Object-Oriented Programming Finest Practices with Kotlin

Object-Oriented Programming (OOP) is the preferred laptop programming paradigm. Utilizing it correctly could make your life, and your coworkers’, lives simpler. On this tutorial, you’ll construct a terminal app to execute shell instructions on Android.

Within the course of, you’ll study the next:

• Key ideas of Object-Oriented Programming.
• SOLID ideas and the way they make your code higher.
• Some Kotlin particular good-to-knows.

Getting began

To start with, obtain the Kodeco Shell venture utilizing the Obtain Supplies button on the prime or backside of this tutorial.

Open the starter venture in Android Studio 2022.2.1 or later by choosing Open on the Android Studio welcome display screen:

The app consists of a single display screen just like Terminal on Home windows/Linux/MacOS. It helps you to enter instructions and present their output and errors. Moreover, there are two actions, one to cease a operating command and one to clear the output.

Construct and run the venture. You must see the principle, and solely, display screen of the app:

Whoa, what’s happening right here? As you possibly can see, the app at the moment refuses to run any instructions, it simply shows a non-cooperative message. Subsequently, your job might be to make use of OOP Finest Practices and repair that! You’ll add the power to enter instructions and show their output.

Understanding Object-Oriented Programming?

Earlier than including any code, you need to perceive what OOP is.

Object-Oriented Programming is a programming mannequin based mostly on information. Every part is modeled as objects that may carry out sure actions and talk with one another.

For instance, for those who have been to signify a automotive in object-oriented programming, one of many objects could be a Automotive. It might comprise actions equivalent to:

• Speed up
• Brake
• Steer left
• Steer proper

Lessons and Objects

Probably the most vital distinctions in object-oriented programming is between courses and objects.

Persevering with the automotive analogy, a category could be a concrete automotive mannequin and make you should buy, for instance — Fiat Panda.

A category describes how the automotive behaves, equivalent to its prime pace, how briskly it might probably speed up, and so on. It is sort of a blueprint for the automotive.

An object is an occasion of a automotive, for those who go to a dealership and get your self a Fiat Panda, the Panda you’re now driving in is an object.

Let’s check out courses in KodecoShell app:

• MainActivity class represents the display screen proven once you open the app.
• TerminalCommandProcessor class processes instructions that you just’ll enter on the display screen and takes care of capturing their output and errors.
• Shell class executes the instructions utilizing Android runtime.
• TerminalItem class represents a piece of textual content proven on the display screen, a command that was entered, its output or error.

MainActivity makes use of TerminalCommandProcessor to course of the instructions the consumer enters. To take action, it first must create an object from it, known as “creating an object” or “instantiating an object of a category”.

To realize this in Kotlin, you utilize:

non-public val commandProcessor: TerminalCommandProcessor = TerminalCommandProcessor()

Afterward, you would use it by calling its capabilities, for instance:

commandProcessor.init()

Key Ideas of OOP

Now that the fundamentals, it’s time to maneuver on to the important thing ideas of OOP:

• Encapsulation
• Abstraction
• Inheritance
• Polymorphism

These ideas make it potential to construct code that’s straightforward to know and preserve.

Understanding Encapsulation and Kotlin Lessons

Knowledge inside a category might be restricted. Be sure different courses can solely change the info in anticipated methods and forestall state inconsistencies.

Briefly, the skin world doesn’t must know how a category does one thing, however what it does.

In Kotlin, you utilize visibility modifiers to manage the visibility of properties and capabilities inside courses. Two of crucial ones are:

• non-public: property or operate is just seen inside the category the place it’s outlined.
• public: default visibility modifier if none is specified, property or operate is seen all over the place.

Marking the inner information of a category as non-public prevents different courses from modifying it unexpectedly and inflicting errors.

To see this in motion, open TerminalCommandProcessor class and add the next import:

import com.kodeco.android.kodecoshell.processor.shell.Shell

Then, add the next inside the category:

non-public val shell = Shell(
    outputCallback = { outputCallback(TerminalItem(it)) },
    errorCallback = { outputCallback(TerminalItem(it)) }
)

You instantiated a Shell to run shell instructions. You may’t entry it outdoors of TerminalCommandProcessor. You need different courses to make use of course of() to course of instructions by way of TerminalCommandProcessor.

Word you handed blocks of code for outputCallback and errorCallback parameters. Shell will execute certainly one of them when its course of operate is known as.

To check this, open MainActivity and add the next line on the finish of the onCreate operate:

commandProcessor.shell.course of("ps")

This code tries to make use of the shell property you’ve simply added to TerminalCommandProcessor to run the ps command.

Nonetheless, Android Studio will present the next error:
Can't entry 'shell': it's non-public in 'TerminalCommandProcessor'

Delete the road and return to TerminalCommandProcessor. Now change the init() operate to the next:

enjoyable init() {
  shell.course of("ps")
}

This code executes when the applying begins as a result of MainActivity calls TerminalViews‘s LaunchEffect.

Construct and run the app.

Consequently, now you need to see the output of the ps command, which is the listing of the at the moment operating processes.

Abstraction

That is just like encapsulation, it permits entry to courses via a selected contract. In Kotlin, you possibly can outline that contract utilizing interfaces.

Interfaces in Kotlin can comprise declarations of capabilities and properties. However, the principle distinction between interfaces and courses is that interfaces can’t retailer state.

In Kotlin, capabilities in interfaces can have implementations or be summary. Properties can solely be summary; in any other case, interfaces might retailer state.

Open TerminalCommandProcessor and change class key phrase with interface.

Word Android Studio’s error for the shell property: Property initializers aren't allowed in interfaces.

As talked about, interfaces can’t retailer state, and you can not initialize properties.

Delete the shell property to get rid of the error.

You’ll get the identical error for the outputCallback property. On this case, take away solely the initializer:

var outputCallback: (TerminalItem) -> Unit

Now you may have an interface with three capabilities with implementations.

Exchange init operate with the next:

enjoyable init()

That is now an summary operate with no implementation. All courses that implement TerminalCommandProcessor interface should present the implementation of this operate.

Exchange course of and stopCurrentCommand capabilities with the next:

enjoyable course of(command: String)

enjoyable stopCurrentCommand()

Lessons in Kotlin can implement a number of interfaces. Every interface a category implements should present implementations of all its summary capabilities and properties.

Create a brand new class ShellCommandProcessor implementing TerminalCommandProcessor in processor/shell bundle with the next content material:

bundle com.kodeco.android.kodecoshell.processor.shell

import com.kodeco.android.kodecoshell.processor.TerminalCommandProcessor
import com.kodeco.android.kodecoshell.processor.mannequin.TerminalItem

class ShellCommandProcessor: TerminalCommandProcessor { // 1
  // 2  
  override var outputCallback: (TerminalItem) -> Unit = {}

  // 3
  non-public val shell = Shell(
    outputCallback = { outputCallback(TerminalItem(it)) },
    errorCallback = { outputCallback(TerminalItem(it)) }
  )

  // 4
  override enjoyable init() {
    outputCallback(TerminalItem("Welcome to Kodeco shell - enter your command ..."))
  }

  override enjoyable course of(command: String) {
    shell.course of(command)
  }

  override enjoyable stopCurrentCommand() {   
    shell.stopCurrentCommand()
  }
}

Let’s go over this step-by-step.

1. You implement TerminalCommandProcessor interface.
2. You declare a property named outputCallback and use the override key phrase to declare that it’s an implementation of property with the identical title from TerminalCommandProcessor interface.
3. You create a non-public property holding a Shell object for executing instructions. You cross the code blocks that cross the command output and errors to outputCallback wrapped in TerminalItem objects.
4. Implementations of init, course of and stopCurrentCommand capabilities name applicable Shell object capabilities.

You want yet one more MainActivity change to check the brand new code. So, add the next import:

import com.kodeco.android.kodecoshell.processor.shell.ShellCommandProcessor

Then, change commandProcessor property with:

non-public val commandProcessor: TerminalCommandProcessor = ShellCommandProcessor()

Construct and run the app.

Inheritance and Polymorphism

It’s time so as to add the power to enter instructions. You’ll do that with the assistance of one other OOP precept — inheritance. MainActivity is ready as much as present a listing of TerminalItem objects. How will you present a unique merchandise if a listing is ready as much as present an object of a sure class? The reply lies in inheritance and polymorphism.

Inheritance lets you create a brand new class with all of the properties and capabilities “inherited” from one other class, often known as deriving a category from one other. The category you’re deriving from can be known as a superclass.

Another vital factor in inheritance is you can present a unique implementation of a public operate “inherited” from a superclass. This leads us to the following idea.

Polymorphism is said to inheritance and lets you deal with all derived courses as a superclass. For instance, you possibly can cross a derived class to TerminalView, and it’ll fortunately present it considering it’s a TerminalItem. Why would you do this? Since you might present your personal implementation of View() operate that returns a composable to indicate on display screen. This implementation might be an enter discipline for getting into instructions for the derived class.

So, create a brand new class named TerminalCommandPrompt extending TerminalItem in processor/mannequin bundle and change its contents with the next:

bundle com.kodeco.android.kodecoshell.processor.mannequin

import androidx.compose.material3.ExperimentalMaterial3Api
import androidx.compose.runtime.Composable
import com.kodeco.android.kodecoshell.processor.CommandInputWriter
import com.kodeco.android.kodecoshell.processor.TerminalCommandProcessor
import com.kodeco.android.kodecoshell.processor.ui.CommandInputField

class TerminalCommandPrompt(
    non-public val commandProcessor: TerminalCommandProcessor
) : TerminalItem() {

}

It takes one constructor parameter, a TerminalCommandProcessor object, which it’ll use to cross the instructions to.

Android Studio will present an error. Should you hover over it, you’ll see: This sort is ultimate, so it can't be inherited from.

It is because, by default, all courses in Kotlin are ultimate, which means a category can’t inherit from them.
Add the open key phrase to repair this.

Open TerminalItem and add the open key phrase earlier than class, so your class seems like this:

open class TerminalItem(non-public val textual content: String = "") {

  open enjoyable textToShow(): String = textual content

  @Composable
  open enjoyable View() {
    Textual content(
        textual content = textToShow(),
        fontSize = TextUnit(16f, TextUnitType.Sp),
        fontFamily = FontFamily.Monospace,
    )
  }
}

Now, again to TerminalCommandPrompt class.

It’s time to offer its View() implementation. Add the next operate override to the brand new class:

@Composable
@ExperimentalMaterial3Api
// 1
override enjoyable View() {
  CommandInputField(
      // 2
      inputWriter = object : CommandInputWriter {
        // 3
        override enjoyable sendInput(enter: String) {
          commandProcessor.course of(enter)
        }
      }
  )
}

Let’s go over this step-by-step:

1. Returns a CommandInputField composable. This takes the enter line by line and passes it to the CommandInputWriter.
2. An vital idea to notice right here is that you just’re passing an nameless object that implements CommandInputWriter.
3. Implementation of sendInput from nameless CommandInputWriter handed to CommandInputField passes the enter to TerminalCommandProcessor object from class constructor.

There’s one ultimate factor to do, open MainActivity and add the next import:

import com.kodeco.android.kodecoshell.processor.mannequin.TerminalCommandPrompt

Now, change the TerminalView instantiation with:

TerminalView(commandProcessor, TerminalCommandPrompt(commandProcessor))

This units the merchandise used for getting into instructions on TerminalView to TerminalCommandPrompt.

Construct and run the app. Yay, now you can enter instructions! For instance, pwd.

Word that you just gained’t have permission for some instructions, and also you’ll get errors.

SOLIDifying your code

Moreover, 5 extra design ideas will allow you to make sturdy, maintainable and easy-to-understand object-oriented code.

The SOLID ideas are:

• Single Accountability Precept: Every class ought to have one accountability.
• Open Closed Precept: You must have the ability to lengthen the habits of a part with out breaking its utilization.
• Liskov Substitution Precept: In case you have a category of 1 kind, you need to have the ability to signify the bottom class utilization with the subclass with out breaking the app.
• Interface Segregation Precept: It’s higher to have a number of small interfaces than solely a big one to forestall courses from implementing strategies they don’t want.
• Dependency Inversion Precept: Elements ought to depend upon abstractions quite than concrete implementations.

Understanding the Single Accountability Precept

Every class ought to have just one factor to do. This makes the code simpler to learn and preserve. You too can consult with this precept as “decoupling” code.

In the identical means, every operate ought to carry out one process if potential. A superb measure is that you need to have the ability to know what every operate does from its title.

Listed here are some examples of this precept from the KodecoShell app:

• Shell class: Its process is to ship instructions to Android shell and notify the outcomes utilizing callbacks. It doesn’t care the way you enter the instructions or show the consequence.
• CommandInputField: A Composable that takes care of command enter and nothing else.
• MainActivity: Reveals a terminal window UI utilizing Jetpack Compose. It delegates the dealing with of instructions to TerminalCommandProcessor implementation.

Understanding the Open Closed Precept

You’ve seen this precept in motion once you added TerminalCommandPrompt merchandise. Extending the performance by including new kinds of objects to the listing on the display screen doesn’t break current performance. No additional work in TerminalItem or MainActivity was wanted.

It is a results of utilizing polymorphism by offering an implementation of View operate in courses derived from TerminalItem. MainActivity doesn’t should do any additional work for those who add extra objects. That is what the Open Closed Precept is all about.

For observe, take a look at this precept as soon as extra by including two new TerminalItem courses:

• TerminalCommandErrorOutput: for exhibiting errors. The brand new merchandise ought to look the identical as TerminalItem however have a unique shade.
• TerminalCommandInput: for exhibiting instructions that you just entered. The brand new merchandise ought to look the identical as TerminalItem however have “>” prefixed.

Right here’s the answer:

[spoiler title=”Solution”]

bundle com.kodeco.android.kodecoshell.processor.mannequin

import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Textual content
import androidx.compose.runtime.Composable
import androidx.compose.ui.textual content.font.FontFamily
import androidx.compose.ui.unit.TextUnit
import androidx.compose.ui.unit.TextUnitType

/** Represents command error output in Terminal. */
class TerminalCommandErrorOutput(
    non-public val errorOutput: String
) : TerminalItem() {
  override enjoyable textToShow(): String = errorOutput

  @Composable
  override enjoyable View() {
    Textual content(
        textual content = textToShow(),
        fontSize = TextUnit(16f, TextUnitType.Sp),
        fontFamily = FontFamily.Monospace,
        shade = MaterialTheme.colorScheme.error
    )
  }
}

bundle com.kodeco.android.kodecoshell.processor.mannequin

class TerminalCommandInput(
    non-public val command: String
) : TerminalItem() {
  override enjoyable textToShow(): String = "> $command"
}

Replace ShellCommandProcessor property initializer:

non-public val shell = Shell(
  outputCallback = { outputCallback(TerminalItem(it)) },
  errorCallback = { outputCallback(TerminalCommandErrorOutput(it)) }
)

Then, course of operate:

override enjoyable course of(command: String) {
  outputCallback(TerminalCommandInput(command))
  shell.course of(command)
}

Import the next:

import com.kodeco.android.kodecoshell.processor.mannequin.TerminalCommandErrorOutput
import com.kodeco.android.kodecoshell.processor.mannequin.TerminalCommandInput

[/spoiler]

Construct and run the app. Sort a command that wants permission or an invalid command. You’ll see one thing like this:

Understanding the Liskov Substitution Precept

This precept states that for those who change a subclass of a category with a unique one, the app shouldn’t break.

For instance, for those who’re utilizing a Record, the precise implementation doesn’t matter. Your app would nonetheless work, though the occasions to entry the listing parts would differ.

To check this out, create a brand new class named DebugShellCommandProcessor in processor/shell bundle.
Paste the next code into it:

bundle com.kodeco.android.kodecoshell.processor.shell

import com.kodeco.android.kodecoshell.processor.TerminalCommandProcessor
import com.kodeco.android.kodecoshell.processor.mannequin.TerminalCommandErrorOutput
import com.kodeco.android.kodecoshell.processor.mannequin.TerminalCommandInput
import com.kodeco.android.kodecoshell.processor.mannequin.TerminalItem
import java.util.concurrent.TimeUnit

class DebugShellCommandProcessor(
    override var outputCallback: (TerminalItem) -> Unit = {}
) : TerminalCommandProcessor {

  non-public val shell = Shell(
      outputCallback = {
        val elapsedTimeMs = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - commandStartNs)
        outputCallback(TerminalItem(it))
        outputCallback(TerminalItem("Command success, time: ${elapsedTimeMs}ms"))
      },
      errorCallback = {
        val elapsedTimeMs = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - commandStartNs)
        outputCallback(TerminalCommandErrorOutput(it))
        outputCallback(TerminalItem("Command error, time: ${elapsedTimeMs}ms"))
      }
  )

  non-public var commandStartNs = 0L

  override enjoyable init() {
    outputCallback(TerminalItem("Welcome to Kodeco shell (Debug) - enter your command ..."))
  }

  override enjoyable course of(command: String) {
    outputCallback(TerminalCommandInput(command))
    commandStartNs = System.nanoTime()
    shell.course of(command)
  }

  override enjoyable stopCurrentCommand() {
    shell.stopCurrentCommand()
  }
}

As you’ll have observed, that is just like ShellCommandProcessor with the added code for monitoring how lengthy every command takes to execute.

Go to MainActivity and change commandProcessor property with the next:

non-public val commandProcessor: TerminalCommandProcessor = DebugShellCommandProcessor()

You’ll should import this:

import com.kodeco.android.kodecoshell.processor.shell.DebugShellCommandProcessor

Now construct and run the app.

Attempt executing the “ps” command.

Your app nonetheless works, and also you now get some further debug data — the time that command took to execute.

Understanding the Interface Segregation Precept

This precept states it’s higher to separate interfaces into smaller ones.

To see the advantages of this, open TerminalCommandPrompt. Then change it to implement CommandInputWriter as follows:

class TerminalCommandPrompt(
    non-public val commandProcessor: TerminalCommandProcessor
) : TerminalItem(), CommandInputWriter {

  @Composable
  @ExperimentalMaterial3Api
  override enjoyable View() {
    CommandInputField(inputWriter = this)
  }

  override enjoyable sendInput(enter: String) {
    commandProcessor.course of(enter)
  }
}

Construct and run the app to ensure it’s nonetheless working.

Should you used just one interface – by placing summary sendInput operate into TerminalItem – all courses extending TerminalItem must present an implementation for it though they don’t use it. As a substitute, by separating it into a unique interface, solely TerminalCommandPrompt can implement it.

Understanding the Dependency Inversion Precept

As a substitute of relying on concrete implementations, equivalent to ShellCommandProcessor, your courses ought to depend upon abstractions: interfaces or summary courses that outline a contract. On this case, TerminalCommandProcessor.

You’ve already seen how highly effective the Liskov substitution precept is — this precept makes it tremendous straightforward to make use of. By relying on TerminalCommandProcessor in MainActivity, it’s straightforward to switch the implementation used. Additionally, this is useful when writing assessments. You may cross mock objects to a examined class.

Kotlin Particular Ideas

Lastly, listed below are a number of Kotlin-specific ideas.

Kotlin has a helpful mechanism for controlling inheritance: sealed courses and interfaces. Briefly, for those who declare a category as sealed, all its subclasses should be throughout the identical module.

For extra info, examine the official documentation.

In Kotlin, courses can’t have static capabilities and properties shared throughout all situations of your class. That is the place companion objects are available.

For extra info have a look at the official documentation.

The place to Go From Right here?

If you wish to know extra about commonest design patterns utilized in OOP, take a look at our sources on patterns utilized in Android.

Should you want a useful listing of design patterns, ensure that to examine this.

One other useful resource associated to design patterns is Design Patterns: Components of Reusable Object-Oriented Software program, by the Gang of 4.

You’ve discovered what Object-Oriented Programming greatest practices are and leverage them.

Now go and write readable and maintainable code and unfold the phrase! In case you have any feedback or questions, please be a part of the discussion board dialogue under!