added MqttUtils class

common/build.gradle

@@ -28,6 +28,8 @@ dependencies {
     implementation fileTree(dir: 'libs', include: ['*.jar'])
 
     implementation 'com.android.support:appcompat-v7:26.1.0'
+    // https://mvnrepository.com/artifact/commons-lang/commons-lang
+    implementation group: 'commons-lang', name: 'commons-lang', version: '2.6'
     testImplementation 'junit:junit:4.12'
     androidTestImplementation 'com.android.support.test:runner:1.0.2'
     androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'

common/src/main/java/de/tudresden/inf/st/sensorsharing/common/MqttUtils.java

+package de.tudresden.inf.st.sensorsharing.common;
+
+import android.os.Build;
+
+import org.apache.commons.lang.RandomStringUtils;
+
+public class MqttUtils {
+
+    public static String createClientId() {
+        return String.format("%s-%s", Build.DEVICE, RandomStringUtils.randomAlphabetic(6));
+    }
+}