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Learn the basics of object-oriented programming in Java by exploring classes and objects.

Java: Introduction to Classes

Java is an object-oriented programming language where every program has at least one class. Programs are often built from many classes and objects, which are the instances of a class. 

Classes define the state and behavior of their instances. Behavior comes from methods defined in the class. State comes from instance fields declared inside the class.

Classes are modeled on the real-world things we want to represent in our program. Later we will explore how a program can be made from multiple classes. For now, our programs are a single class.

```java
public class Dog {
  // instance field
  String breed;
  // constructor method
  public Dog(String dogBreed) {
    /* 
    value of parameter dogBreed 
    assigned to instance field breed
    */
    breed = dogBreed;
  }
  public static void main(String[] args) {
    /* 
    create instance: 
    use 'new' operator and invoke constructor
    */
    Dog fido = new Dog("poodle");
    /* 
    fields are accessed using: 
    the instance name, `.` operator, and the field name.
    */
    System.out.println(fido.breed);
    // Prints "poodle"
  }
}
```


Classes: Review

Objects are not limited to a single instance field. We can declare as many fields as are necessary for the requirements of our program.

Let's change `Car` instances so they have multiple fields.

We'll add a `boolean isRunning`, that indicates the car engine is on and an `int velocity`, that indicates the speed at which the car is traveling.

```java
public class Car {
  String color;
  // new fields!
  boolean isRunning;
  int velocity;
  
  // new parameters that correspond to the new fields
  public Car(String carColor, boolean carRunning, int milesPerHour) {
    color = carColor;
    // assign new parameters to the new fields
    isRunning = carRunning;
    velocity = milesPerHour;
  }

  public static void main(String[] args) {
    // new values passed into the method call
    Car ferrari = new Car("red", true, 27);
    Car renault = new Car("blue", false, 70);
    
    System.out.println(renault.isRunning);
    // false
    System.out.println(ferrari.velocity);
    // 27
  }
}
```

The constructor now has multiple parameters to receive values for the new fields. We still specify the type as well as the name for each parameter. 

Ordering matters! We must pass values into the constructor invocation in the same order that they're listed in the parameters.

```java
// values match types, no error
Car honda = new Car("green", false, 0);

// values do not match types, error!
Car junker = new Car(true, 42, "brown");

```


Classes: Multiple Fields

The fundamental concept of object-oriented programming is the class.

A _class_ is the set of instructions that describe how an instance can behave and what information it contains.

Java has pre-defined classes such as `System`, which we've used in logging text to our screen, but we also need to write our own classes for the custom needs of a program.
 
Here's a definition of a Java class:

```java
public class Car {
// scope of Car class starts after curly brace

  public static void main(String[] args) {
    // scope of main() starts after curly brace

    // program tasks

  }
  // scope of main() ends after curly brace

}
// scope of Car class ends after curly brace
```
 
This example defines a `class` named  `Car`. `public` is an _access level modifier_ that allows **other** classes to interact with this class. For now, all classes will be `public`.
 
This class has a `main()` method, which lists the tasks performed by the program. `main()` runs when we execute the compiled **Car.class** file.


Classes: Syntax

Our last exercise ended with printing an instance of `Store`, which looked something like `Store@6bc7c054`. The first part, `Store`, refers to the class, and the second part `@6bc7c054` refers to the instance's location in the computer's memory.
 
We don't care about memory location, but our instances have no other characteristics! When an object is created, the constructor sets the initial state of the object. The _state_ is made up of associated data that represents the characteristics of an object.
 
We'll add data to an object by introducing _instance variables_, or _instance fields_. 

We want `Car` instances of different colors, so we declare a `String color` instance field. Often times, instance variables are described as a "has-a" relationship with the object. For example, a `Car` "has-a" `color`. Another way to think of this is that instance variables are the nouns and adjectives associated with an object. What qualities other than `color` might a car _have_?
 
```java
public class Car {
  /*
  declare fields inside the class
  by specifying the type and name
  */
  String color;

  public Car() {
    /* 
    instance fields available in
    scope of constructor method
    */
  }

  public static void main(String[] args) {
    // body of main method
  }
}
```
The declaration is **within** the class and the instance variable will be available for assignment inside the constructor.
 
Fields are a type of state each instance will possess. One instance may have `"red"` as its `color`, another `"blue"`, etc. It's the job of the constructor to give these instance fields initial value. We'll learn how to do this in the next exercise.

Classes: Instance Fields

In order to create an object (an instance of a class), we need a [constructor](https://www.codecademy.com/resources/docs/java/constructors?page_ref=catalog) method. The constructor is defined within the class.
 
Let's take a look at the `Car` class with a constructor. The constructor, `Car()`, shares the same name as the class:
```java
public class Car {
  // Constructor method
  public Car() {
    // instructions for creating a Car instance
  }  

  public static void main(String[] args) {
    // body of main method
  }
}
```
 
To create an instance, we need to _call_ or _invoke_ the constructor within `main()`. The following example assigns a `Car` instance to the variable `ferrari`:

```java
public class Car {

  public Car() {
    // instructions for creating a Car instance
  }
 
  public static void main(String[] args) {
    // Invoke the constructor
    Car ferrari = new Car(); 
  }
}
```
In this example, instead of being declared with a primitive data type like `int` or `boolean`, our variable `ferrari` is declared as a _reference data type_. This means that the value of our variable is a reference to an instance's memory address. During its declaration, the class name is used as the variable's type. In this case, the type is `Car`.

After the assignment operator, (`=`), we invoke the constructor method: `Car()`, and use the keyword `new` to indicate that we're creating an instance. **Omitting `new` causes an error.**

If we were to output the value of `ferrari` we would see its memory address:
```
Car@76ed5528
```
**Keep Reading: AP Computer Science A Students**

We can initialize a reference-type variable without assigning it a reference if we utilize the special value `null`. Something that is `null` has no value; if we were to assign `null` to an object, it would have a void reference. 

For example, in the following code snippet, we'll create an instance of `Car`, assign it a reference, and then change its value to `null`:
```java
Car thunderBird = new Car();
System.out.println(thunderBird); // Prints: Car@76ed5528

thunderBird = null; // change value to null
System.out.println(thunderBird); // Prints: null
```
It's important to note that if we use a `null` reference to call a method or access an instance variable, we will receive a `NullPointerException` error.


Classes: Constructors

All programs require one or more [classes](https://www.codecademy.com/resources/docs/java/classes?page_ref=catalog) that act as a model for the world.

For example, a program to track student test scores might have `Student`, `Course`, and `Grade` classes. Our real-world concerns, students and their grades, are inside the program as classes. 

We represent each student as an _instance_, or _object_, of the `Student` class.
 
This is _object-oriented programming_ because programs are built around objects and their interactions. An object contains state and behavior.
 
![Diagram of a generic software object](https://content.codecademy.com/courses/learn-java/revised-2019/diagram%20of%20an%20object-01.png)
 
Classes are a blueprint for objects. Blueprints detail the general structure. For example, all students have an ID, all courses can enroll a student, etc.

An instance is the thing itself. *This* student has an ID of `42`, *this* course enrolled *that* student, etc.
 
Let's review with another example, a savings account at a bank.
 
What should a savings account know?
* The balance of money available.
 
What should a savings account do?
* Deposit money.
* Withdraw money.
 
![Diagram of a Savings Account as software object](https://content.codecademy.com/courses/learn-java/revised-2019/diagram%20of%20an%20object-02.png)
 
Imagine two people have accounts that are instances of the `SavingsAccount` class. They share behavior (how they deposit and withdraw) but have individual state (their balances), and even with the same balance amount these accounts are separate entities. 


Introduction to Classes

To create objects with dynamic, individual states, we'll use a combination of the constructor method and instance fields.
 
In order to assign a value to an instance variable, we need to alter our constructor method to include parameters so that it can access the data we want to assign to an instance. 

We've already seen a parameter in the `main()` method: `String[] args`, but this is the first time we're using the parameter value within a method body.

The `Car` constructor now has a parameter: `String carColor`:
```java
public class Car {
  String color;
  // constructor method with a parameter
  public Car(String carColor) {
    // parameter value assigned to the field
    color = carColor;
  }
  public static void main(String[] args) {
    // program tasks
  }
}
```
When a String value gets passed into `Car()`, it is assigned to the parameter `carColor`. Then, inside the constructor, `carColor` will be assigned as the value to the instance variable `color`.

Our method also has a _signature_ which defines the name and parameters of the method. In the above example, the signature is `Car(String carColor)`.

In the next exercise, we'll learn how to pass values into a method!

**Keep Reading: AP Computer Science A Students** 

There are two types of parameters: formal and actual. A _formal parameter_ specifies the type and name of data that can be passed into a method. In our example above, `String carColor` is a formal parameter; `carColor` will represent whatever `String` value is passed into the constructor. We'll learn about actual parameters in the next exercise.

In Java, because of _constructor overloading_, a class can have multiple constructors as long as they have different parameter values. The signature is useful in that it helps the compiler differentiate between the different methods. For example:

```java
public class Car {
  String color;
  int mpg;
  boolean isElectric;

  // constructor 1
  public Car(String carColor, int milesPerGallon) {
    color = carColor;
    mpg = milesPerGallon;
  }
  // constructor 2
  public Car(boolean electricCar, int milesPerGallon) {
    isElectric = electricCar;
    mpg = milesPerGallon;
  }
}
```
In the example above, there are two constructors. When we initialize an object, the compiler will know which constructor to use because of the values we pass into it. For example, `Car myCar = new Car(true, 40)` will be created by the second constructor because the arguments match the type and order of the second constructor's signature.

If we do not define a constructor, the Java compiler will generate a default constructor that contains no arguments and assigns the object default values. Default values can be created by assigning values to the instance fields during their declaration:

```java
public class Car {
  String color = "red";
  boolean isElectric = false;
  int cupHolders = 4;

  public static void main(String[] args) {
    Car myCar = new Car();
    System.out.println(myCar.color); // Prints: red
  }
}
```



Classes: Constructor Parameters

Now that our constructor has a parameter, we must pass values into the method call. These values are referred to as _arguments_; once they are passed in, they will be used to give the instance fields initial value.

Here we create an instance, `ferrari`, in the `main()` method with `"red"` as its `color` field:
 
```java
public class Car {
  String color;

  public Car(String carColor) {
    // assign parameter value to instance field
    color = carColor;
  }

  public static void main(String[] args) {
    // parameter value supplied when calling constructor
    Car ferrari = new Car("red");
  }
}
```

We pass the String value `"red"` to our constructor method call: `new Car("red");`. 

The type of the value given to the invocation **must match** the type declared by the parameter.
 
Inside the constructor, the parameter `carColor` refers to the value passed in during the invocation: `"red"`. This value is assigned to the instance field `color`.

`color` has already been declared, so we don't specify the type during assignment.
 
The object, `ferrari`, holds the state of `color` as an instance field referencing the value `"red"`. 

We access the value of this field with the _dot operator_ (`.`):
 
```java
/*
accessing a field:
objectName.fieldName
*/

ferrari.color;
// "red"
```
**Keep Reading: AP Computer Science A Students**

An _actual parameter_, or argument, refers to the value being passed during a method call. 

_Call by value_ is the process of calling a method with an argument value. When an argument is passed, the formal parameter is initialized with a copy of the argument value. For example, when we declared the `ferrari` object, the `String` value `"red"` is passed as an argument; then, the formal parameter `carColor` is assigned a copy of that value.
