单击按钮后我的应用程序崩溃

Question

When I click the button my app crashes and I have no idea why. I've looked up my code a lot of times but can't seem to find any "major flaws". The purpose of the app is just clicking the button twice as fast as possible and it displays the amount of time needed from the first tap to the second tap. Some sort of reaction thing. I used a timer because I didn't know what else to use. (Value of start time) - (value of current timer) = (time between first and second click).

    <?xml version="1.0" encoding="utf-8"?>

<RelativeLayout

    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#000000"
    tools:context="com.keklabs.reactiontimer.MainActivity">


    <TextView
        android:id="@+id/scoreText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="78dp"
        android:text="Click below to start!"
        android:textColor="#39FF14"
        android:textSize="30dp" />

    <Button
        android:id="@+id/startStopButton"
        android:layout_width="match_parent"
        android:layout_height="350dp"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:background="#000000"
        android:text="Start / Stop"
        android:textSize="25dp"
        android:textColor="#39FF14" />

</RelativeLayout>

public class MainActivity extends AppCompatActivity {

    private final long startTime = 0;
    private final long interval = 100;
    private boolean buttonClicked;
    private Button startStopButton;
    private TextView scoreText;

    CountDownTimer timer = new CountDownTimer(30000, 100) {

        @Override
        public void onTick(long millisUntilFinished) {
            scoreText.setText("kek"); //displayed text is value of starttime (30000) - current value of timer,
        }                             //some sort of reaction game thing

        @Override
        public void onFinish() {
        }
    };

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

        TextView scoreText = (TextView) findViewById(R.id.scoreText);
        Button startStopButton = (Button) findViewById(R.id.startStopButton);


        startStopButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (!buttonClicked) {
                    timer.start();
                    buttonClicked = true;
                }
                else {
                    timer.cancel();
                    buttonClicked = false;
                }
            }
        });

    }

}

EDIT

How do i do it the right way in the line scoreText.setText(startTime - millisUntilFinished); to display startTime-current timer value?

CountDownTimer timer = new CountDownTimer(startTime, interval) {

    @Override
    public void onTick(long millisUntilFinished) {
        scoreText.setText(startTime - millisUntilFinished);
    }

    @Override
    public void onFinish() {
    }
};

Answer 1

Actually, you are declaring your TextView scoreText twice. One gloabally and other locally. You can replace TextView scoreText = (TextView) findViewById(R.id.scoreText); to scoreText = (TextView) findViewById(R.id.scoreText); in your onCreate()