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Dig deeper into object-oriented Java with inheritance and polymorphism.

Inheritance and Polymorphism

In Java, if `Orange` is a `Fruit` through inheritance, you can then use `Orange` in the same contexts as `Fruit` like this:
```java
String makeJuice(Fruit fruit) {

  return "Apple juice and " + fruit.squeeze();

}

// inside main()
Orange orange = new Orange();
System.out.println(juicer.makeJuice(orange));
```
Wait, how does that work? 

This is because Java incorporates the object-oriented programming principle of _polymorphism_. Polymorphism, which derives from Greek meaning "many forms", allows a child class to share the information and behavior of its parent class while also incorporating its own functionality.

The main advantages of polymorphic programming:
- Simplifying syntax
- Reducing cognitive overload for developers

These benefits are particularly helpful when we want to develop our own Java packages for other developers to import and use.

For example, the built-in operator `+` can be used for both `double`s and `int`s. To the computer, the `+` means something like `addDouble()` for one and `addInt()` for the other, but the creators of Java (and of other languages) didn't want to burden us as developers with recalling each individual method.

Note that the reverse situation is not true; you cannot use a generic parent class instance where a child class instance is required. So an `Orange` can be used as a `Fruit`, but a `Fruit` cannot be used as an `Orange`.

The Orange class extends the Fruit class, so you can use Orange objects where Fruit objects are required (for example, as an argument in the method makeJuice(Fruit fruit).

Introducing Polymorphism

Excellent work! You've learned quite a bundle about inheritance and polymorphism in Java:
- A Java class can inherit fields and methods from another class.
- Each Java class requires its own file, but only one class in a Java package needs a `main()` method.
- Child classes inherit the parent constructor by default, but it's possible to modify the constructor using `super()` or override it completely.
- You can use `protected` and `final` to control child class access to parent class members.
- Java's OOP principle of polymorphism means you can use a child class object like a member of its parent class, but also give it its own traits.
- You can override parent class methods in the child class, ideally using the `@Override` keyword.
- It's possible to use objects of different classes that share a parent class together in an array or `ArrayList`.


Review of Inheritance and Polymorphism

Usually, when we create an array or an `ArrayList`, the list items all need to be the same type. But polymorphism puts a new spin on what is considered the same type...

In fact, we can put instances of different classes that share a parent class together in an array or `ArrayList`! For example, let's say we have a `Monster` parent class with a few child classes: `Vampire`, `Werewolf`, and `Zombie`. We can set up an array with instances of each:
```java
Monster dracula, wolfman, zombie1;

dracula = new Vampire();
wolfman = new Werewolf();
zombie1 = new Zombie();

Monster[] monsters = {dracula, wolfman, zombie1};
```

We can even iterate through the list of items — regardless of subclass — and perform the same action with each item:
```java
for (Monster monster : monsters) {

  monster.attack();

}
```
In the code above, we were able to call `attack()` on each monster in `monsters` despite the fact that, in the for-each loop, `monster` is declared as the parent class type `Monster`.

Child Classes in Arrays and ArrayLists

When we call a method that contains parameters, the arguments we place in our method call must match the parameter type. Similar to the previous exercise, polymorphism gives us a little more flexibility with the arguments we can use.

If we use a superclass reference as a method parameter, we can call the method using subclass reference arguments!

For example, imagine the class `ScaryStory`, whose constructor takes in a reference to the `Monster` class:
```java
class ScaryStory {
  Monster monster;
  String setting;

  public ScaryStory(Monster antagonist, String place) {
    monster = antagonist;
    setting = place;
  }

  public void tellStory(){
    System.out.println("Once upon a time, " + monster.name + " was at " + setting + " looking to scare some mortals.");
  }

  public static void main(String[] args) {
    Monster dracula;
    dracula = new Vampire("Dracula");
    ScaryStory countDracula = new ScaryStory(dracula, "Dracula Castle");
    countDracula.tellStory();
  }
}
```
In the `main()` method, we used a reference of the class `Vampire` as our argument even though the constructor requested an object of class `Monster`. This is allowed because `Vampire` is a subclass of the `Monster` class.


Child Classes in Method Parameters

One common use of polymorphism with Java classes is something we mentioned earlier &mdash; _overriding_ parent class methods in a child class. Like the `+` operator, we can give a single method slightly different meanings for different classes. This is useful when we want our child class method to have the same name as a parent class method but behave a bit differently in some way.

Let's say we have a `BankAccount` class that allows us to print the current balance. We want to build a `CheckingAccount` class that inherits the functionality of a `BankAccount` but with a modified `printBalance()` method. We can do the following:

```java
class BankAccount {
  protected double balance;

  public BankAccount(double balanceIn){
    balance = balanceIn;
  }

  public void printBalance() {
    System.out.println("Your account balance is $" + balance);
  }
}

class CheckingAccount extends BankAccount {
  
  public CheckingAccount(double balance) {
    super(balance);
  }

  @Override
  public void printBalance() {
    System.out.println("Your checking account balance is $" + balance);
  }
}
```
Notice that in order to properly override `printBalance()`, in `CheckingAccount` the method has the following in common with the corresponding method in `BankAccount`:
- Method name
- Return type
- Number and type of parameters

You may have also noticed the `@Override` keyword above `printBalance()` in `CheckingAccount`. This annotation informs the compiler that we want to override a method in the parent class. If the method doesn't exist in the parent class, we'll get a helpful error when we compile the program.

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

In a previous exercise, we learned that the `super` keyword can be used to call the constructor of a superclass. That's not the only use of `super`; we can also use this keyword to call the methods of a parent class. While we now have the ability to override methods from a superclass, we may find ourselves in a unique situation where we want to use the superclass method instead of the subclass' overridden method. 

If that's the case, we can call the parent class method by prepending `super` followed by the dot operator (`.`) to the method call. Note that this only works if we pass in the proper method parameters. Let's see this in action by adding a `checkBalances()` method to `CheckingAccount` that calls both versions of `printBalance()`:

```java
class CheckingAccount extends BankAccount {
  public CheckingAccount(double balance) {
    super(balance);
  }

  @Override
  public void printBalance() {
    System.out.println("Your checking account balance is $" + balance);
  }

  public void checkBalances() {
    // calls method from CheckingAccount
    printBalance();
    // calls method from BankAccount
    super.printBalance();
  }

  public static void main(String[] args) {
    CheckingAccount myCheckings = new CheckingAccount(5000);
    myCheckings.checkBalances();
  }
}
```
This program will output:
```
Your checking account balance is $5000
Your account balance is $5000
```



Method Overriding

Hang on, you might be thinking, if the child class inherits its parent's fields and methods, does it also inherit the constructor? Let's take a look at how the `super()` constructor works!

Let's say `Shape` has a `numSides` field that is set by passing an integer into the constructor. If we're instantiating a `Triangle`, we would want that number to always be `3`, so we'd want to modify the constructor to automatically assign `numSides` with a value of `3`.

Can we do that?

As it happens, Java has a trick up its sleeve for just this occasion: using the `super()` method which acts like the parent constructor inside the child class constructor:
```java
class Triangle extends Shape {

  Triangle() {
    super(3);
  }

  // additional Triangle class members

}
```
By passing `3` to `super()`, we are making it possible to instantiate a `Triangle` without passing in a value for `numSides`. 

Meanwhile, `super(3)` (behaving as `Shape(3)`) will shoulder the responsibility of setting `numSides` to `3` for our `Triangle` object. It's like we called `Shape(3)`.

It is also possible to write a constructor without making a call to any `super()` constructor:
```java
class Triangle extends Shape {

  Triangle() {
    this.numSides = 3;
  }

  // additional Triangle class methods

}
```
In this situation, Java secretly calls the parent class' no-argument constructor (`super()`). So in this specific example, the `Triangle()` constructor first calls the `Shape()` constructor. That `Shape()` takes care of whatever business it needs to take care of. And then after that is complete, we go in and set `this.numSides` to `3`.

If you're writing a constructor of a child class, and don't explicitly make a call to a constructor from a parent class using `super`, it's important to remember that Java will automatically (and secretly) call `super()` as the first line of your child class constructor.

Inheriting the Constructor

You may recall that Java class members use `private` and `public` access modifiers to determine whether they can be accessed from outside the class. So does a child class inherit its parent's `private` members? 

Well, no. But there is another access modifier we can use to keep a parent class member accessible to its child classes and to files in the package it's contained in &mdash; and otherwise private: `protected`.
<img src="https://content.codecademy.com/courses/learn-java/revised-2019/access-modifiers-chart.png" style="max-width: 600px; width: 100%;" alt="public is visible everywhere; protected is visible in the class, the package, and child classes; a member with no modifier is visible in the class and package; private is visible only in the class itself" />
Here's what `protected` looks like in use:

```java
class Shape {

  protected double perimeter;

}

// any child class of Shape can access perimeter
```

In addition to access modifiers, there's another way to establish how child classes can interact with inherited parent class members: using the `final` keyword. If we add `final` after a parent class method's access modifier, we disallow any child classes from changing that method. This is helpful in limiting bugs that might occur from modifying a particular method.

Though it is not required, there is an established order when two or more field modifiers are used (eg. `public final`). To learn more about this read the [documentation](https://docs.oracle.com/javase/specs/jls/se11/html/jls-8.html#jls-8.3.1).


Parent Class Aspect Modifiers

When you hear the word "inheritance," code may not be the first thing that springs to mind; you're probably more likely to think of inheriting genetic traits, like eye color from your mother or a smile from your grandfather. But [inheritance](https://www.codecademy.com/resources/docs/java/inheritance?page_ref=catalog) is also an important feature of object-oriented programming in Java.

Suppose we are building a `Shape` class in Java. We might give it some points in 2D, a method for calculating the area, and another method for displaying the shape. But what happens if we want a class for a triangle that has some triangle-specific methods? Do we need to redefine all of the same methods that we created for `Shape`?

No! (Phew.) Lucky for us, a Java class can also _inherit_ traits from another class. Because a `Triangle` is a `Shape`, we can define `Triangle` so that it inherits fields and methods directly from `Shape`. A reference of type `Shape` can refer to an object of `Shape` or an object of `Triangle`. The object-oriented principle of inheritance saves us the headache of redefining the same class members all over again.

Our `Triangle` class will inherit all the traits of `Shape`, but `Triangle` can also contain its own unique methods and variables. For example, we could have an instance variable called `hypotenuse` and a method called `findHypotenuse()` that can only be accessed by `Triangle` class references. Objects of `Triangle` can call any method contained in `Triangle` or `Shape`. This gives us a bunch of possibilities!

There are several terms you'll encounter frequently: 
- _Parent class_, _superclass_, and _base class_ refer to the class that another class inherits from (like `Shape`).
- _Child class_, _subclass_, and _derived class_ refer to a class that inherits from another class (like `Triangle`).

A DeadlineTask class can inherit fields and methods from a Task class. In this case, Task is a parent class and DeadlineTask is a child class. The DeadlineTask class may also have its own functionality like a deadline field or an isOverdue() method.

Introducing Inheritance

An important facet of polymorphism is the ability to use a child class object where an object of its parent class is expected. 

One way to do this explicitly is to instantiate a child class object as a member of the parent class. We can instantiate a `CheckingAccount` object as a `BankAccount` like this:
```java
BankAccount kaylasAccount = new CheckingAccount(600.00);
```
We can use `kaylasAccount` as if it were an instance of `BankAccount`, in any situation where a `BankAccount` object would be expected. (This would be true even if `kaylasAccount` were instantiated as a `CheckingAccount`, but using the explicit child as parent syntax is most helpful when we want to declare objects in bulk.)

It is important to note here that the compiler just considers `kaylasAccount` to be any old `BankAccount`. But because method overriding is handled at runtime, if we call `printBalance()`, we'll see something `CheckingAccount` specific:
```
Your checking account balance is $600.00
```
This is because at runtime, `kaylasAccount` is recognized as the `CheckingAccount` it is. So, what if `CheckingAccount` has a method `transferToSavings()` that `BankAccount` does not have? Can `kaylasAccount` still use that method? 

Well, no. The compiler believes that `kaylasAccount` is just a `BankAccount` that doesn't have some fancy child class `transferToSavings()` method, so it would throw an error.


Using a Child Class as its Parent Class

So how do we define a child class so that it inherits from a parent class? We use the keyword `extends` like this:
```java
class Shape {

  // Shape class members

}

class Triangle extends Shape {

  // additional Triangle class members

}
```
Now `Triangle` has inherited traits from `Shape`, meaning it copied over class members from `Shape`. When we use inheritance to extend a subclass from a superclass, we create an "is-a" relationship from the subclass to the superclass. For example, an object of `Triangle` _is a_ member of the `Shape` class; however, an object of `Shape` is not necessarily an object of `Triangle`.

Until now, we've only been working with one class and one file. However, most Java programs utilize multiple classes, each of which requires its own file. Only one file needs a `main()` method — this is the file we will run.

Note: the various classes in our Java package — even though they are in different files — will have access to each other, so we can instantiate one class inside of another.
