This will first search for the specified contact and then open a chat window.
Note: phone_number and str are variables.
Uri mUri = Uri.parse("https://api.whatsapp.com/send?
phone=" + phone_no + "&text=" + str);
Intent mIntent = new Intent("android.intent.action.VIEW", mUri);
mIntent.setPackage("com.whatsapp");
startActivity(mIntent);