Managing google cloud instances using jcloud api - google-cloud-platform

Question

I want to add and list all the instances/VM under my project in google cloud using Jcloud api. In this code, I am assuming a node to be an instance.
I have set all the variables as required and extracted the private key from the json file. The context build takes place successfully.
images = compute.listImages() => lists all the images provided by google.
nodes = compute.listNodes() =>should list nodes but instead give null pointer exception.
Output=>
No of images 246
Exception in thread "main" java.lang.NullPointerException: group
at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:229)
at org.jclouds.compute.internal.FormatSharedNamesAndAppendUniqueStringToThoseWhichRepeat.checkGroup(FormatSharedNamesAndAppendUniqueStringToThoseWhichRepeat.java:124)
at org.jclouds.compute.internal.FormatSharedNamesAndAppendUniqueStringToThoseWhichRepeat.sharedNameForGroup(FormatSharedNamesAndAppendUniqueStringToThoseWhichRepeat.java:120)
at org.jclouds.googlecomputeengine.compute.functions.FirewallTagNamingConvention$Factory.get(FirewallTagNamingConvention.java:39)
at org.jclouds.googlecomputeengine.compute.functions.InstanceToNodeMetadata.apply(InstanceToNodeMetadata.java:68)
at org.jclouds.googlecomputeengine.compute.functions.InstanceToNodeMetadata.apply(InstanceToNodeMetadata.java:43)
at com.google.common.base.Functions$FunctionComposition.apply(Functions.java:211)
at com.google.common.collect.Iterators$8.transform(Iterators.java:794)
at com.google.common.collect.TransformedIterator.next(TransformedIterator.java:48)
at com.google.common.collect.Iterators$7.computeNext(Iterators.java:646)
at com.google.common.collect.AbstractIterator.tryToComputeNext(AbstractIterator.java:143)
at com.google.common.collect.AbstractIterator.hasNext(AbstractIterator.java:138)
at com.google.common.collect.Iterators.addAll(Iterators.java:356)
at com.google.common.collect.Iterables.addAll(Iterables.java:350)
at com.google.common.collect.Sets.newLinkedHashSet(Sets.java:328)
at org.jclouds.compute.internal.BaseComputeService.listNodes(BaseComputeService.java:335)
at org.jclouds.examples.compute.basics.Example.main(Example.java:54)
package org.jclouds.examples.compute.basics;
import static com.google.common.base.Charsets.UTF_8;
import static org.jclouds.compute.config.ComputeServiceProperties.TIMEOUT_SCRIPT_COMPLETE;
import java.io.File;
import java.io.IOException;
import java.util.Properties;
import java.util.Set;
import java.util.concurrent.TimeUnit;
import org.jclouds.ContextBuilder;
import org.jclouds.compute.ComputeService;
import org.jclouds.compute.ComputeServiceContext;
import org.jclouds.compute.domain.ComputeMetadata;
import org.jclouds.compute.domain.Image;
import org.jclouds.domain.Credentials;
import org.jclouds.enterprise.config.EnterpriseConfigurationModule;
import org.jclouds.googlecloud.GoogleCredentialsFromJson;
import org.jclouds.logging.slf4j.config.SLF4JLoggingModule;
import org.jclouds.sshj.config.SshjSshClientModule;
import com.google.common.base.Supplier;
import com.google.common.collect.ImmutableSet;
import com.google.common.io.Files;
import com.google.inject.Module;
public class Example {
public static void main(String[] args)
{
String provider = "google-compute-engine";
String identity = "***#developer.gserviceaccount.com";
String credential = "path to private key file ";
credential = getCredentialFromJsonKeyFile(credential);
Properties properties = new Properties();
long scriptTimeout = TimeUnit.MILLISECONDS.convert(20, TimeUnit.MINUTES);
properties.setProperty(TIMEOUT_SCRIPT_COMPLETE, scriptTimeout + "");
Iterable<Module> modules = ImmutableSet.<Module> of(
new SshjSshClientModule(),new SLF4JLoggingModule(),
new EnterpriseConfigurationModule());
ContextBuilder builder = ContextBuilder.newBuilder(provider)
.credentials(identity, credential)
.modules(modules)
.overrides(properties);
ComputeService compute=builder.buildView(ComputeServiceContext.class).getComputeService();
Set<? extends Image> images = compute.listImages();
System.out.printf(">> No of images %d%n", images.size());
Set<? extends ComputeMetadata> nodes = compute.listNodes();
System.out.printf(">> No of nodes/instances %d%n", nodes.size());
compute.getContext().close();
}
private static String getCredentialFromJsonKeyFile(String filename) {
try {
String fileContents = Files.toString(new File(filename), UTF_8);
Supplier<Credentials> credentialSupplier = new GoogleCredentialsFromJson(fileContents);
String credential = credentialSupplier.get().credential;
return credential;
} catch (IOException e) {
System.err.println("Exception reading private key from '%s': " + filename);
e.printStackTrace();
System.exit(1);
return null;
}
}
}