Simple calculator always displaying "0"

Question

I am trying to code a simple calculator. My code showed up no errors and also uploaded successfully, but when I try to enter numbers the display always remains zero. The code is like this: Coding for android 2.3.3

public class MainActivity extends Activity {
    TextView res;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        res = (TextView)findViewById(R.id.textView1);
        res.setText("0");
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    static boolean empty = true;

    public void num_clicked(View sender)
    {
        Button bt = (Button)sender;
        if(res.getText().length() > 8) return;
        if(empty)
        {   
            if(bt.getText().toString().equals("0")) return;
            res.setText(bt.getText());
            empty = false;
        }
        else
        {
            res.append(bt.getText());
        }
    }

    static int acc = 0;
    static short op;

    public void op_clicked(View sender)
    {  
        Button bt = (Button)sender;
        switch(op)
        {
            case 0:
                acc+=Integer.parseInt(res.getText().toString());
                break;
            case 1:
                acc-=Integer.parseInt(res.getText().toString());
                break;  
            case 2:
                acc*=Integer.parseInt(res.getText().toString());
                break;  
            case 3:
                acc/=Integer.parseInt(res.getText().toString());
                break;  
        }
        res.setText(Integer.toString(acc));

        if(bt.getText().toString().equals("+")) op = 0;
        if(bt.getText().toString().equals("-")) op = 1;
        if(bt.getText().toString().equals("*")) op = 2;
        if(bt.getText().toString().equals("/")) op = 3;

        empty = true;
    }
}

Answer 1

First of all, you have some incorrect code at the bottom:

if(bt.getText().toString().equals("+")) op = 0;
if(bt.getText().toString().equals("+")) op = 1;
if(bt.getText().toString().equals("+")) op = 2;
if(bt.getText().toString().equals("+")) op = 3;

empty = true;

because you test for "+" every time so op will always be 3 when a plus sign is entered and 0 otherwise.

Secondly, you are resetting empty to be true at the end, no matter what. So, this will make the display 0:

if(empty)
{   
    if(bt.getText().toString().equals("0")) return;
    res.setText(bt.getText());
    empty = false;
}

I don't know how you've set up your XML layout so I don't know what if the button listeners are properly implemented. This is a start of your problems.

---

The variable acc is only ever touched in your switch statement, so since it is 0 by default, /= and *= will still leave it at 0. That's a problem.

This code is probably also wrong:

switch(op)
{
    case 0:
        acc+=Integer.parseInt(res.getText().toString());
        break;
    case 1:
        acc-=Integer.parseInt(res.getText().toString());
        break;  
    case 2:
        acc*=Integer.parseInt(res.getText().toString());
        break;  
    case 3:
        acc/=Integer.parseInt(res.getText().toString());
        break;  
}
res.setText(Integer.toString(acc));

if(bt.getText().toString().equals("+")) op = 0;
if(bt.getText().toString().equals("-")) op = 1;
if(bt.getText().toString().equals("*")) op = 2;
if(bt.getText().toString().equals("/")) op = 3;

because your are assigning a value to op after you have already used a switch statement with op. Put the if statements before the switch statement.

Answer 2

It seems that you didn't use setOnClickListenerto set the OnClickListener of Buttons. Or write the android:onClick in your layout file.