copyright © 2002, systems and computer engineering, carleton university. 94.204-08-hashtable.ppt 1...

Copyright © 2002, Systems and Computer Engineering, Carleton University. 94.204-08-Hashtable.ppt

1

94.204* Object-Oriented Software Development

Unit 8

Hashtables

(Case Study of Using and Overriding Methods Inherited from Class Object)

• revised January 2002


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.


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


4

Example

• An FGR posted outside the Registrar’s office:

Student # Grade

107312

168904

221655

...

B+

A+

B-

valueskeys


5

Example

• A departmental telephone directory

Name Ext.

Homer

Marge

Lisa

...

1786

8113

4321

valueskeys


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


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 ID Employee

98765

Example

:Employeeid=98765…


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


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


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• 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


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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){…


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;


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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


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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.


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More java.util.Hashtable methods

public Object remove(Object key)Deletes a key/value association from the

tablepublic 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();


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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


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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


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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


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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);

...

0

1

2

3

99

k1.hashCode() == 1

k1 v1ht


20



...

0

1

2

3

99k2.hashCode() == 3

k1 v1

k2 v2

ht


21



...

0

1

2

3

99

k1.hashCode() == 1

k1 v3

k2 v2

htreplaces k1/v1


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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)


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Storing Key/Value Pairs in a Hashtable Object


...

0

1

2

3

99

k3.hashCode() == 1

k1 v3

k2 v2

ht k3 v4


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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


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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


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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.”


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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.”


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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()


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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)


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(); } …}

copyright © 2002, systems and computer engineering, carleton university. 94.204-08-hashtable.ppt 1...

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