Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt * Object-Oriented Software Development Unit 8 Hashtables (Case Study of Using and Overriding Methods Inherited from Class Object) revised January 2002

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 2 Class Object Comparing, copying and printing are just three of the behaviours defined in the class Object. There are other behaviours that only become apparent as your programming experience broadens. Example : public native int hashCode(); –We must first introduce hashtables, then discuss how we should override hashCode() from class Object.

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 3 Tables A table is a collection of pairs of items (key, value) When you put a value in the table, you associate the value with a key To retrieve a value from the table, you provide a key, and the table looks up and returns the value associated with the key

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 4 Example An FGR posted outside the Registrar’s office: Student # Grade B+ A+ B- values keys

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 5 Example A departmental telephone directory Name Ext. Homer Marge Lisa values keys

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 6 Java's Hashtable Class The Java API provides class Hashtable in package java.util –(note the lower-case "t". Yes, this is inconsistent with the normal Java naming conventions.) public class Hashtable extends Dictionary

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 7 Suppose we want to keep a table of employee records –each employee has an ID number, which will be used as the key Employee engineer = new Employee(); engineer.setID( "98765“ ); Question : Employee ID is the key AND part of the value. Why don’t we just use a list of Employee objects? Employee IDEmployee Example :Employee id=98765 …

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 8 Java.util.Hashtable Constructors public Hashtable(); public Hashtable (int initialCapacity, float loadFactor); public Hashtable (int initialCapacity); In our example : Hashtable staff = new Hashtable(); // According to JBuilder, // a default capacity=101

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 9 java.util.Hashtable put() public Object put (Object key, Object value) put() associates a value (any object) with a key (an object), and stores the value in the hash table –later, we will discuss how to design a class so that its instances can be used as keys

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 10 java.util.Hashtable put() Keys must be unique –You cannot store two values with the same key. If you call put() twice with the same key, the second value replaces the first value –put() uses the equals() method of the key object to compare –put() returns the previous value stored with that key. If put() returns a non-null value, you know that you have replaced a previous entry

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 11 java.util.Hashtable put() In our example : String id = engineer.getID(); Object old = staff.put( id, engineer ); if (old != null) { // id over-wrote oldID String oldId = (String)old; … } // or simply: if (staff.put( engineer.getID(), engineer ) != null) {…

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 12 java.util.Hashtable get() public Object get(Object key) get() returns the value associated with the specified key, and leaves the value in the table –get() returns null if the table does not contain a value associated with the specified key To retrieve information about an employee, we need to know the key (the employee ID): String s = "98765"; Object o = staff.get( s ); if ((o != null) && (o instanceof Employee)) { Employee e = (Employee)o;

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 13 java.util.Hashtable Search Methods public boolean contains ( Object value) contains() returns true if the hash table contains the specified value Employee e2 = new Employee(…); if ( staff.contains( e2 ) { … } // Uses Employee#equals() to compare e2 // to all employee records stored in the // table

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 14 java.util.Hashtable Search Methods public boolean containsKey ( Object key ) containsKey() returns true if the hash table contains the specified key if ( staff.containsKey( “98765”) ) { … } // Uses String#equals() to compare // “98765” to all keys used in the table.

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 15 More java.util.Hashtable methods public Object remove(Object key) Deletes a key/value association from the table public void clear(); Clears table so it has no keys public int size(); Returns the number of entries in the table public boolean isEmpty(); Returns true if the table is empty And the methods inherited from Class Object: public boolean equals(Object o) public String toString(); public Object clone();

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 16 More java.util.Hashtable Methods Tables are not “ordered” like lists. We cannot index through the elements. for (i=0;i<staff.size();i++) Employee e=(Employee)staff.get(i) public Enumeration keys(); public Enumeration elements (); Return Enumeration objects to allow you to iterate through the complete set of keys or values stored in a table We will discuss enumerations (or iterators) later, but a quick example is given Enumeration e = staff.elements(); while (e.hasMoreElements() ) {Employee e = (Employee)e.nextElement();} Can’t do this

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 17 Hash Table Theory Consider how we could implement the FGR or dept. phone list tables If we used an array or linked list to store key/value pairs, retrieving a value would require a sequential search through the keys –for large tables, this is inefficient

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 18 Hash Table Theory Think of a hash table as an array of “buckets” When we pass a key object to a hash table object, the table asks the key to compute its hash code public native int hashCode(); The hash code is used as the index into the array (it selects one bucket), and the key/value pair is stored in the bucket

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 19 Storing Key/Value Pairs in a java.util.Hashtable Object e.g., add key/value objects k1/v1 to the hash table: ht.put(k1, v1); k1.hashCode() == 1 k1 v1 ht

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 20 Storing Key/Value Pairs in a java.util.Hashtable Object e.g., add key/value objects k2/v2 to the hash table: ht.put(k2, v2); k2.hashCode() == 3 k1 v1 k2 v2 ht

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 21 Storing Key/Value Pairs in a java.util.Hashtable Object e.g., add key/value objects k1/v3 to the hash table: ht.put(k1, v3); k1.hashCode() == 1 k1 v3 k2 v2 ht replaces k1/v1

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 22 Hash Table Theory: Collisions What if two different key objects compute the same hash code? We say that there is a collision (multiple key/value pairs should be stored in the same bucket) This can be handled several ways, but a common approach is “chain” all key/value pairs with the same hashcode (i.e., the bucket is a linked list of key/value pairs)

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 23 Storing Key/Value Pairs in a Hashtable Object e.g., add key/value objects k3/v4 to the hash table: ht.put(k3, v4); k3.hashCode() == 1 k1 v3 k2 v2 ht k3 v4

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 24 Retrieving Values From a Hashtable Object ht.get(k1); returns v3 ht.get(k2); returns v2 ht.get(k3); returns v4 ht.get(k4); returns null –no such key in the hash table Facilities for minimizing collisions : protected void rehash(); –Resizes & re-organizes the hashtable The Load Factor A measure of “how full” the table is. Default = 0.75 Set as argument in constructor Rehash() is automatically called when this threshold is reached

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 25 Retrieving Values From a Hashtable Object Value retrieval is efficient –get() asks the key that is passed to it to compute its hash code –if the table contains a value associated with that key, it must be in the bucket that is selected by the hash code –get() has to search at most one bucket (list) for the specified key

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 26 What Objects Can Be Used As Keys? If a class overrides equals() and hashCode() as appropriate for the class, instances of the class can be used as keys in a Hashtable public native int hashCode() Extract from the javadoc page for Object: –“Returns a hash code value for the object. This method is supported for the benefit of hashtables such as those provided by java.util.Hashtable.”

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 27 hashCode() “The general contract of hashCode is: –Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer. This integer need not remain consistent from one execution of an application to another execution of the same application. –If two objects are equal according to the equals method, then calling the hashCode method on each of the two objects must produce the same integer result.”

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 28 Using Strings as Keys Why did our employee example work ? –If you look at the javadoc page for class String, you’ll see that it overrides hashCode() and equals()

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 29 Designing hashCode() If you want to use instances of one of your classes as keys in a hashtable, that class will have to provide a hashCode() method. Approach 1: Write your own hashCode() method –study the theory that underlies designing a good hashing function and write the method from scratch (not that difficult to do, but beyond the scope of this course)

Copyright © 2002, Systems and Computer Engineering, Carleton University Hashtable.ppt 30 Designing hashCode() Approach 2: Delegation –if your class has an association or aggregation relationship with an object that implements hashCode(), forward the hashCode() message to it, and use the value returned by the object; e.g., class SomeClass { HashableClass o;... public int hashCode() { // return o’s hash code as mine return o.hashCode(); } … }