If you want to launch a application B from your application A then you can launch that by using intent Like this :

First define application B package name so that we can set launch intent :

Intent myIntent = getPackageManager().getLaunchIntentForPackage("com.package.B");
if (myIntent != null) {
    // We found the activity now start the activity
    myIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    startActivity(myIntent);
} else {
    // redirect to play store to choose an app?
    myIntent = new Intent(Intent.ACTION_VIEW);
    myIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    myIntent.setData(Uri.parse("market://details?id=" + "com.package.name"));
    startActivity(myIntent);
}

here you can check package name of B application but if doesn't exist then you can redirect user to google play store to download that B application and then you can start.