Question

Assume Applications Foo an Eggs are on the same Android device. It’s possible for either application to get a list of all applications on the device. Is it possible for one application to know if another application has run and for how long?

Answer 1

You can get a list of installed applications by using the PackageManager.
Code from here:

public class AppList extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        PackageManager pm = this.getPackageManager();

        Intent intent = new Intent(Intent.ACTION_MAIN, null);
        intent.addCategory(Intent.CATEGORY_LAUNCHER);

        List list = pm.queryIntentActivities(intent, PackageManager.PERMISSION_GRANTED);
        for (ResolveInfo rInfo : list) {
            Log.w("Installed Applications", rInfo.activityInfo.applicationInfo.loadLabel(pm).toString());
        }
    }
}

To see the currently running apps, you can use the ActivityManager.