为什么在第二个界面和java里面声明的button 执行不了啊

import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
import android.widget.TextView;
import org.jsoup.Jsoup;
import org.jsoup.nodes.Document;
import java.io.IOException;
import java.util.ArrayDeque;
public class Thesecondactivity extends ActionBarActivity {
private Button btnclass;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.thesecondactivity_main);
btnclass = (Button) findViewById(R.id.btn1);
btnclass.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
setContentView(R.layout.class_main);
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
}
// 想接着转到第三个 这个button

也声明了 <activity android:name=".Thesecondactivity">

嗯，但是如果是button，命令执行不了。之前在第二页没写button，写了listview，listview干脆就是不现实，textview到时显示

没，ID没重复