Untitled

public class MainActivity extends AppCompatActivity {

    private Integer result = 0;
    private Integer number;
    private TextView numberTW;
    private TextView resultTW;

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

        this.numberTW = (TextView) findViewById(R.id.tempNo);
        this.resultTW = (TextView)findViewById(R.id.result);
        this.number = Integer.valueOf(numberTW.getText().toString());

    }

    public void subtract(View view){
        this.number = this.number - 1;
        this.numberTW.setText(this.number.toString());
    }

    public void add(View view){
        this.number = this.number + 1;
        this.numberTW.setText(this.number.toString());
    }

    public void clear(View view){
        this.number = 0;
        this.numberTW.setText(this.number.toString());
        this.result = 0;
        this.resultTW.setText(this.result.toString());

        resultTW.setBackgroundResource(R.color.black);
        resultTW.setTextColor(Color.WHITE);
    }

    public void calculate(View view){
        this.result = this.result + this.number;
        this.resultTW.setText(this.result.toString());
        resultTW.setBackgroundResource(R.color.white);
        resultTW.setTextColor(Color.BLACK);

    }
}