file activity_regist.xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_login"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.novyan.app1.login"
    android:background="@color/abu"
    android:gravity="center">

    <LinearLayout
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true">

        <ImageView
            android:layout_width="wrap_content"
            android:layout_height="192dp"
            app:srcCompat="@drawable/logo_lab"
            android:id="@+id/imageView" />

        <LinearLayout
            android:orientation="vertical"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:weightSum="1">

            <TextView
                android:text="NPM"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:id="@+id/textView"
                android:textColor="@color/abucerah"
                android:textSize="18sp" />

            <EditText
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:inputType="text"
                android:ems="10"
                android:id="@+id/fil_npm"
                android:textColor="@android:color/black"
                android:background="@android:color/background_light"
                android:singleLine="true"
                android:gravity="center"
                android:textSize="24sp" />

            <TextView
                android:text="Username"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:id="@+id/textView2"
                android:textColor="@color/abucerah"
                android:textSize="18sp" />

            <EditText
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:inputType="text"
                android:ems="10"
                android:id="@+id/fil_uname"
                android:textColor="@android:color/black"
                android:background="@android:color/background_light"
                android:singleLine="true"
                android:gravity="center"
                android:textSize="24sp" />

            <TextView
                android:text="Password"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:id="@+id/textView3"
                android:textColor="@color/abucerah"
                android:textSize="18sp" />

            <EditText
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:inputType="textPassword"
                android:ems="10"
                android:id="@+id/fil_pass"
                android:textColor="@android:color/black"
                android:background="@android:color/background_light"
                android:singleLine="true"
                android:gravity="center"
                android:textSize="24sp" />

        </LinearLayout>

        <LinearLayout
            android:orientation="vertical"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:gravity="center">

            <Button
                android:id="@+id/button1"
                android:layout_height="wrap_content"
                android:layout_width="wrap_content"
                android:text="self_destruct" />


            <TextView
                android:text="Sudah Registrasi?"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:id="@+id/textView4" />

            <Button
                android:text="Log In"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:id="@+id/login_btn"
                android:onClick="login" />

        </LinearLayout>

    </LinearLayout>

</RelativeLayout>

--------------------------------------------------------------------------------

file Regist.java:

package com.example.novyan.app1;

import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class Regist extends AppCompatActivity {
    EditText ETnpm,ETuname,ETpass;
    String npm, uname, pass, tipe;

    /*@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_regist);

    }*/

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_regist);
        ETuname = (EditText) findViewById(R.id.fil_user);
        ETpass = (EditText) findViewById(R.id.fil_pass);
        ETnpm = (EditText) findViewById(R.id.fil_npm);


        Button button = (Button) findViewById(R.id.button1);

        button.setOnClickListener( new View.OnClickListener()
        {
            public void onClick (View v){
                next_page(v);
            }
        });
    }

    public void masukin(){
        npm = ETnpm.getText().toString();
        uname = ETuname.getText().toString();
        pass = ETpass.getText().toString();
    }

    public void next_page(View v) {

        tipe = "regist";
        Toast.makeText(getApplicationContext(), tipe, Toast.LENGTH_LONG).show();
        masukin();
        //uname = ETuname.getText().toString();
        //pass = ETpass.getText().toString();
        //tipe = "regist";

        /*BackgroundWorker bgworker = new BackgroundWorker(this);
        bgworker.execute(type, uname, pass, npm);*/
    }

    /*public void daftar (View view) {


    }*/


    public  void login (View view){
        Intent intent = new Intent(this, login.class);
        startActivity(intent);
    }
}

--------------------------------------------------------------------------------


file BackgroundWorker.java:

package com.example.novyan.app1;

import android.app.AlertDialog;
import android.app.ProgressDialog;
import android.content.Context;
import android.os.AsyncTask;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;

/**
 * Created by Novyan on 6/6/2017.
 */

class BackgroundWorker extends AsyncTask <String,String,String> {
    Context context;
    AlertDialog ad;
    String result;

    BackgroundWorker (Context ctx){
        context = ctx;
    }
    @Override
    protected String doInBackground(String... params) {
        String type = params[0];
        String login_url = "http://projeknovyan.net16.net/login.php";
        String regist_url = "http://projeknovyan.net16.net/registrasi.php";
        if(type.equals("login")){
            try {
                String username = params[1];
                String password = params[2];
                URL url = new URL(login_url);
                HttpURLConnection httpURLConnection = (HttpURLConnection)url.openConnection();
                httpURLConnection.setRequestMethod("POST");
                httpURLConnection.setDoOutput(true);
                httpURLConnection.setDoInput(true);
                OutputStream outputStream = httpURLConnection.getOutputStream();
                BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
                String post_data = URLEncoder.encode("user_android","UTF-8")+"="+URLEncoder.encode(username,"UTF-8")+"&"
                        +URLEncoder.encode("pass_user_anroid","UTF-8")+"="+URLEncoder.encode(password,"UTF-8");
                bufferedWriter.write(post_data);
                bufferedWriter.flush();
                bufferedWriter.close();
                InputStream inputStream = httpURLConnection.getInputStream();
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream, "iso-8859-1"));
                String result = "", line;
                while ((line = bufferedReader.readLine()) != null){
                    result+=line;
                }
                bufferedReader.close();
                inputStream.close();
                httpURLConnection.disconnect();
                return result;
            } catch (IOException e) {
                e.printStackTrace();
            }
        }else {
            if (type.equals("regist")) {
                try {
                    String username = params[1];
                    String password = params[2];
                    String npm = params[3];
                    URL url = new URL(regist_url);
                    HttpURLConnection httpURLConnection = (HttpURLConnection)url.openConnection();
                    httpURLConnection.setRequestMethod("POST");
                    httpURLConnection.setDoOutput(true);
                    httpURLConnection.setDoInput(true);
                    OutputStream outputStream = httpURLConnection.getOutputStream();
                    BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
                    String post_data = URLEncoder.encode("user_android","UTF-8")+"="+URLEncoder.encode(username,"UTF-8")+"&"
                            +URLEncoder.encode("pass_user_anroid","UTF-8")+"="+URLEncoder.encode(password,"UTF-8")+"&"
                            +URLEncoder.encode("npm_android","UTF-8")+"="+URLEncoder.encode(npm,"UTF-8");
                    bufferedWriter.write(post_data);
                    bufferedWriter.flush();
                    bufferedWriter.close();
                    InputStream inputStream = httpURLConnection.getInputStream();
                    BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream, "iso-8859-1"));
                    String result = "", line;
                    while ((line = bufferedReader.readLine()) != null){
                        result+=line;
                    }
                    bufferedReader.close();
                    inputStream.close();
                    httpURLConnection.disconnect();
                    return result;
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        return null;
    }


    @Override
    protected void onPreExecute() {
        ad = new AlertDialog.Builder(context).create();
        ad.setTitle("Status");
    }

    @Override
    protected void onPostExecute(String result) {
        ad.setMessage(result);
        ad.show();
    }

    @Override
    protected void onProgressUpdate(String... values) {
        super.onProgressUpdate(values);
    }
}

--------------------------------------------------------------------------------------------------------------------

file registrasi.php

<?php
require_once "conn.php";

$key= $_POST["npm_android"];
$username = $_POST["user_android"];
$userpassword = $_POST["pass_user_android"];
$qry_select = "select nama from mahasiswa where npm like $key";
$res = mysqli_query($conn, $qry_select);
if(mysqli_num_rows($res) > 0){
 $qry_select2 = "select uname_mhs from mahasiswa where npm like $key";
 $res2 = mysqli_query($conn, $qry_select2);
 if(mysqli_num_rows($res2) < 1){
  $qry = "update mahasiswa set uname_mhs = '$username', pass_mhs = '$userpassword' where npm = '$key'";
   if($conn->query($qry) === TRUE){
    echo "Registrasi Berhasil, Silahkan Login";
   }else{
    echo $conn->error;
   }
  }else{
   echo "User Telah Terdaftar, Registrasi Dibatalkan";
  }
}else{
  echo "NPM anda tidak terdaftar";
}

$conn->close();
?>