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 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 mission utilizing the Obtain Supplies button on the prime or backside of this tutorial.

Open the starter mission 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 permits you to enter instructions and present their output and errors. Moreover, there are two actions, one to cease a working command and one to clear the output.

Construct and run the mission. You need to see the primary, and solely, display screen of the app:

Whoa, what’s occurring right here? As you may see, the app presently refuses to run any instructions, it simply shows a non-cooperative message. Due to this fact, your job might be to make use of OOP Greatest Practices and repair that! You’ll add the flexibility 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 primarily based on information. Every little thing is modeled as objects that may carry out sure actions and talk with one another.

For instance, should you had been to characterize a automotive in object-oriented programming, one of many objects could be a Automotive. It might comprise actions akin to:

• Speed up
• Brake
• Steer left
• Steer proper

Courses and Objects

Some of the essential distinctions in object-oriented programming is between lessons 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, akin to its prime velocity, how briskly it may well speed up, and so forth. It is sort of a blueprint for the automotive.

An object is an occasion of a automotive, 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 if 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 bit 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 person 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:

personal val commandProcessor: TerminalCommandProcessor = TerminalCommandProcessor()

Afterward, you can use it by calling its features, for instance:

commandProcessor.init()

Key Rules of OOP

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

• Encapsulation
• Abstraction
• Inheritance
• Polymorphism

These ideas make it attainable to construct code that’s straightforward to grasp and preserve.

Understanding Encapsulation and Kotlin Courses

Knowledge inside a category could be restricted. Make certain different lessons can solely change the information in anticipated methods and stop 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 features inside lessons. Two of a very powerful ones are:

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

Marking the interior information of a category as personal 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:

personal val shell = Shell(
    outputCallback = { outputCallback(TerminalItem(it)) },
    errorCallback = { outputCallback(TerminalItem(it)) }
)

You instantiated a Shell to run shell instructions. You’ll be able to’t entry it exterior of TerminalCommandProcessor. You need different lessons to make use of course of() to course of instructions through TerminalCommandProcessor.

Be aware you handed blocks of code for outputCallback and errorCallback parameters. Shell will execute considered one of them when its course of perform known as.

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

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 personal in 'TerminalCommandProcessor'

Delete the road and return to TerminalCommandProcessor. Now change the init() perform 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 you need to see the output of the ps command, which is the listing of the presently working processes.

Abstraction

That is just 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 comprise declarations of features and properties. However, the primary distinction between interfaces and lessons is that interfaces can’t retailer state.

In Kotlin, features 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.

Be aware 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’ve gotten an interface with three features with implementations.

Exchange init perform with the next:

enjoyable init()

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

Exchange course of and stopCurrentCommand features 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 features and properties.

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

package deal 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
  personal 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 identify from TerminalCommandProcessor interface.
3. You create a non-public property holding a Shell object for executing instructions. You move the code blocks that move the command output and errors to outputCallback wrapped in TerminalItem objects.
4. Implementations of init, course of and stopCurrentCommand features name applicable Shell object features.

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:

personal val commandProcessor: TerminalCommandProcessor = ShellCommandProcessor()

Construct and run the app.

Inheritance and Polymorphism

It’s time so as to add the flexibility 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 allows you to create a brand new class with all of the properties and features “inherited” from one other class, also called deriving a category from one other. The category you’re deriving from can also be known as a superclass.

Another essential factor in inheritance is that you would be able to present a unique implementation of a public perform “inherited” from a superclass. This leads us to the subsequent idea.

Polymorphism is expounded to inheritance and allows you to deal with all derived lessons as a superclass. For instance, you may move 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 personal implementation of View() perform that returns a composable to indicate on display screen. This implementation might be an enter subject for coming into instructions for the derived class.

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

package deal 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(
    personal val commandProcessor: TerminalCommandProcessor
) : TerminalItem() {

}

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

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

It’s because, by default, all lessons in Kotlin are last, that 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 to be like this:

open class TerminalItem(personal 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 perform 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 last 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 coming into instructions on TerminalView to TerminalCommandPrompt.

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

Be aware that you simply 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 strong, maintainable and easy-to-understand object-oriented code.

The SOLID ideas are:

• Single Duty Precept: Every class ought to have one duty.
• Open Closed Precept: You need to be capable of prolong the conduct of a part with out breaking its utilization.
• Liskov Substitution Precept: If in case you have a category of 1 sort, you need to be capable of 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 stop lessons from implementing strategies they don’t want.
• Dependency Inversion Precept: Elements ought to depend upon abstractions somewhat 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. You too can check with this precept as “decoupling” code.

In the identical method, every perform ought to carry out one activity if attainable. A great measure is that you need to be capable of know what every perform does from its identify.

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

• Shell class: Its activity 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 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 if you added TerminalCommandPrompt merchandise. Extending the performance by including new kinds of gadgets to the listing on the display screen doesn’t break present performance. No further work in TerminalItem or MainActivity was wanted.

This can be a results of utilizing polymorphism by offering an implementation of View perform in lessons derived from TerminalItem. MainActivity doesn’t must do any further work should you add extra gadgets. 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 lessons:

• TerminalCommandErrorOutput: for exhibiting errors. The brand new merchandise ought to look the identical as TerminalItem however have a unique coloration.
• TerminalCommandInput: for exhibiting 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”]

package deal 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(
    personal 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
    )
  }
}

package deal com.kodeco.android.kodecoshell.processor.mannequin

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

Replace ShellCommandProcessor property initializer:

personal val shell = Shell(
  outputCallback = { outputCallback(TerminalItem(it)) },
  errorCallback = { outputCallback(TerminalCommandErrorOutput(it)) }
)

Then, course of perform:

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. Kind 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 change 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, regardless that the occasions to entry the listing components would differ.

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

package deal 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 {

  personal 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"))
      }
  )

  personal 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 seen, 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:

personal 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 extra 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(
    personal 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.

For those who used just one interface – by placing summary sendInput perform into TerminalItem – all lessons extending TerminalItem must present an implementation for it regardless that they don’t use it. As an alternative, by separating it into a unique interface, solely TerminalCommandPrompt can implement it.

Understanding the Dependency Inversion Precept

As an alternative of relying on concrete implementations, akin 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 straightforward to make use of. By relying on TerminalCommandProcessor in MainActivity, it’s straightforward to exchange the implementation used. Additionally, this turns out to be useful when writing assessments. You’ll be able to move mock objects to a examined class.

Kotlin Particular Ideas

Lastly, listed here are a number of Kotlin-specific suggestions.

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

For extra info, test the official documentation.

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

For extra info 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 assets on patterns utilized in Android.

For those who want a helpful listing of design patterns, ensure that to test 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! If in case you have any feedback or questions, please be part of the discussion board dialogue under!