Classes and Objects

This lesson explains classes, objects, instance variables, instance methods, object declaration, object creation, and the difference between instance and class members.

A class is a blueprint for creating objects. It describes what data an object can store and what actions it can perform.

For example, a Phone class might describe data such as brand, model, and screen size, and behavior such as power on, call, and send message.

An object is a specific instance of a class. If Phone is the class, then a specific phone model owned by a person can be represented as an object.

Objects usually have:

• state: values stored in variables
• behavior: actions implemented as methods

Software objects are modeled after real-world objects by combining data and behavior.

A class header includes an optional access modifier, the keyword class, and the class name.

public class Student {
    // fields and methods go here
}

Fields are variables that belong to the class or object.

public class Student {
    int age = 18;
}

Fields are often marked private so other classes cannot change them directly. This protects object data and encourages controlled access through methods.

Common modifiers:

Instance methods belong to objects. They are usually called after creating an object with new.

Setter methods store values. Getter methods return values.

Declaring a variable of a class type does not create an object yet.

Student learner;          // declaration only
learner = new Student();  // object creation

Both steps can be combined.

Student learner = new Student();

The new operator reserves memory and calls a constructor.

An instance variable belongs to one object. Each object gets its own copy.

A static variable belongs to the class. All objects share the same static variable.

public class SchoolRecord {
    static int schoolCode = 100;
    int studentNumber;
}

If one object changes a static variable, all objects see the updated value.

Create a second Student object with different values and call displayInfo() on it.