Classes and Objects

Java is object-oriented. A class is a blueprint that describes the data (fields) and behavior (methods) of something. An object is a concrete instance of that class, created with new.

Defining a class

A class can have fields (variables that belong to each object) and methods. Here is a simple class:

java
public class Car {
    String brand;
    int year;

    public Car(String brand, int year) {
        this.brand = brand;
        this.year = year;
    }
}

• brand and year are instance fields: each Car object has its own copy.
• public Car(String brand, int year) is a constructor. It has the same name as the class and no return type. It runs when you create a new Car with new Car(...).
• this.brand and this.year refer to the fields of the object being constructed. Using this avoids confusion when parameter names match field names.

Creating objects

You create an object with the new keyword and the constructor:

java
Car myCar = new Car(\"Toyota\", 2020);
System.out.println(myCar.brand);  // Toyota
System.out.println(myCar.year);   // 2020

myCar is a reference to a Car object. You access fields and methods with a dot: myCar.brand, myCar.year.

Multiple constructors

A class can have more than one constructor. Each has a different parameter list. This is another form of overloading.

java
public class Car {
    String brand;
    int year;

    public Car(String brand, int year) {
        this.brand = brand;
        this.year = year;
    }

    public Car(String brand) {
        this.brand = brand;
        this.year = 2020;  // default
    }
}

Then you can call new Car(\"Tesla\", 2023) or new Car(\"Tesla\").

null

A reference that does not point to any object has the value null. Using a null reference (e.g. calling a method on it) causes a NullPointerException at runtime. Always ensure an object is not null before using it, or use optional types and checks as you learn more. In the next lesson we will use lists (collections) to store many objects.