Kotlin GeoQuiz Example

The Java code is from Big Nerd Ranch Android Programing Chapter 2. I will walk you through how to enable and use kotlin in the main activity

If you would like to see entire code I will post it on my Github

package com.givechuckajob.geoquiz;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
public class QuizActivity extends AppCompatActivity {
    private Button mTrueButton;
    private Button mFalseButton;
    private Button mNextButton;
    private TextView mQuestionTextView;

    private Question[] mQuestionBank = new Question[]
            {
                    new Question(R.string.question_australia,true),
                    new Question(R.string.question_oceans,true),
                    new Question(R.string.question_mideast,false),
                    new Question(R.string.question_africa,false),
                    new Question(R.string.question_americas,true),
                    new Question(R.string.question_asia,true),
            };
    private int mCurrentIndex = 0;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_quiz);

        mQuestionTextView = findViewById(R.id.question_text_view);

        mNextButton = findViewById(R.id.next_button);
        mNextButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                mCurrentIndex = (mCurrentIndex +1) % mQuestionBank.length;
                updateQuestion();
            }
        });

        mTrueButton = findViewById(R.id.true_button);
        mTrueButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                checkAnswer(true);            }
        });
        mFalseButton =findViewById(R.id.false_button);
        mFalseButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                checkAnswer(false);            }
        });
        updateQuestion();
    }
    private void updateQuestion() {
        int question = mQuestionBank[mCurrentIndex].getTextResId();
        mQuestionTextView.setText(question);
    }
    private void checkAnswer(boolean userPressedTrue){
        boolean answerIsTrue = mQuestionBank[mCurrentIndex].isAnswerTrue();

        int messageResId = 0;
        if(userPressedTrue == answerIsTrue) {
            messageResId = R.string.correct_toast;
        }
        else {
            messageResId = R.string.incorrect_toast;
        }
        Toast.makeText(this, messageResId, Toast.LENGTH_SHORT).show();
    }
}

Conversion Step 1

Download Android Studio 3 (You can use older android studio but you have more steps to enable kotlin)
In menu go to Tools - Kotlin - Configure Kotlin in Project - Android with Gradle click ok (Project will compile fine without any kotlin files)
Create a new Kotlin file named QuizActivity.kt
Copy all code from QuizActity java to QuizActivity kt, it will prompt you to covert code to kotlin, select yes
Congrats you are now using kotlin try compiling and running and you will see that it works!
PS if you delete java files make sure your manifest is referencing correct activity


package com.givechuckajob.geoquiz

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.view.View
import android.widget.Button
import android.widget.TextView
import android.widget.Toast

class QuizActivity : AppCompatActivity() {
    private var mTrueButton: Button? = null
    private var mFalseButton: Button? = null
    private var mNextButton: Button? = null
    private var mQuestionTextView: TextView? = null

    private val mQuestionBank = arrayOf(Question(R.string.question_australia, true), Question(R.string.question_oceans, true), Question(R.string.question_mideast, false), Question(R.string.question_africa, false), Question(R.string.question_americas, true), Question(R.string.question_asia, true))
    private var mCurrentIndex = 0

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_quiz)

        mQuestionTextView = findViewById(R.id.question_text_view)

        mNextButton = findViewById(R.id.next_button)
        mNextButton!!.setOnClickListener {
            mCurrentIndex = (mCurrentIndex + 1) % mQuestionBank.size
            updateQuestion()
        }
        mTrueButton = findViewById(R.id.true_button)
        mTrueButton!!.setOnClickListener { checkAnswer(true) }
        mFalseButton = findViewById(R.id.false_button)
        mFalseButton!!.setOnClickListener { checkAnswer(false) }
        updateQuestion()
    }

    private fun updateQuestion() {
        val question = mQuestionBank[mCurrentIndex].textResId
        mQuestionTextView!!.setText(question)
    }

    private fun checkAnswer(userPressedTrue: Boolean) {
        val answerIsTrue = mQuestionBank[mCurrentIndex].isAnswerTrue
        var messageResId = 0
        if (userPressedTrue == answerIsTrue) {
            messageResId = R.string.correct_toast
        } else {
            messageResId = R.string.incorrect_toast
        }
        Toast.makeText(this, messageResId, Toast.LENGTH_SHORT).show()
    }
}

AAAA what happened to my semi colons hehe. It looks good but we can improve it more. Notice how we are have properties like mTruebutton up top being declared null let's see if we can fix that.

Conversion Step 2 - Cleanup Variable Declarations


package com.givechuckajob.geoquiz

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.widget.Button
import android.widget.TextView
import android.widget.Toast

class QuizActivity : AppCompatActivity() {

    lateinit private var mQuestionTextView:TextView
    lateinit private var mNextButton:Button
    lateinit private var mTrueButton:Button
    lateinit private var mFalseButton:Button

    private val mQuestionBank = arrayOf(Question(R.string.question_australia, true), Question(R.string.question_oceans, true), Question(R.string.question_mideast, false), Question(R.string.question_africa, false), Question(R.string.question_americas, true), Question(R.string.question_asia, true))
    private var mCurrentIndex = 0

    override fun onCreate(savedInstanceState: Bundle?) {

        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_quiz)

        mQuestionTextView = findViewById(R.id.question_text_view)
        mNextButton = findViewById(R.id.next_button)
        mTrueButton = findViewById(R.id.true_button)
        mFalseButton = findViewById(R.id.false_button)

        mNextButton.setOnClickListener {
            mCurrentIndex = (mCurrentIndex + 1) % mQuestionBank.size
            updateQuestion()
        }
        mTrueButton.setOnClickListener { checkAnswer(true) }
        mFalseButton.setOnClickListener { checkAnswer(false) }
        updateQuestion()
    }
    private fun updateQuestion() {
        val question = mQuestionBank[mCurrentIndex].textResId
        mQuestionTextView.setText(question)
    }
    private fun checkAnswer(userPressedTrue: Boolean) {
        val answerIsTrue = mQuestionBank[mCurrentIndex].isAnswerTrue

        var messageResId = 0;
        if (userPressedTrue == answerIsTrue) {
            messageResId = R.string.correct_toast
        }
        else {
            messageResId = R.string.incorrect_toast
        }
        Toast.makeText(this, messageResId, Toast.LENGTH_SHORT).show()
    }
}

I could do variable inits inline but decided to keep them up top because some are used in other classes. I was faced with how not to declare them null up top but found lateinit on google example. More info can be found at Kotlin Lateinit

Conversion Step 3 - Kotlin Extensions

Ever get annoyed by the constant findviewbyid, I have a fix for you -- Kotlin Extensions

Go to your app build.gradle and add the following line to your plugins - apply plugin: 'kotlin-android-extensions', and resync
Next step import your view, you could import individual compements of your xml but why not do it all

Look at code below to see how your code will change


package com.givechuckajob.geoquiz

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.widget.Toast
import kotlinx.android.synthetic.main.activity_quiz.*

class QuizActivity : AppCompatActivity() {

    private val mQuestionBank = arrayOf(Question(R.string.question_australia, true), Question(R.string.question_oceans, true), Question(R.string.question_mideast, false), Question(R.string.question_africa, false), Question(R.string.question_americas, true), Question(R.string.question_asia, true))
    private var mCurrentIndex = 0

    override fun onCreate(savedInstanceState: Bundle?) {

        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_quiz)

        next_button.setOnClickListener {
            mCurrentIndex = (mCurrentIndex + 1) % mQuestionBank.size
            updateQuestion()
        }
        true_button.setOnClickListener { checkAnswer(true) }
        false_button.setOnClickListener { checkAnswer(false) }
        updateQuestion()
    }
    private fun updateQuestion() {
        val question = mQuestionBank[mCurrentIndex].textResId
        question_text_view.setText(question)
    }
    private fun checkAnswer(userPressedTrue: Boolean) {
        val answerIsTrue = mQuestionBank[mCurrentIndex].isAnswerTrue
        var messageResId = 0
        if (userPressedTrue == answerIsTrue) {
            messageResId = R.string.correct_toast
        }
        else {
            messageResId = R.string.incorrect_toast
        }
        Toast.makeText(this, messageResId, Toast.LENGTH_SHORT).show()
    }
}

One import statement then no more declaring view variables, they can just be assigned on the fly