Object-Oriented Programming Greatest Practices with Kotlin

Object-Oriented Programming (OOP) is the most well-liked 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 high or backside of this tutorial.

Open the starter challenge 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 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 working command and one to clear the output.

Construct and run the challenge. It’s best to see the principle, and solely, display screen of the app:

Whoa, what’s occurring right here? As you’ll be able to see, the app presently refuses to run any instructions, it simply shows a non-cooperative message. Subsequently, your job will probably 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, you must perceive what OOP is.

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

For instance, in case you have been to symbolize a automobile in object-oriented programming, one of many objects can be a Automotive. It could comprise actions reminiscent of:

• Speed up
• Brake
• Steer left
• Steer proper

Lessons and Objects

One of the vital necessary distinctions in object-oriented programming is between courses and objects.

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

A category describes how the automobile behaves, reminiscent of its high velocity, how briskly it will probably speed up, and so on. It is sort of a blueprint for the automobile.

An object is an occasion of a automobile, in case 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 courses 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 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 employ:

personal val commandProcessor: TerminalCommandProcessor = TerminalCommandProcessor()

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

commandProcessor.init()

Key Ideas 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 attainable to construct code that’s straightforward to grasp and keep.

Understanding Encapsulation and Kotlin Lessons

Knowledge inside a category will be restricted. Be sure different courses can solely change the information in anticipated methods and stop state inconsistencies.

In brief, the surface world doesn’t have to know how a category does one thing, however what it does.

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

• personal: property or perform is barely 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 knowledge of a category as personal 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:

personal 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.

Observe you handed blocks of code for outputCallback and errorCallback parameters. Shell will execute one among them when its course of perform is 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.

Nevertheless, 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 appliance begins as a result of MainActivity calls TerminalViews‘s LaunchEffect.

Construct and run the app.

Because of this, now you must see the output of the ps command, which is the listing of the presently working processes.

Abstraction

That is much like encapsulation, it permits entry to courses via a selected contract. In Kotlin, you’ll be able to 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 may retailer state.

Open TerminalCommandProcessor and substitute class key phrase with interface.

Observe 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 eradicate 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 might have an interface with three capabilities with implementations.

Substitute init perform with the next:

enjoyable init()

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

Substitute 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
  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 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 acceptable 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, substitute commandProcessor property with:

personal 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 special 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 necessary factor in inheritance is which you could present a special implementation of a public perform “inherited” from a superclass. This leads us to the following idea.

Polymorphism is expounded to inheritance and allows you to deal with all derived courses as a superclass. For instance, you’ll be able to cross a derived class to TerminalView, and it’ll fortunately present it considering it’s a TerminalItem. Why would you try this? Since you may present your personal implementation of View() perform that returns a composable to point out on display screen. This implementation will probably 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 substitute 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(
    personal 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. For those who hover over it, you’ll see: This sort is remaining, so it can't be inherited from.

It is because, by default, all courses in Kotlin are remaining, 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 offer 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 necessary 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, substitute 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.

Observe 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 assist you make strong, maintainable and easy-to-understand object-oriented code.

The SOLID rules are:

• Single Accountability Precept: Every class ought to have one duty.
• Open Closed Precept: It’s best to be capable of prolong the conduct of a element with out breaking its utilization.
• Liskov Substitution Precept: When you’ve got a category of 1 kind, you must be capable of symbolize 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 courses from implementing strategies they don’t want.
• Dependency Inversion Precept: Parts ought to depend upon abstractions slightly 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 keep. It’s also possible to discuss with this precept as “decoupling” code.

In the identical approach, every perform ought to carry out one process if attainable. A superb measure is that you must be capable of know what every perform 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 the way 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 if you added TerminalCommandPrompt merchandise. Extending the performance by including new sorts of objects to the listing on the display screen doesn’t break current performance. No further work in TerminalItem or MainActivity was wanted.

It is a results of utilizing polymorphism by offering an implementation of View perform in courses derived from TerminalItem. MainActivity doesn’t must do any further work in case you add extra objects. That is what the Open Closed Precept is all about.

For follow, check 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 special colour.
• 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”]

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(
    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,
        colour = MaterialTheme.colorScheme.error
    )
  }
}

bundle 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. 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 in case you substitute a subclass of a category with a special one, the app shouldn’t break.

For instance, in case you’re utilizing a Record, the precise implementation doesn’t matter. Your app would nonetheless work, though the instances to entry the listing components would fluctuate.

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 {

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

Go to MainActivity and substitute 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.

Attempt 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(
    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 courses extending TerminalItem must present an implementation for it though they don’t use it. As a substitute, by separating it into a special interface, solely TerminalCommandPrompt can implement it.

Understanding the Dependency Inversion Precept

As a substitute of relying on concrete implementations, reminiscent of 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 interchange the implementation used. Additionally, this is useful when writing checks. You may cross mock objects to a examined class.

Kotlin Particular Suggestions

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

Kotlin has a helpful mechanism for controlling inheritance: sealed courses and interfaces. In brief, in case you declare a category as sealed, all its subclasses should be inside the identical module.

For extra info, test the official documentation.

In Kotlin, courses can’t have static capabilities and properties shared throughout all cases 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, try our assets on patterns utilized in Android.

For those who want a useful listing of design patterns, make sure to check this.

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

You’ve discovered what Object-Oriented Programming greatest practices are and the way 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 a part of the discussion board dialogue beneath!