Getting started with GoogleCloudPlatform / Java - basic app fails

Question

I'm getting started with GoogleCloudPlatform - I want to use it from Java on my Mac.

I have followed the various installation instructions and have (presumably) all relevant material on my machine. However, launching a basic test app I get the error below.

Quite honestly don't know where to start...

bash-3.2$ mvn exec:java -DQuickstart -X
Apache Maven 3.6.3 (cecedd343002696d0abb50b32b541b8a6ba2883f)
Maven home: /opt/apache-maven
Java version: 14.0.1, vendor: Oracle Corporation, runtime: /Library/Java/JavaVirtualMachines/jdk-14.0.1.jdk/Contents/Home
Default locale: en_CH, platform encoding: UTF-8
OS name: "mac os x", version: "10.14.6", arch: "x86_64", family: "mac"
[DEBUG] Created new class realm maven.api

(... lot of intermediate trace not allowed by post cop)

[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:3.0.0:java (default-cli) on project google-api-grpc: The parameters 'mainClass' for goal org.codehaus.mojo:exec-maven-plugin:3.0.0:java are missing or invalid -> [Help 1]
org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal org.codehaus.mojo:exec-maven-plugin:3.0.0:java (default-cli) on project google-api-grpc: The parameters 'mainClass' for goal org.codehaus.mojo:exec-maven-plugin:3.0.0:java are missing or invalid
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:215)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:156)
    at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:148)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:117)
    at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:81)
    at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build (SingleThreadedBuilder.java:56)
    at org.apache.maven.lifecycle.internal.LifecycleStarter.execute (LifecycleStarter.java:128)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:305)
    at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:192)
    at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:105)
    at org.apache.maven.cli.MavenCli.execute (MavenCli.java:957)
    at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:289)
    at org.apache.maven.cli.MavenCli.main (MavenCli.java:193)
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
    at jdk.internal.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62)
    at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke (Method.java:564)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:282)
    at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:225)
    at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:406)
    at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:347)
Caused by: org.apache.maven.plugin.PluginParameterException: The parameters 'mainClass' for goal org.codehaus.mojo:exec-maven-plugin:3.0.0:java are missing or invalid
    at org.apache.maven.plugin.internal.DefaultMavenPluginManager.populatePluginFields (DefaultMavenPluginManager.java:644)