1. Generic Classes and Methods
Chapter 5
Generic Classes and Methods
© 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

2. The Object Superclass
The Object class defines a variety of methods, some of which have applications in operating systems and systems programming.
© 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

3. Object Class Methods
Boolean equals(Object obj) Indicates whether some other object is “equal to” this one. A class implements the method by first checking whether obj is an instance of the class.
String toString() Returns a string representation of the object
© 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

4. Time24 Class Design
A Time24 object has integer data variables, hour and minute, where the hour is in the range 0 to 23 and minute is in the range 0 to 59.
The interface has methods to access and update the data members.
© 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

5. Student Question
How would you check equality of two times?

6. Time24 equals() public boolean equals(Object item){
// check the obj is a Time24 object
if (item instanceof Time24){
// convert objects to minutes
// and compare as integers
Time24 t = (Time24)item;
int time, ttime;
time = hour * 60 + minute;
ttime = t.hour * 60 + t.minute;
// compare the two times
return time == ttime; }
// argument is not a Time24 object
return false;
© 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

7. Generalized toString(arr)
// returns a string that represents
// an array of objects; found in Arrays class
public static String toString(Object[] arr){
if (arr == null)
return "null";
else if (arr.length == 0)
return "[]";
// start with the left bracket
String str = "[" + arr[0];
// output all but the last element,
// separating items with a comma.
for (int i = 1; i < arr.length; i++)
str += ", " + arr[i];
str += "]";
return str; }
© 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

8. Generalized Sequential Search
A generalized version of the sequential search specifies an array and a target of type Object. The array type must override the Object equals() method. The scan of the array uses the equals() method and polymorphism to locate the index of the target value.
© 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

9. Sequential Search with int
public static int seqSearch(int[] arr, int first,
int last, int target) {
// scan elements in the range
// first <= i < last; test for a match
for (int i = first; i < last; i++)
if (arr[i] == target)
return i;
// match not found
return -1; }
© 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

10. Sequential Search with Object
public static int seqSearch(Object[] arr,
int first, int last, Object target) {
// scan elements in the range
// first <= i < last; test for a match
for (int i = first; i < last; i++)
if (arr[i].equals(target))
return i;
// match not found
return -1; }
© 2005 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

11. Design of a Canonical Class
Every class in Java is an Object; this means you may override some higher level methods

12. No Argument Constructor
Normal creation of classes
The class and its constructors are known
The new is used to create an instance
Dynamic creation of classes at runtime
In some applications, particularly when processing is distributed between machines, the class itself may not be known until runtime
The JVM can still create instances of the class provided a no argument constructor is provided
This ability to load classes and create instances at runtime is a very powerful feature in Java

13. Object Equality Conditions to be satisfied by an equals( ) method
public boolean equals(Object other) {
if (this == other)
return true;
if (other instanceof C) {
C otherObj = (C) other;
-- compare each field and return false if not equal –
return true; }
else return false; }

14. Student Question
Given a linked list implementation of a list structures, how would you override the equals method?

15. Equals for the Linked List
public boolean equals(Object other) {
if (other != null &&
other instanceof LinkedList) {
LinkedList otherlist = (LinkedList) other;
if (this.size() == otherlist.size()) {
Node thisnode = this.head;
Node othernode = otherlist.head;
while (thisnode!=null && othernode!=null) {
if (!thisnode.item.equals(othernode.item))
return false;
thisnode = thisnode.next;
othernode = othernode.next; }
return true; }}
Equals for the Linked List

16. Overriding hashcode
If equals is overriden, hashcode should be overriden
A hashcode for two equal objects must return the same value; here is the hashcode for LinkedList

17. Please explain the following method
Student Question
Please explain the following method

18. Calculating a Hashcode
Common ways to combine hash values

19. Cloning Objects

20. Student Question
How would you clone a linked list?

21. Cloning Objects

22. Implementing toString
The toString method is defined for every object
For complex objects, the default is to print the class name
The programmer can override the toString method to make a more detailed printout of information
How would you override toString for a linked list?

23. toString for LinkedList
public String toString() {
StringBuffer s = new StringBuffer();
int i = 0;
for (Node n = head; n != null; n = n.next, i++) {
s.append("[" + i + "] = " + n.item + "\n"); }
return s.toString();

24. Implementing Serialization
What is serialization?
Transform an object to and from a byte stream
Important if the object is to be sent over a network to a remote site or to be stored in a file
If an object is to be serialized, the class should implement the java.io.Seralizable interface to custom define the methods writeObject( ) and readObject( )
Java provides a default implementation for objects
This implementation is adequate for our LinkedList example