Error: setContentView(R.layout.main) in Android Studio

I'm developing a simple app for learning purpose, In which there is a single check box and a text line, which shows it is checked or unchecked. But it shows an error in line setContentView(R.layout.main) and cb=(CheckBox)findViewById(R.id.check) this is my java code:

package com.example.kaushal.checkbox;
import android.app.Activity;
import android.os.Bundle;
import android.widget.CheckBox;
import android.widget.CompoundButton;
import android.R;

public class Checkbox extends Activity
    implements CompoundButton.OnCheckedChangeListener {
    CheckBox cb;
    @Override
    public void onCreate(Bundle icicle) {
    super.onCreate(icicle);

    setContentView(R.layout.main);
    cb=(CheckBox)findViewById(R.id.check);
    cb.setOnCheckedChangeListener(this);
    }
    public void onCheckedChanged(CompoundButton buttonView,
                             boolean isChecked) {
        if (isChecked) {
            cb.setText("This checkbox is: checked");
        }
        else {
            cb.setText("This checkbox is: unchecked");
        }
    }
}

It is not a duplicate question,I've seen the answers of the same questions but it was all for eclipse.I'm working with android studio.

My activity_ckeckbox.xml file has no errors.I'm new to android so please guide me.