curr is null, it means that
          // the key was not found in the table, so there is nothing
          // to do.

          if (curr != null) {
             prev.next = curr.next;
             count--;  // Record new number of items in the table.
          }
       }

       /**
        * Test whether the specified key has an associated value in the table.
        * @param key The key that we want to search for.
        * @return true if the key exists in the table, false if not
        */
       public boolean containsKey(String key) {

          int bucket = hash(key);  // In what location should key be?

          ListNode list = table[bucket];  // For traversing the list.
          while (list != null) {
                // If we find the key in this node, return true.
             if (list.key.equals(key))
                return true;
             list = list.next;
          }

          // If we get to this point, we know that the key does
          // not exist in the table.

          return false;
       }

       /**
        * Return the number of key/value pairs in the table.
        */
       public int size() {
          return count;
       }

       /**
        * Compute a hash code for the key; key cannot be null.
        * The hash code depends on the size of the table as
        * well as on the value returned by key.hashCode().
        */
       private int hash(Object key) {
          return (Math.abs(key.hashCode())) % table.length;
       }

       /**
        * Double the size of the table, and redistribute the
        * key/value pairs to their proper locations in the
        * new table.
        */
       private void resize() {
          ListNode[] newtable = new ListNode[table.length*2];
          for (int i = 0; i < table.length; i++) {
                 // Move all the nodes in linked list number i into the new table.
                 // No new ListNodes are created.  The existing ListNode for each
                 // key/value pair is moved to the newtable.  This is done by
                 // changing the "next" pointer in the node and by making a pointer
                 // in the new table point to the node.
             ListNode list = table[i]; // For traversing linked list number i.
             while (list != null) {
                   // Move the node pointed to by list to the new table.
                ListNode next = list.next;  // The is the next node in the list.
                   // Remember it, before changing the value of list!
                int hash = (Math.abs(list.key.hashCode())) % newtable.length;
                   // hash is the hash code of list.key that is
                   // appropriate for the new table size.  The
                   // next two lines add the node pointed to by list
                   // onto the head of the linked list in the new table
                   // at position number hash.
                list.next = newtable[hash];
                newtable[hash] = list;
                list = next;  // Move on to the next node in the OLD table.
             }
          }
          table = newtable;  // Replace the table with the new table.
       } // end resize()

    } // end class HashTable

A Class for Testing:

    /**
     * A little program to test the HashTable class.  Note that I
     * start with a really small table so that I can easily test
     * the resize() method.
     */

    public class TestHashTable {

       public static void main(String[] args){
          HashTable table = new HashTable(2);  // Initial size of table is 2.
          String key,value;
          while (true) {
             System.out.println("\nMenu:");
             System.out.println("   1. test put(key,value)");
             System.out.println("   2. test get(key)");
             System.out.println("   3. test containsKey(key)");
             System.out.println("   4. test remove(key)");
             System.out.println("   5. show complete contents of hash table.");
             System.out.println("   6. EXIT");
             System.out.print("Enter your command:  ");
             switch ( TextIO.getlnInt()) {
             case 1:
                System.out.print("\n   Key = ");
                key = TextIO.getln();
                System.out.print("   Value = ");
                value = TextIO.getln();
                table.put(key,value);
                break;
             case 2:
                System.out.print("\n   Key = ");
                key = TextIO.getln();
                System.out.println("   Value is " + table.get(key));
                break;
             case 3:
                System.out.print("\n   Key = ");
                key = TextIO.getln();
                System.out.println("   containsKey(" + key + ") is "
                      + table.containsKey(key));
                break;
             case 4:
                System.out.print("\n   Key = ");
                key = TextIO.getln();
                table.remove(key);
                break;
             case 5:
                table.dump();
                break;
             case 6:
                return;  // End program by returning from main()
             default:
                System.out.println("   Illegal command.");
             break;
             }
             System.out.println("\nHash table size is " + table.size());
          }
       }

    } // end class TestHashTable

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