Object-Oriented Programming Greatest Practices with Kotlin

Object-Oriented Programming (OOP) is the preferred pc 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 rules of Object-Oriented Programming.
• SOLID rules and the way they make your code higher.
• Some Kotlin particular good-to-knows.

Getting began

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

Open the starter challenge in Android Studio 2022.2.1 or later by deciding on Open on the Android Studio welcome display screen:

The app consists of a single display screen much 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 challenge. It’s best to see the primary, and solely, display screen of the app:

Whoa, what’s occurring right here? As you may see, the app at present refuses to run any instructions, it simply shows a non-cooperative message. Subsequently, your job shall be to make use of OOP Greatest 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, it is best to perceive what OOP is.

Object-Oriented Programming is a programming mannequin based mostly on knowledge. All the things is modeled as objects that may carry out sure actions and talk with one another.

For instance, should you have been to characterize a automobile in object-oriented programming, one of many objects could be a Automotive. It will include actions comparable to:

• Speed up
• Brake
• Steer left
• Steer proper

Courses and Objects

One of the vital essential distinctions in object-oriented programming is between lessons and objects.

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

A category describes how the automobile behaves, comparable to its prime velocity, how briskly it might speed up, and so forth. It is sort of a blueprint for the automobile.

An object is an occasion of a automobile, should you 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 lessons in KodecoShell app:

• MainActivity class represents the display screen proven while you open the app.
• TerminalCommandProcessor class processes instructions that you simply’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 attain this in Kotlin, you utilize:

non-public val commandProcessor: TerminalCommandProcessor = TerminalCommandProcessor()

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

commandProcessor.init()

Key Rules of OOP

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

• Encapsulation
• Abstraction
• Inheritance
• Polymorphism

These rules make it doable to construct code that’s simple to grasp and preserve.

Understanding Encapsulation and Kotlin Courses

Knowledge inside a category may be restricted. Ensure different lessons can solely change the info in anticipated methods and stop state inconsistencies.

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

In Kotlin, you utilize visibility modifiers to regulate the visibility of properties and capabilities inside lessons. Two of a very powerful ones are:

• non-public: property or operate is barely seen inside the category the place it’s outlined.
• public: default visibility modifier if none is specified, property or operate is seen in every single place.

Marking the interior knowledge of a category as non-public prevents different lessons 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 lessons to make use of course of() to course of instructions through TerminalCommandProcessor.

Notice you handed blocks of code for outputCallback and errorCallback parameters. Shell will execute one among them when its course of operate 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 not 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.

In consequence, now it is best to see the output of the ps command, which is the checklist of the at present operating processes.

Abstraction

That is much like encapsulation, it permits entry to lessons by means of a particular contract. In Kotlin, you may outline that contract utilizing interfaces.

Interfaces in Kotlin can include declarations of capabilities and properties. However, the primary distinction between interfaces and lessons 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 exchange class key phrase with interface.

Notice 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’t 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’ve 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 lessons 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()

Courses 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 personal property holding a Shell object for executing instructions. You go the code blocks that go 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 another MainActivity change to check the brand new code. So, add the next import:

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

Then, exchange 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 an inventory of TerminalItem objects. How are you going to present a unique merchandise if an inventory is ready as much as present an object of a sure class? The reply lies in inheritance and polymorphism.

Inheritance allows you to 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.

Yet one more essential factor in inheritance is that you may present a unique implementation of a public operate “inherited” from a superclass. This leads us to the following idea.

Polymorphism is expounded to inheritance and allows you to deal with all derived lessons as a superclass. For instance, you may go a derived class to TerminalView, and it’ll fortunately present it considering it’s a TerminalItem. Why would you try this? Since you might present your individual implementation of View() operate that returns a composable to indicate on display screen. This implementation shall be an enter area for getting into instructions for the derived class.

So, create a brand new class named TerminalCommandPrompt extending TerminalItem in processor/mannequin bundle and exchange 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 go the instructions to.

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

It is because, by default, all lessons in Kotlin are remaining, 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 appears 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 supply 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 essential idea to notice right here is that you simply’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 remaining factor to do, open MainActivity and add the next import:

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

Now, exchange 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.

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

SOLIDifying your code

Moreover, 5 extra design rules will provide help to make sturdy, maintainable and easy-to-understand object-oriented code.

The SOLID rules are:

• Single Duty Precept: Every class ought to have one duty.
• Open Closed Precept: It’s best to be capable to lengthen the habits of a element with out breaking its utilization.
• Liskov Substitution Precept: When you’ve got a category of 1 kind, it is best to be capable to characterize 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 lessons from implementing strategies they don’t want.
• Dependency Inversion Precept: Parts ought to depend upon abstractions relatively than concrete implementations.

Understanding the Single Duty Precept

Every class ought to have just one factor to do. This makes the code simpler to learn and preserve. It’s also possible to check with this precept as “decoupling” code.

In the identical approach, every operate ought to carry out one job if doable. A very good measure is that it is best to be capable to know what every operate does from its title.

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

• Shell class: Its job is to ship instructions to Android shell and notify the outcomes utilizing callbacks. It doesn’t care the way you enter the instructions or methods to show the end result.
• 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 while you added TerminalCommandPrompt merchandise. Extending the performance by including new forms of gadgets to the checklist on the display screen doesn’t break present 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 lessons derived from TerminalItem. MainActivity doesn’t must do any additional work should you add extra gadgets. That is what the Open Closed Precept is all about.

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

• TerminalCommandErrorOutput: for displaying errors. The brand new merchandise ought to look the identical as TerminalItem however have a unique coloration.
• TerminalCommandInput: for displaying instructions that you simply 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,
        coloration = 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 should you exchange a subclass of a category with a unique one, the app shouldn’t break.

For instance, should you’re utilizing a Record, the precise implementation doesn’t matter. Your app would nonetheless work, though the occasions to entry the checklist parts would range.

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 might have seen, that is much like ShellCommandProcessor with the added code for monitoring how lengthy every command takes to execute.

Go to MainActivity and exchange commandProcessor property with the next:

non-public val commandProcessor: TerminalCommandProcessor = DebugShellCommandProcessor()

You’ll must import this:

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

Now construct and run the app.

Strive executing the “ps” command.

Your app nonetheless works, and also you now get some further debug information — 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.

When you used just one interface – by placing summary sendInput operate into TerminalItem – all lessons 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, comparable to ShellCommandProcessor, your lessons ought to depend upon abstractions: interfaces or summary lessons 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 simple to make use of. By relying on TerminalCommandProcessor in MainActivity, it’s simple to exchange the implementation used. Additionally, this is useful when writing checks. You may go mock objects to a examined class.

Kotlin Particular Suggestions

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

Kotlin has a helpful mechanism for controlling inheritance: sealed lessons and interfaces. Briefly, should you declare a category as sealed, all its subclasses have to be inside the similar module.

For extra data, examine the official documentation.

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

For extra data take a look at the official documentation.

The place to Go From Right here?

If you wish to know extra about most typical design patterns utilized in OOP, try our sources on patterns utilized in Android.

When you want a useful checklist of design patterns, make certain to examine this.

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

You’ve discovered what Object-Oriented Programming finest practices are and methods to leverage them.

Now go and write readable and maintainable code and unfold the phrase! When you’ve got any feedback or questions, please be part of the discussion board dialogue beneath!