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Write elegantly designed code by learning about access, encapsulation, and scope. 

Access, Encapsulation, and Scope

Now that we've seen how methods can call other methods using `this.`, let's look at a situation where you might want to use `private` methods. Oftentimes, `private` methods are helper methods &mdash; that is to say that they're methods that other, bigger methods use. 

For example, for our `CheckingAccount` example, we might want a public method like `getAccountInformation()` that prints information like the name of the account owner, the amount of money in the account, and the amount of interest the account will make in a month. That way, another class, like a `Bank`, could call that public method to get all of that information quickly.

Well, in order to get that information, we might want to break that larger method into several helper methods. For example, inside `getAccountInformation()`, we might want to call a function called `calculateNextMonthInterest()`. That helper method should probably be `private`. There's no need for a `Bank` to call these smaller helper methods &mdash; instead, a `Bank` can call the one `public` method, and rely on that method to do all of the complicated work by calling smaller `private` methods.


Other Private Methods

In addition to access modifiers like `public` and `private`, the _scope_ of the variable also determines what parts of your code can access that variable.

The _scope_ of a variable is determined by where the variable is declared. For example, because instance variables are declared inside a class but outside any methods or constructors, all methods and constructors are within the scope of that variable. For example, in the code block below, constructors and methods of the `Dog` class are using the `Dog` instance variables like `name` and `age`:

```java
class Dog{
  public String name;
  public int age;
  public int weight;

  public Dog(){
    name = "Winston";
    age = 8;
    weight = 30;
  }

  public void speak(){
    System.out.println("My name is " + name);
  }
}
```

However, if we have a variable declared inside a method, that variable can only be used inside that method. The same is true for parameters. The scope of those parameters is only the method they're associated with. If you try to use a parameter outside the function it's defined in, you'll get an error. These variables are often called local variables. Note that we don't use `public` or `private` when declaring local variables.

This idea of scope extends to conditionals and loops as well. If you declare a variable inside the body of a conditional or in a loop, that variable can only be used inside that structure. This also includes the variable you're using as your looping variable. For example, consider the following block of code:

```java
for(int i = 0; i < 10; i++){
  // You can use i here
}
// i is out of scope here
```
You can only use `i` between the curly braces of the for loop. In general, whenever you see curly braces, be aware of scope. If a variable is defined inside curly braces, and you try to use that variable outside of those curly braces, you will likely see an error!

Scope: Local Variables

Nice work! In this lesson, we dove into some of the more subtle features of classes with a focus on access, encapsulation, and scope. Here are some of the main takeaways from this lesson:

- The `public` and `private` keywords are used to define what parts of code have access to other classes, methods, constructors, and instance variables.
- Encapsulation is a technique used to keep implementation details hidden from other classes. Its aim is to create small bundles of logic.
- The `this` keyword can be used to designate the difference between instance variables and local variables.
- Local variables can only be used within the scope that they were defined in.
- The `this` keyword can be used to call methods when writing classes.



Review

After running the code in the last exercise, you should be developing an intuition on what the `public` and `private` keywords are doing. These keywords are defining what parts of your code have _access_ to other parts of your code. 

We can define the access of many different parts of our code including instance variables, methods, constructors, and even a class itself. If we choose to declare these as `public` this means that any part of our code can interact with them - even if that code is in a different class!

The way we declare something to be `public` is to use the `public` keyword in the declaration statement. In the code block below, we have a public class, constructor, instance variables, and method. Notice the five different uses of `public`.

```java
public class Dog{
  public String name;
  public int age;

  public Dog(String input_name, int input_age){
    name = input_name;
    age = input_age;
  }
	
  public void speak() {
    System.out.println("Arf Arf! My name is " + name + " and I am a good dog!");
  }
}
```

Since everything about a `Dog` is public, any other class can access anything about a `Dog`. For example, let’s say there was a `DogSchool` class. Any method of the `DogSchool` class could make a new `Dog` using the `public` `Dog` constructor, directly access that `Dog`’s instance variables, and directly use that `Dog`’s methods:

```java
public class DogSchool{
  public void makeADog(){
    Dog cujo = new Dog("Cujo", 7);
    System.out.println(cujo.age);
    cujo.speak();
  }
}
```
Notice that the `DogSchool` class and the `makeADog()` method are also public. This means that if some other class created a `DogSchool`, they would have access to these methods as well! We have public methods calling public methods!

One final thing to note is that for the purposes of this lesson, we'll almost always make our classes and constructors `public`. While you can set them to `private`, it's fairly uncommon to do so. Instead, we'll focus on why you might make your instance variables and methods `private`. We'll start looking into the `private` keyword in the next exercise.


The public Keyword

Often times when creating classes, programmers will create local variables with the same name as instance variables. For example, consider the code block below:

```java
public class Dog{
  public String name;

  public Dog(String inputName){
    name = inputName;
  }

  public void speakNewName(String name){
    System.out.println("Hello, my new name is" + name);
  }

  public static void main(String[] args){
    Dog myDog = new Dog("Winston");
    myDog.speakNewName("Darla"); // Prints "Darla" - "Winston" ignored

  }
}
```

We have an instance variable named `name`, but the method `speakNewName` has a parameter named `name`. So when the method tries to print `name`, which variable will be printed? By default, Java refers to the local variable name. So in this case, the value passed to the parameter will be printed and not the instance variable.

If we wanted to access the instance variable and not the local variable, we could use the `this` keyword.

```java
public class Dog{
  public String name;

  public Dog(String inputName){
    name = inputName;
  }

  public void speakNewName(String name){
    System.out.println("Hello, my new name is" + this.name);
  }
    
  public static void main(String[] args){
    Dog a = new Dog("Fido");
    Dog b = new Dog("Odie");

    a.speakNewName("Winston");
    // "Fido", the instance variable of Dog a is printed. "Winston" is ignored

    b.speakNewName("Darla");
    // "Odie", the instance variable of Dog b is printed. "Darla" is ignored.
  }
}
```
The `this` keyword is a reference to the current object. We used `this.name` in our `speakNewName()` method. This caused the method to print out the value stored in the instance variable `name` of whatever `Dog` Object called `speakNewName()`. (Note that in this somewhat contrived example, the local variable `name` used as a parameter gets completely ignored).

Oftentimes, you'll see constructors have parameters with the same name as the instance variable. For example, you might see something like:

```java
public Dog(String name){
  this.name = name;
}
```

You can read this as "set `this` `Dog`'s instance variable `name` equal to the variable passed into the constructor". While this naming is a common convention, it can also be confusing. There's nothing wrong with naming your parameters something else to be more clear. Sometimes you will see something like:

```java
public Dog(String inputName){
  this.name = inputName;
}
```
This is now a little clearer &mdash; we're setting the `Dog`'s instance variable `name` equal to the name we give the constructor.

Finally, mutator methods also usually follow this pattern:

```java
public void setName(String name){
  this.name = name;
}
```
We reset the instance variable to the value passed into the parameter.

Throughout the rest of this lesson, we'll use `this.` when referring to an instance variable. This isn't always explicitly necessary &mdash; if there's no local variable with the same name, Java will know to use the instance variable with that name. That being said, it is a good habit to use `this.` when working with your instance variables to avoid potential confusion.

Scope: The this Keyword

When writing classes, we often make all of our instance variables `private`. However, we still might want some other classes to have access to them, we just don't want those classes to know the _exact_ variable name. To give other classes access to a private instance variable, we would write an _accessor method_ (sometimes also known as a "getter" method).

```java
public class Dog{
  private String name;
	
  //Other methods and constructors

  public String getName() {
    return name;
  }
}
```

Even though the instance variable `name` is `private`, other classes could call the `public` method `getName()` which returns the value of that instance variable. Accessor methods will always be `public`, and will have a return type that matches the type of the instance variable they're accessing.

Similarly, `private` instance variables often have mutator methods (sometimes known as "setters"). These methods allow other classes to reset the value stored in `private` instance variables.

```java
public class Dog{
  private String name;
	
  //Other methods and constructors

  public void setName(String newName) {
    name = newName;
  }

  public static void main(String[] args){
    Dog myDog = new Dog("Cujo");
    myDog.setName("Lassie");
  }
}
```
Mutator methods, or "setters", often are `void` methods &mdash; they don't return anything, they just reset the value of an existing variable. Similarly, they often have one parameter that is the same type as the variable they're trying to change.


Accessor and Mutator Methods

We've seen how the `this` works with variables, but we can also use the `this` with methods.

Recall how we've been calling methods up to this point:

```java
public static void main(String[] args){
  Dog myDog = new Dog("Odie");
  myDog.speak();
}
```
Here we're creating an instance of a `Dog` and using that `Dog` to call the `speak()` method. However, when defining methods, we can also use the `this` keyword to call other methods. Consider the code block below:

```java
public class Computer{
  public int brightness;
  public int volume;
  
  public void setBrightness(int inputBrightness){
    this.brightness = inputBrightness;
  }

  public void setVolume(int inputVolume){
    this.volume = inputVolume;
  }

  public void resetSettings(){
    this.setBrightness(0);
    this.setVolume(0);
  }
}
```

Take a look at the `resetSettings()` method in particular. This method calls other methods from the class. But it needs an object to call those methods! Rather than create a new object (like we did with the `Dog` named `myDog` earlier), we use `this` as the object. What this means is that the object that calls `resetSettings()` will be used to call `setBrightness(0)` and `setVolume(0)`.

```java
public static void main(String[] args){
  Computer myComputer = new Computer();
  myComputer.resetSettings();
}
```
In this example, calling `myComputer.resetSettings()` is as if we called `myComputer.setBrightness(0)` and `myComputer.setVolume(0)`. `this` serves as a placeholder for whatever object was used to call the original method.

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

Finally, `this` can be used as a value for a parameter. Let's say a method exists that takes a `Computer` as a parameter (that method's signature might be something like `public void pairWithOtherComputer(Computer other)`). If you're writing another method in `Computer`, and want to call the `pairWithOtherComputer()` method, you could use `this` as the parameter. That call might look something like `this.pairWithOtherComputer(this)`. You're using the current object to call the method _and_ are passing that object as that method's parameter.

```java
public void pairWithOtherComputer(Computer other){
  // Code for method that uses the parameter other
}

public void setUpConnection(){
  // We use "this" to call the method and also pass "this" to the method so it can be used in that method
  this.pairWithOtherComputer(this);
}
```

Using this With Methods

As our Java programs begin to get bigger and we begin to have multiple Objects and Classes that interact with each other, the concepts of _access_ and _scope_ come into play. To oversimplify things, the concepts of access and scope both center around what parts of your programs can interact with specific variables or methods from other parts of your program. Let’s take a brief look at some of the concepts we’ll cover:

#### Access

- The `public` and `private` keywords and how they relate to Classes, variables, constructors, and methods
- The concept of encapsulation
- Accessor methods, sometimes known as “getters”
- Mutator methods, sometimes known as “setters”

#### Scope 

- Local variables vs. instance variables
- The `this` keyword

Let’s get started!


What are Access and Scope?

By now you're probably catching onto what the `private` keyword does. When a Class' instance variable or method is marked as `private`, that means that you can only access those structures from elsewhere inside that same class. Let's look back at our `DogSchool` example:

```java
public class DogSchool{

  public void makeADog(){
    Dog cujo = new Dog("Cujo", 7);
    System.out.println(cujo.age);
    cujo.speak();
  }
}
```

`makeADog` is trying to directly access `Dog`'s `.age` variable. It's also trying to use the `.speak()` method. If those are marked as `private` in the `Dog` class, the `DogSchool` class won't be able to do that. Other methods within the `Dog` class would be able to use `.age` or `.speak()` (for example, we could use `cujo.age` within the `Dog` class), but other classes won't have access.

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

At this point, you might be thinking to yourself "Why even bother with any of this? In the last exercise, my code was broken until I flipped some variables and methods to `public`. Why don't I just make everything `public`?"

While those are valid points, sometimes restricting our code is actually useful from a design perspective. This is one of the core ideas behind _encapsulation_. By making our instance variables (and some methods) `private`, we encapsulate our code into nice little bundles of logic. 

For example, a `Bank` object doesn't necessarily need to know the inner workings of a `CheckingAccount` object. It doesn't need to know that the money is stored in a field named `money`, or that interest is added to an account by using a method named `.addInterest()`. In fact, if it had access to those fields or methods, it's possible that someone using a `Bank` object could change things in a `CheckingAccount` without realizing it. By limiting access by using the `private` keyword, we are able to segment, or encapsulate, our code into individual units.

Note that we don't necessarily want to completely block everything from other classes. In the next exercise, we'll get into when you might want to make methods public &mdash; we'll take a look at getter and setter methods.
