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public class MainActivity extends AppCompatActivity {

    RadioGroup radioGroup;
    TextView resultTextView;
    EditText valueEditText;

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

        radioGroup = findViewById(R.id.radioGroup);
        resultTextView = findViewById(R.id.resultTextView);
        valueEditText = findViewById(R.id.valueEditText);
    }

    public void onCalculateButtonClicked(View view) {
        if (valueEditText.getText().toString().isEmpty()) return;

        BigDecimal multiplier;
        switch (radioGroup.getCheckedRadioButtonId()) {
            case R.id.cmRadioButton:
                multiplier = BigDecimal.valueOf(100);
                break;
            case R.id.kmRadioButton:
                multiplier = BigDecimal.valueOf(0.001);
                break;
            case R.id.mileRadioButton:
                multiplier = BigDecimal.valueOf(0.00062137);
                break;
            case R.id.lyRadioButton:
                multiplier = BigDecimal.valueOf(1057, 19);
                break;
            default:
                resultTextView.setText("Wybierz rodzaj konwersji.");
                return;
        }
        double valueToConvert = Double.valueOf(valueEditText.getText().toString());
        BigDecimal valueBigDecimal = BigDecimal.valueOf(valueToConvert);
        BigDecimal result = valueBigDecimal.multiply(multiplier);
        resultTextView.setText(String.valueOf(result));
    }
}