> For the complete documentation index, see [llms.txt](https://codehs.gitbook.io/apjava/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://codehs.gitbook.io/apjava/classes-and-object-oriented-programming/introduction-to-classes-and-objects.md).

# Introduction To Classes and Objects

Classes and Objects are utilized in Java as part of the object-oriented programming model. This model focuses on objects and the data and actions associated with the objects.

## Objects

Objects are structures that contain a state and behavior. Every day objects we commonly use have states and behaviors. For example, a car is an object with both a state and a behavior.

### State

The state contains the information about the specific object.

Thinking back to the car in this case. An example of a state, with the car, would be how much fuel it has.

### Behavior

The behavior is the actions that can be performed on the specific object.

A behavior of the car may be to get the mileage from the remaining fuel.

## Classes

Classes are the templates we use for creating objects.

### Rectangle Class

Here is an example of a class that lets us create a rectangle, and get its area:

```java
public class Rectangle
{
    private double width;
    private double height;

    public Rectangle(double rectWidth, double rectHeight)
    {
        width = rectWidth;
        height = rectHeight;
    }

    public int getWidth()
    {
        return width;
    }

    public int getHeight()
    {
        return height;
    }

    public int getArea()
    {
        return width * height;
    }

    public String toString()
    {
        String rectInfo = "Rectangle with width: " + width + " and height: " + height + 
            " and area: " + getArea();

        return rectInfo;
    }
}
```

### Animal Class

Here is another example of a class that lets us create new animal objects:

```java
public class Animal
{
    private String name;
    private boolean isPet;
    private int age;

    public Animal(String animalName, boolean isAnimalPet, int animalAge)
    {
        name = animalName;
        isPet = isAnimalPet;
        age = animalAge;
    }

    public String getName()
    {
        return name;
    }

    public boolean getPetStatus()
    {
        return isPet;
    }

    public int getAge()
    {
        return age;
    }

    public String toString()
    {
        String aInfo = "This animal's name is: " + name + " they are currently a pet: " +
            isPet + ". The animal is " + age + " years old.";

        return aInfo;
    }
}
```

### Vehicle Class

Here is another example of a class that takes in a vehicle type, its age, and how many miles it has:

```java
public class Vehicle
{
    private String vehicleType;
    private int vehicleAge;
    private double vehicleMiles;

    public Vehicle(String vType, int vAge, double vMiles)
    {
        vehicleType = vType;
        vehicleAge = vAge;
        vehicleMiles = vMiles;
    }

    public String getType()
    {
        return vehicleType;
    }

    public int getAge()
    {
        return vehicleAge;
    }

    public double getMiles()
    {
        return vehicleMiles;
    }

    public double estimateMilesPerYear()
    {
        return vehicleMiles / vehicleAge;
    }
}
```

## Creating Objects

When creating a new object from a class you will use the following format:

```java
[Class Name] [Object Name] = new [Class Name] (params);

// Here are some examples using the classes we created earlier

Rectangle rect = new Rectangle(20, 8);

Animal pet = new Animal("Cujo", true, 7);

Vehicle myTruck = new Vehicle("Truck", 10, 173600.4);
```

## Using Objects

Here are some examples on how to create objects with our previous defined classes, and use them.

```java
// Rectangle Class
public class StateBehavior_Rectangle
{
    // Here we will create a new rectangle, and then print its information.
    public void run()
    {
        // Create the rectangle
        Rectangle rect = new Rectangle(20, 8);
        // This will print "Rectangle with width: 20 and height: 8 and area: 160"
        System.out.println(rect);
    }
}

// Animal Class
public class StateBehavior_Animal
{
    // Here we will create a new animal, and then print its information.
    public void run()
    {
        // Create the animal and assign its attributes
        Animal myPet = new Animal("Cujo", true, 7);
        // Print out the animal info
        System.out.println(myPet);

        // This will print: "This animal's name is: Cujo they are currently a pet: true.
        // The animal is 5 years old."
    }
}

// Vehicle Class
public class StateBehavior_Vehicle
{
    // Here we will create a new vehicle, and then calculate how many miles/year it has.
    public void run()
    {
        // Create the vehicle and assign its attributes
        Vehicle myTruck = new Vehicle("Truck", 15, 173600.4);

        // Now we will use the `estimateMilesPerYear()` method to determine how many miles
        // per year, it has been driven.
        System.out.println("Your yearly mileage is: " + myTruck.estimateMilesPerYear());
    }
}
```