Android - prawidłowe budowanie aktywności, klasy, metody

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public class main extends AppCompatActivity {
    ...
    Button undoButton;
    ...
    protected void onCreate(Bundle savedInstanceState) {
        ...
        undoButton = (Button) findViewById(R.id.undo);
        ...
    }
}

undoButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {                
                Cursor undo = matchDb.undo(count_round - 1);
                if (undo.getCount() > 0) {
                    count = undo.getInt(0);
                    playerName1.setText(undo.getString(1));
                    total1After.setText(undo.getString(3));
                    score1 = undo.getString(4);
                    coins1.setText(undo.getString(5));
                    queen1.setChecked(false);
                    if (undo.getInt(6) == 1) {
                        queen1.setChecked(true);
                    } else {
                        queen1.setChecked(false);
                    }
                    if (undo.getInt(7) == 1) {
                        break1.setChecked(true);
                    } else {
                        break1.setChecked(false);
                    }
                    playerName2.setText(undo.getString(8));
                    total2After.setText(undo.getString(10));
                    score2 = undo.getString(11);
                    coins2.setText(undo.getString(12));
                    queen2.setChecked(false);
                    if (undo.getInt(13) == 1) {
                        queen2.setChecked(true);
                    } else {
                        queen2.setChecked(false);
                    }
                    if (undo.getInt(14) == 1) {
                        break2.setChecked(true);
                    } else {
                        break2.setChecked(false);
                    }                    
                }
                if (count == 1) {
                    undoButton.setVisibility(View.INVISIBLE);
                }
            }

        });

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        undoButton = (Button) findViewById(R.id.undo);

        undoButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                MyClass fields = matchDb.undo(count_round - 1);
                if (fields.count > 0) {
                    setUpTextViews(fields);
                    setUpScores(fields);
                    setUpCheckBoxes(fields);
                    if (count == 1) {
                        undoButton.setVisibility(View.INVISIBLE);
                    }
                }
            }

        });

    }

    private void setUpCheckBoxes(MyClass fields) {
        queen1.setChecked(fields.queen1);
        break1.setChecked(fields.break1);
        queen2.setChecked(fields.queen2);
        break2.setChecked(fields.break2);
    }

    private void setUpScores(MyClass fields) {
        score1 = fields.score1; //Nie wiem co to score1 ale też powinno mieć znaczącą nazwe
        score2 = fields.score2;
    }

    private void setUpTextViews(MyClass fields) {
        playerName1TextView.setText(fields.playerName);
        playerName2TextView.setText(fields.playerName2);
        total1AfterTextView.setText(fields.total1After);
        total1After2TextView.setText(fields.total2After);
        coins1TextView.setText(fields.coins1);
        coins2TextView.setText(fields.coins2);
    }

    class MyClass{
        String playerName,total1After,score1,coins1,playerName2,total2After,score2,coins2;
        Boolean queen1,queen2,break1,break2;
        int count;
        MyClass(Cursor undo){
            count  = undo.getInt(0);
            playerName = undo.getString(1);
            total1After =  undo.getString(3);
            score1 = undo.getString(4);
            coins1 = undo.getString(5);
            queen1 = (undo.getInt(6) == 1);
            break1 = (undo.getInt(7) == 1);
            playerName2 = undo.getString(8);
            total2After = undo.getString(10);
            score2 = undo.getString(11);
            coins2 = undo.getString(12);
            queen2 = (undo.getInt(13) == 1);
            break2 = (undo.getInt(14) == 1);
        }

    }

Undo fields = matchDb.undo(count_round-1);

Undo fields = (Undo) matchDb.undo(count_round-1);

public Cursor undo(int id) {
        SQLiteDatabase db = this.getReadableDatabase();
        String sqlSelect = "SELECT " + COL_2 + ", " + COL_3 + ", " + COL_4 + ", " + COL_5 + ", " + COL_6 + ", " + COL_7 + ", " + COL_8 + ", " + COL_9 + ", " + COL_10 + ", " + COL_11 + ", " + COL_12 + ", " + COL_13 + ", " + COL_14 +  ", " + COL_15 + ", " + COL_16 + " FROM " + TABLE_NAME + " WHERE " + COL_2 + "=" + id;
        Cursor cursor = db.rawQuery(sqlSelect, null);

        if(cursor != null) {
            cursor.moveToFirst();
        }
        return cursor;
    }

Undo fields = new Undo(matchDb.undo(count - 1));

if(queen1Checkbox.isChecked() && Integer.parseInt(coins1TextView.getText().toString()) > 0 && Integer.parseInt(total1AfterButton.getText().toString()) < 22 && Integer.parseInt(coins2TextView.getText().toString()) == 0){
        total1AfterButton.setText(Integer.toString(Integer.parseInt(total1AfterButton.getText().toString()) + Integer.parseInt(coins1TextView.getText().toString()) + 3));
        score1 = Integer.toString(Integer.parseInt(coins1TextView.getText().toString()) + 3);

    }
    else if(queen1Checkbox.isChecked() && Integer.parseInt(coins1TextView.getText().toString()) == 0 && Integer.parseInt(total1AfterButton.getText().toString()) < 22 && Integer.parseInt(coins2TextView.getText().toString()) == 0) {
        total1AfterButton.setText(Integer.toString(Integer.parseInt(total1AfterButton.getText().toString()) + Integer.parseInt(coins1TextView.getText().toString()) + 3));
        score1 = Integer.toString(Integer.parseInt(coins1TextView.getText().toString()) + 3);
    }
    else if(queen1Checkbox.isChecked() && Integer.parseInt(coins1TextView.getText().toString()) == 0 && Integer.parseInt(total1AfterButton.getText().toString()) >= 22 && Integer.parseInt(coins2TextView.getText().toString()) == 0) {
        total1AfterButton.setText(Integer.toString(Integer.parseInt(total1AfterButton.getText().toString()) + Integer.parseInt(coins1TextView.getText().toString()) + 1));
        score1 = Integer.toString(Integer.parseInt(coins1TextView.getText().toString()) + 1);
    }
    else {
        total1AfterButton.setText(Integer.toString(Integer.parseInt(total1AfterButton.getText().toString()) + Integer.parseInt(coins1TextView.getText().toString())));
        score1 = Integer.toString(Integer.parseInt(coins1TextView.getText().toString()));
    }

 private boolean calculatePlayer1Var1() {
        if(queen1Checkbox.isChecked() && Integer.parseInt(coins1TextView.getText().toString()) > 0 && Integer.parseInt(total1AfterButton.getText().toString()) < 22 && Integer.parseInt(coins2TextView.getText().toString()) == 0){
            total1AfterButton.setText(Integer.toString(Integer.parseInt(total1AfterButton.getText().toString()) + Integer.parseInt(coins1TextView.getText().toString()) + 3));
            score1 = Integer.toString(Integer.parseInt(coins1TextView.getText().toString()) + 3);
            return true;
        }
        return false;
    }

    private boolean calculatePlayer1Var2() {
        if(queen1Checkbox.isChecked() && Integer.parseInt(coins1TextView.getText().toString()) == 0 && Integer.parseInt(total1AfterButton.getText().toString()) < 22 && Integer.parseInt(coins2TextView.getText().toString()) == 0) {
            total1AfterButton.setText(Integer.toString(Integer.parseInt(total1AfterButton.getText().toString()) + Integer.parseInt(coins1TextView.getText().toString()) + 3));
            score1 = Integer.toString(Integer.parseInt(coins1TextView.getText().toString()) + 3);
            return true;
        }
        return false;
    }

    private boolean calculatePlayer1Var3() {
        if(queen1Checkbox.isChecked() && Integer.parseInt(coins1TextView.getText().toString()) == 0 && Integer.parseInt(total1AfterButton.getText().toString()) >= 22 && Integer.parseInt(coins2TextView.getText().toString()) == 0) {
            total1AfterButton.setText(Integer.toString(Integer.parseInt(total1AfterButton.getText().toString()) + Integer.parseInt(coins1TextView.getText().toString()) + 1));
            score1 = Integer.toString(Integer.parseInt(coins1TextView.getText().toString()) + 1);
            return true;
        }
        return false;
    }

    private boolean calculatePlayer1Var4() {
        total1AfterButton.setText(Integer.toString(Integer.parseInt(total1AfterButton.getText().toString()) + Integer.parseInt(coins1TextView.getText().toString())));
        score1 = Integer.toString(Integer.parseInt(coins1TextView.getText().toString()));
        return true;
    }

next.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {                
               if(calculatePlayer1Var1()) {
                }
                else if(calculatePlayer1Var2()) {
                }
                else if(calculatePlayer1Var3()) {
                }
                else {
                    calculatePlayer1Var4();
                }
});

    private boolean calculatePlayer1Var3() {
       return (queen1Checkbox.isChecked() && Integer.parseInt(coins1TextView.getText().toString()) == 0 && Integer.parseInt(total1AfterButton.getText().toString()) >= 22 &&         Integer.parseInt(coins2TextView.getText().toString()) == 0)
    }

public class Timer extends CountDownTimer{
    String start, hms;
    public Timer(long miliseconds, long interval) {
        super(miliseconds, interval);

    }

    @Override
    public void onTick(long millisUntilFinished) {
        hms = String.format("%02d:%02d", TimeUnit.MILLISECONDS.toMinutes(millisUntilFinished),
                TimeUnit.MILLISECONDS.toSeconds(millisUntilFinished) - TimeUnit.MINUTES.toSeconds(TimeUnit.MILLISECONDS.toMinutes(millisUntilFinished)));
        System.out.println(hms);
        start = hms;

    }

    @Override
    public void onFinish() {
        start = "00:00";
    }
}

private void timer() {
    Timer timer = new Timer(milliseconds, interval);
    timer.start();
    start.setText(timer.hms);
}

start.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        timer();
    }
});

interface OnTick{
void setTextOnTick(String text);
}

public class MyActivity extends Activity implements OnTick{
.......
.....

@Override
public void setTextOnTick(String text){
runOnUiThread{//nie wiem czy to tu potrzebne sprawdź sam
 start.setText(text);
}

}


}

OnTick myTickListener;
    public Timer(long miliseconds, long interval,OnTick myTickListener) {
        super(miliseconds, interval);
        this.myTickListener=myTickListener;

    }

  @Override
    public void onTick(long millisUntilFinished) {
        hms = String.format("%02d:%02d", TimeUnit.MILLISECONDS.toMinutes(millisUntilFinished),
                TimeUnit.MILLISECONDS.toSeconds(millisUntilFinished) - TimeUnit.MINUTES.toSeconds(TimeUnit.MILLISECONDS.toMinutes(millisUntilFinished)));
        System.out.println(hms);
        this.myTickListener.setTextOnTick(hms)

    }

public Timer(long miliseconds, long interval, OnTick myTickListener) {

 
Attempt to invoke interface method 'void com.example.user.apk.OnTick.setTextOnTick(java.lang.String)' on a null object reference