android: how to change layout on button click?-Collection of common programming errors

I have to following code for selecting layout on button click.

View.OnClickListener handler = new View.OnClickListener(){
    public void onClick(View v) {

        switch (v.getId()) {

            case R.id.DownloadView: 
                // doStuff
                setContentView(R.layout.main);
                break;
            case R.id.AppView: 
                // doStuff
                setContentView(R.layout.app);
                break;
        }
    }
};

findViewById(R.id.DownloadView).setOnClickListener(handler);
findViewById(R.id.AppView).setOnClickListener(handler);

When I click the “AppView” button, the layout changes, but when I click the “DownloadView “button, nothing happens.

This link says that I have to start a new activity.

But I don’t know how to use the code there of intent to start new activity, will a new file be added? I’m new to java and android dev, many thanks in advanced for any help! 🙂

EDIT: I have my code on the new activity:

package com.example.engagiasync;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.TextView;

public class AppView extends Activity implements OnClickListener{


    @Override
    public void onCreate(Bundle savedInstanceState){

        setContentView(R.layout.app);

        TextView tv = (TextView) this.findViewById(R.id.thetext);
        tv.setText("App View yo!?\n");
    }

    @Override
    public void onClick(View v) {
        // TODO Auto-generated method stub

    }
}

but it does not work, it force closes, the logcat says: