How to build a simple Android app

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Activity Lifecycle

package cs.ecl.mobile.android.simple;

import android.app.Activity;
import android.os.Bundle;
import android.widget.Toast;

public class ActivityLifecycle extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        Toast.makeText(this, " onCreate", Toast.LENGTH_SHORT).show();
    }

    @Override
    protected void onStart() {
        super.onStart();
        Toast.makeText(this, " onStart", Toast.LENGTH_SHORT).show();
    }

    @Override
    protected void onResume() {
        super.onResume();
        Toast.makeText(this, " onResume", Toast.LENGTH_SHORT).show();
    }

    @Override
    protected void onRestart() {
        super.onRestart();
        Toast.makeText(this, " onRestart", Toast.LENGTH_SHORT).show();
    }

    @Override
    protected void onPause() {
        Toast.makeText(this, "onPause", Toast.LENGTH_SHORT).show();
        super.onPause();
    }

    @Override
    protected void onStop() {
        Toast.makeText(this, "onStop", Toast.LENGTH_SHORT).show();
        super.onStop();
    }

    @Override
    protected void onDestroy() {
        Toast.makeText(this, "onDestroy", Toast.LENGTH_SHORT).show();
        super.onDestroy();
    }
}

Run the app and try to understand the sequences of message based on diagram from android developers site

The entire source code for this application could be found sample that illustrates the activity here