Android Activity findviewbyid() is null

Question

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.widget.Button;

public class FileExplorerActivity extends Activity 
{
    public static final String TAG="ricky";
    Button button;
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        button = (Button) findViewById(R.id.but);<<<------------------
        if(button == null)
        Log.d(TAG, "FileExplorerActivity: button is null");
    }
    public FileExplorerActivity()
    {
       Log.d(TAG, "FileExplorer: constructor()");
    }
}

This is a simple Activity which is brought in front by another activity using Intent

Intent openFileBrowser = new Intent(this, FileExplorerActivity.class);
try
{
    startActivity(openFileBrowser); 
}

After running the code my LogCat file says "button is null" Why ???