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

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.

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

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

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

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: 100

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

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

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

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

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

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

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

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

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!

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)

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

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

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

Preconditions Postconditions Exceptions Javadoc For next time Preconditions Postconditions Exceptions Javadoc