What is the Java equivalent to use https://fcm.googleapis.com/fcm/send REST Api

401 means unauthorized, so no valid Authorization header was sent.

And this line:

httpcon.setRequestProperty("Authorization: key", "AIza...iD9wk");

Is not equivalent to -H "Authorization: key=AIza...iD9wk". The first argument should be the header name, which is Authorization:

httpcon.setRequestProperty("Authorization", "key=AIza...iD9wk");

In conclusion, you misunderstood how HTTP header is formatted. Basically the header name and value are separated by : not =.