Lesson 14 of 20

Classes and Objects

Model real entities with classes, objects, fields, constructors, and instance methods.

Learning objectives

• Explain the difference between a class and an object.
• Declare fields and instance methods.
• Initialize objects with a constructor.
• Create multiple independent instances with new.
• Use object state in a practical program.

1. Class versus object

A class defines a type. An object is a particular instance of that type. If Student is the class, Amina and Daniel can be two different Student objects with different field values.

Example: Student class

public class Student {
    String studentId;
    String name;
    int mark;

    Student(String studentId, String name, int mark) {
        this.studentId = studentId;
        this.name = name;
        this.mark = mark;
    }

    void displayDetails() {
        System.out.println(studentId + " - " + name + " - " + mark);
    }
}

The constructor has the same name as the class and initializes each new object's fields. this.studentId refers to the field that belongs to the current object.

2. Creating objects

public class StudentApp {
    public static void main(String[] args) {
        Student student1 = new Student("S001", "Amina", 82);
        Student student2 = new Student("S002", "Daniel", 74);

        student1.displayDetails();
        student2.displayDetails();
    }
}

Each new Student(...) creates a separate object with its own state.

3. Adding behavior

boolean hasPassed() {
    return mark >= 50;
}

This method belongs naturally to Student because it answers a question about a student's own mark.

Mini project: bank account class

public class BankAccount {
    String accountNumber;
    String holderName;
    double balance;

    BankAccount(String accountNumber, String holderName, double balance) {
        this.accountNumber = accountNumber;
        this.holderName = holderName;
        this.balance = balance;
    }

    void deposit(double amount) {
        balance += amount;
    }

    void displayBalance() {
        System.out.printf("%s balance: RM %.2f%n", holderName, balance);
    }

    public static void main(String[] args) {
        BankAccount account =
            new BankAccount("A1001", "Amina", 500.00);

        account.deposit(125.50);
        account.displayBalance();
    }
}

Common mistakes

Summary

Classes combine state and behavior; objects are the individual instances created from those class definitions. Lesson 15 improves this design by protecting object data through encapsulation.