Welcome! Today's subject is Encapsulation, a cornerstone of Object-Oriented Programming (OOP). Encapsulation bundles data and the operations that we perform on them into one unit, namely, an object. It guards data against unwanted alterations, ensuring the creation of robust and maintainable software.

Prepare yourself for an exciting journey as we delve into how encapsulation works in Kotlin and explore the vital role it plays in data privacy.

Starting with the basics, encapsulation is similar to packing data and the methods that modify this data into a single compartment known as a class. It safeguards the data in an object from external interference.

To illustrate, consider a Kotlin class representing a bank account. Without encapsulation, the account balance could be directly altered. With encapsulation, however, the balance can only change through specified methods, like depositing or withdrawing.

Kotlin
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1class BankAccount {
2    var balance: Double = 0.0 // no encapsulation
3
4    // Method to withdraw
5    fun withdraw(amount: Double) {
6        balance -= amount
7    }
8
9    // Method to deposit
10    fun deposit(amount: Double) {
11        balance += amount
12    }
13}
14
15fun main() {
16    val account = BankAccount()
17    account.balance += 1000 // directly accessing the balance
18}

Let's understand the problem with the code above. The balance property is public, allowing direct access and modification. This unrestricted access can lead to errors or security vulnerabilities. For instance, a negative balance could be set, which is undesirable. Encapsulation resolves this issue by restricting direct access to the balance property. Let's see how.

Encapsulation restricts direct access to an object's data and prevents unwanted data alteration. This principle is comparable to window blinds, allowing you to look out while preventing others from peeping in.

In Kotlin, encapsulation pertains to private and public properties, which are integral to data privacy. Private properties, marked by the private keyword, require caution while being manipulated.

To illustrate, let's consider a Kotlin class named Person, which includes a private property name.

Kotlin
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1class Person(private val name: String) {
2
3    // Accessor method
4    fun getName(): String {
5        return name
6    }
7}
8
9fun main() {
10    val person = Person("Alice")
11    println(person.getName())  // Accessing private property via accessor method. Output: Alice
12    // The following line would cause an error due to private access:
13    // println(person.name)
14}

In this example, name is private, and getName() enables us to access name. However, we don't provide a method to change the name, preventing alterations.

Within encapsulation, Kotlin uses property syntax for getter and setter methods to access or modify private properties. In a class, the property itself can be declared with getters and setters.

Kotlin
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1class Dog(private var name: String) {
2
3    // Getter method
4    fun getName(): String = name
5
6    // Setter method
7    fun setName(name: String) {
8        this.name = name
9    }
10}
11
12fun main() {
13    val myDog = Dog("Max")
14    myDog.setName("Buddy")
15    println(myDog.getName()) // Output: Buddy
16}

Here, setName() and getName() serve as the setter and getter methods, respectively, for the private property name.

Let's apply the principle of encapsulation to our BankAccount class, which includes private properties like account number and balance, along with public functions for withdrawals, deposits, and balance checks.

Kotlin
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1class BankAccount(private val accountNo: Int, private var balance: Double) {
2
3    // Method to withdraw money
4    fun withdraw(amount: Double) {
5        if (amount > 0 && amount <= balance) {
6            balance -= amount
7        } else {
8            println("Invalid amount or insufficient balance.")
9        }
10    }
11
12    // Method to deposit money
13    fun deposit(amount: Double) {
14        if (amount > 0) {
15            balance += amount
16        } else {
17            println("Invalid deposit amount.")
18        }
19    }
20
21    // Method to check balance
22    fun checkBalance(): Double {
23        return balance
24    }
25}
26
27fun main() {
28    val account = BankAccount(1, 500.0)
29    account.withdraw(100.0)
30    account.deposit(50.0)
31    println(account.checkBalance()) // Prints: 450.0
32}

In the above code, the BankAccount class encapsulates account details that we would like to be hidden from the outer scope, and the public methods manipulate the balance in a controlled way. This way, we limit the potential interaction with the account's balance and other private information, improving security.

Admirable! Now it's your turn to apply what you've learned by practicing encapsulation in Kotlin. Remember, practice enhances your comprehension. Enjoy coding! Craft a User class that encapsulates a username and password with private properties. Use appropriate getter and setter methods to secure the data, ensuring that the password can only be set if it meets certain conditions.