1. Software Development Java Classes and Methods
Computer Science 209
Software Development
Java Classes and Methods

2. Objects, Classes, and Methods
There are no functions in Java, only methods
There are instance methods and class methods
Built-in classes include String, Math, and Arrays
Classes in libraries include a host of collections, file streams, resources for GUIs, networks, concurrency, etc.

3. Example: A Student Class
A student has a name and a set of test scores
The name can be accessed and reset
Scores can be accessed and reset by position, counting from 1
The high score and the average score can be accessed
Constructors and string representation are available

4. Default Instantiation and String Representation
Student s = new Student();
System.out.println(s);
Name:
Scores:
Defaults are an empty name string and 3 scores of 0

5. User-Specified Instantiation and String Representation
Student s = new Student("Ken", 5);
System.out.println(s);
Name: Ken
Scores:
Defaults can be overridden with another constructor

6. Access and Mutate Student s = new Student("Ken", 5);
for (int i = 1; i <= 5; i++) // Set all scores to 100
s.setScore(i, 100);
System.out.println("Average: " + s.getAverageScore());
System.out.println("High: " + s.getHighScore());
Average: 100.0
High:

7. Python Class Definition
class Student(object):
# Class variable
DEFAULT_NUM_SCORES = 3
# Constructor
def __init__(self, nm = "", numScores = None):
self.name = nm
if numScores: self.scores = Array(numScores)
else: self.scores = Array(Student.DEFAULT_NUM_SCORES)
# String representation
def __str__(self):
result = "Name: " + self.name + "\nScores:\n"
for score in self scores:
result += str(score) + "\n"
result += "Average: " + getAverage();
return result

8. Java Class Definition
public class Student extends Object{
// Class variable
static public final int DEFAULT_NUM_SCORES = 3;
static variables hold data shared by all instances of the class
final variables behave like constants
Constants use all caps and underscores

9. Java Class Definition
public class Student{
// Class variable
static public final int DEFAULT_NUM_SCORES = 3;
// Instance variables
private String name;
private int[] scores;
Instance variables hold data that belong to each individual object
Instance variables are declared at the same level as methods
Instance variables are normally declared private

10. Java Class Definition Constructors can be chained
public class Student{
// Class variable
static public final int DEFAULT_NUM_SCORES = 3;
// Instance variables
private String name;
private int[] scores;
// Constructors
public Student(String nm, int numScores){
name = nm;
scores = new int[numScores]; }
public Student(){
this("", DEFAULT_NUM_SCORES);
Constructors can be chained

11. Access Modifiers For now, all methods should be declared as public
For now, all instance variables should be declared as private
For now, all class (static) variables should be declared as public final

12. Pubic, Private, Static, and Final
public – anyone can access the item
private – access only within the class definition
static – the item belongs to the class, not to its instances
final – you can initialize but not reset the item

13. The toString Method
// String representation
public String toString(){
String str = "Name: " + name + "\nScores:\n";
for (int score : scores)
str += score + "\n";
str += "Average: " + getAverage();
return str; }
Java automatically runs toString on arguments to println

14. Accessor Methods Just observe state without changing it
public String getName(){
return name; }
public int getScore(int i){
return scores[i - 1];
public int getNumScores(){
return scores.length;
Just observe state without changing it

15. Mutator Methods Change the state! public void setName(String nm){
name = nm; }
public void setScore(int i, int newScore){
scores[i - 1] = newScore;
Change the state!

16. Mutator Methods Change the state!
public void setName(String nm){
name = nm; }
public void setScore(int i, int newScore){
scores[i - 1] = newScore;
Change the state!
Because scores has been declared private, it can only be accessed or modified by running getScore or setScore (data encapsulation)

17. Equality
public boolean equals(Object other){
if (this == other)
return true;
if (! (other instanceof Student))
return false;
Student s = (Student) other;
return this.name.equals(s.name); }
The Object class is the root class in the Java class hierarchy.
You can assign any object to a variable or parameter of type Object
Check
identity
type
structure

18. Comparisons and Ordering
public interface Comparable<T>{
// Returns 0 if receiver equals other
// Returns < 0 if receiver is less than other
// Returns > 0 if receiver is greater than other
public int compareTo(T other); }
public class Student implements Comparable<Student>{
public int compareTo(Student other){
return this.name.compareTo(other.name); } …
More strict than equals; type errors are caught at compile time

19. Now Do Sorting and Searching
Student [] students = new Student[10];
// Add 10 Student objects to the array
java.util.Arrays.sort(students);
Student target = new Student("ken", 10));
int position = java.util.Arrays.binarySearch(students,
target);
The element type in the array must implement the Comparable interface

20. Preconditions Postconditions Exceptions Javadoc
For next time
Preconditions
Postconditions
Exceptions
Javadoc