I'm new to android development and I'm making a simple calculator. Everything is working fine and no errors are showing but `Buttons` aren't working whenever I press them. 

My target SDK is `API 19: Android 4.4(Kitkat)` and compiling with `API 20: Android 4.4(KitKat Wear)`. Here is my code,

**MainActivity.java**

```
package edu.shihank.mycalcv2;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends ActionBarActivity {
public TextView tv;
public float numbBf;
public String Operation;
public ButtonClickListener btnClick;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    tv = (TextView)findViewById(R.id.tv);
    int idList[] = {R.id.zero, R.id.one, R.id.two, R.id.three, R.id.four,
            R.id.five, R.id.six, R.id.seven, R.id.eight, R.id.nine, R.id.add,
            R.id.sub, R.id.div, R.id.multi, R.id.equal, R.id.clear, R.id.history};

    for(int id:idList){
        View v = (View)findViewById(id);
        v.setOnClickListener(btnClick);
    }
}

@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;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

public void getKeyboard(String str) {
    String tvNow = tv.getText().toString();
    tvNow += str;
    tv.setText(tvNow);
}

public void theResult() {
    float numbAf = Float.parseFloat(tv.getText().toString());
    float result = 0;
    if(Operation.equals("+")){
        result = numbAf + numbBf;
    }
    if(Operation.equals("-")){
        result = numbAf - numbBf;
    }
    if(Operation.equals("*")){
        result = numbAf * numbBf;
    }
    if(Operation.equals("/")){
        result = numbAf / numbBf;
    }
    tv.setText(String.valueOf(result));
}

public void theMath(String str) {
    numbBf = Float.parseFloat(tv.getText().toString());
    Operation = str;
    tv.setText("0");
}

private class ButtonClickListener implements OnClickListener{
    @Override
    public void onClick(View v) {       
        switch(v.getId()){
        case R.id.clear:
            tv.setText("0");
            numbBf = 0;
            Operation = "";
            break;
        case R.id.add:
            theMath("+");
            break;
        case R.id.sub:
            theMath("-");
            break;
        case R.id.div:
            theMath("/");
            break;
        case R.id.multi:
            theMath("*");
            break;
        case R.id.equal:
            theResult();
            break;
        default:
            String numb = ((Button) v).getText().toString();
            getKeyboard(numb);
            break;
        }

    }
}
}

```

Can anyone see what I've done wrong?