Einbinden und Aufruf der Klasse WifiCommunication.java in Zusammenhang mit Eingabe, Sendebutton und Anzeige im Layout. Internet-Permission wurde dafür im Manifest eingetragen.

app/src/main/AndroidManifest.xml

 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="com.example.greenwatch">
 
+    <uses-permission android:name="android.permission.INTERNET" />
+
     <application
         android:allowBackup="true"
         android:icon="@mipmap/ic_launcher"

app/src/main/java/com/example/greenwatch/MainActivity.java

 
 import androidx.appcompat.app.AppCompatActivity;
 
+import android.annotation.SuppressLint;
 import android.os.Bundle;
+import android.view.View;
+import android.widget.Button;
+import android.widget.EditText;
+import android.widget.TextView;
+import java.net.SocketException;
 
 public class MainActivity extends AppCompatActivity {
+    WifiCommunication communication; //Member WifiCommunication
+    TextView tvReceivedData; //Test Textview um empfangene Daten anzuzeigen -> Gegen finale Textview in der Anzeige austauschen
+    //ToDo: Falls finale Textview zum Anzeigen der Nachricht anders heißt, muss das in der Klasse WifiCommunication.java angepasst werden!
+    // Wo anpassen: Receive Thread, Zeile 58
+    private EditText etSendMessage; //Test Edittext um eine Nachricht zu senden -> gegen Nachricht von den Sensoren austauschen
+    private Button btnSend; //Test Sendebutton
 
+    private String sendmsg = "default";
+
+    @SuppressLint("MissingInflatedId")
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
+
+        xmlToJava(); //Funktionsaufruf, wo die Testbuttons etc angelegt sind
+    }
+
+    public void xmlToJava(){
+        tvReceivedData = findViewById(R.id.tvRecTxt);       //Test Textview
+        etSendMessage = findViewById(R.id.sendMessage);     //Test Nachricht
+        btnSend = findViewById(R.id.btnSend);               //Test Sendebefehl
+    }
+
+    public void onClickSend(View view) throws SocketException{  //ToDo: Sendebefehl mit boolscher Variable lösen --> wo setzen?
+        sendmsg = etSendMessage.getText().toString();           //ToDo: hier String von Sensoren übergeben
+        communication.setMessage(sendmsg, true);    //übergibt zu sendenden String und setzt die entscheidende Boolsche Variable fürs Senden
+    }
+
+    @Override
+    protected void onResume() {
+        super.onResume();
+        //Erstellen des Members
+        communication = new WifiCommunication(MainActivity.this, MainActivity.this, 1234);
+    }
+
+    @Override
+    protected void onPause() {
+        super.onPause();
+        //schließt socket
+        communication.stopCommunication();
     }
 }

app/src/main/java/com/example/greenwatch/WifiCommunication.java

+package com.example.greenwatch;
+
+import android.annotation.SuppressLint;
+import android.content.Context;
+import android.widget.Toast;
+
+import java.io.IOException;
+import java.net.DatagramPacket;
+import java.net.DatagramSocket;
+import java.net.Inet4Address;
+import java.net.InetAddress;
+import java.net.NetworkInterface;
+import java.net.SocketException;
+import java.net.UnknownHostException;
+import java.util.Enumeration;
+
+public class WifiCommunication {
+    private final MainActivity mainActivity;
+    private final InetAddress address;
+    private final int port;
+    private final Context context;
+    private boolean newMsg;
+    private String rxString;
+    private String sendMsg;
+    //private String sendMsg = "default";
+    private final DatagramSocket socket;
+    volatile private boolean running;
+    @SuppressLint("SetTextI18n")
+    public WifiCommunication(MainActivity mainActivity, Context context, int port) {
+        this.mainActivity = mainActivity;
+        this.port = port;
+        this.context = context;
+        try {
+            socket = new DatagramSocket(this.port);
+            socket.setBroadcast(true);
+            address = InetAddress.getByName("255.255.255.255");
+            running = true;
+            new ReceiveThread().start();
+            new SendThread().start();
+        } catch (SocketException | UnknownHostException e) {
+            throw new RuntimeException(e);
+        }
+        mainActivity.runOnUiThread(() -> Toast.makeText(context,"Communication running", Toast.LENGTH_SHORT).show());
+    }
+    private class ReceiveThread extends Thread {
+        private String rxString = "";
+        private String previousRxString = "";
+        @Override
+        public void run() {
+            try {
+                do {
+
+                    byte[] rxBuffer = new byte[1024];
+                    DatagramPacket packet = new DatagramPacket(rxBuffer, rxBuffer.length);
+                    socket.receive(packet);
+                    rxString = new String(packet.getData(), 0, packet.getLength());
+
+                    if(!previousRxString.equals(rxString))
+                        mainActivity.runOnUiThread(() -> mainActivity.tvReceivedData.setText(rxString));
+
+                    previousRxString = rxString;
+
+                } while (running);
+            }
+            catch (IOException e) {
+                e.printStackTrace();
+            }
+        }
+    }
+
+    public String showMessage(){
+        return rxString;
+    }
+    public void setMessage(String message, boolean newMsg){
+        this.sendMsg=message;
+        this.newMsg = newMsg;
+    }
+    private class SendThread extends Thread {
+        private int tmpCnt = 0;
+        @Override
+        public void run() {
+            try {
+                do {
+
+                    if(newMsg == true)
+                    {
+                        newMsg = false;
+
+                        String txString = getLocalIpAddress() + " sends. #" + tmpCnt++ + sendMsg;
+                        byte[] txBuffer = txString.getBytes();
+
+                        DatagramPacket txPacket = new DatagramPacket(txBuffer, txBuffer.length, address, port);
+
+                        for(int i = 0; i < 20; i++) {
+                            socket.send(txPacket);
+                        }
+                    }
+                } while (running);
+            } catch (IOException e) {
+                e.printStackTrace();
+            }
+        }
+    }
+    public static String getLocalIpAddress() {
+        try {
+            for (Enumeration<NetworkInterface> en = NetworkInterface.getNetworkInterfaces(); en.hasMoreElements();) {
+                NetworkInterface networkInterface = (NetworkInterface) ((Enumeration<?>) en).nextElement();
+                for (Enumeration<InetAddress> addresses = networkInterface.getInetAddresses(); addresses.hasMoreElements();) {
+                    InetAddress inetAddress = addresses.nextElement();
+                    if (!inetAddress.isLoopbackAddress() && inetAddress instanceof Inet4Address) {
+                        return inetAddress.getHostAddress();
+                    }
+                }
+            }
+        } catch (SocketException ex) {
+            ex.printStackTrace();
+        }
+        return null;
+    }
+    public void stopCommunication() {
+        running = false;
+        socket.close();
+    }
+}

app/src/main/res/layout/activity_main.xml

     tools:context=".MainActivity">
 
     <TextView
+        android:id="@+id/tvRecTxt"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
-        android:text="Hello World!"
-        app:layout_constraintBottom_toBottomOf="parent"
-        app:layout_constraintLeft_toLeftOf="parent"
-        app:layout_constraintRight_toRightOf="parent"
-        app:layout_constraintTop_toTopOf="parent" />
+        android:text="Received Message"
+        app:layout_constraintTop_toBottomOf="@id/sendMessage"
+        app:layout_constraintStart_toStartOf="parent"/>
+
+    <EditText
+        android:id="@+id/sendMessage"
+        android:layout_width="wrap_content"
+        android:layout_height="50dp"
+        android:ems="10"
+        android:inputType="textPersonName"
+        android:text="Send Message"
+        app:layout_constraintTop_toTopOf="parent"
+        app:layout_constraintStart_toStartOf="parent"/>
+
+    <Button
+        android:id="@+id/btnSend"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:text="Send"
+        android:onClick="onClickSend"
+        app:layout_constraintTop_toTopOf="parent"
+        app:layout_constraintStart_toEndOf="@id/sendMessage"/>
 
 </androidx.constraintlayout.widget.ConstraintLayout>