Prevent NPE when group has no default command

src/main/java/io/airlift/airline/Cli.java

@@ -32,6 +32,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 
 import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.collect.ImmutableList.toImmutableList;
@@ -86,7 +87,9 @@ private Cli(String name,
                 .map(group -> loadCommandGroup(
                         group.name,
                         group.description,
-                        loadCommand(group.defaultCommand),
+                        Optional.ofNullable(group.defaultCommand)
+                                .map(MetadataLoader::loadCommand)
+                                .orElse(null),
                         loadCommands(group.commands)))
                 .collect(toImmutableList());
 

src/main/java/io/airlift/airline/model/MetadataLoader.java

@@ -22,6 +22,7 @@
 import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.collect.ImmutableList.toImmutableList;
 import static com.google.common.collect.Streams.stream;
+import static java.util.Objects.requireNonNull;
 
 public final class MetadataLoader
 {
@@ -71,6 +72,7 @@ public static <T> ImmutableList<CommandMetadata> loadCommands(Iterable<Class<? e
 
     public static CommandMetadata loadCommand(Class<?> commandType)
     {
+        requireNonNull(commandType, "commandType is null");
         Command command = null;
         for (Class<?> cls = commandType; command == null && !Object.class.equals(cls); cls = cls.getSuperclass()) {
             command = cls.getAnnotation(Command.class);

src/test/java/io/airlift/airline/TestCli.java

@@ -0,0 +1,43 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.airlift.airline;
+
+import io.airlift.airline.Cli.CliBuilder;
+import org.testng.annotations.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+public class TestCli
+{
+    @Test
+    public void testGroupWithoutDefaultCommand()
+    {
+        CliBuilder<Runnable> builder = Cli.builder("command");
+        builder.withGroup("subcommand")
+                .withCommand(Help.class);
+
+        Cli<Runnable> cli = builder.build();
+
+        assertThatThrownBy(cli::parse)
+                .isInstanceOf(ParseCommandMissingException.class)
+                .hasMessage("No command specified");
+
+        assertThatThrownBy(() -> cli.parse("subcommand"))
+                .isInstanceOf(ParseCommandMissingException.class)
+                .hasMessage("No command specified");
+
+        assertThat(cli.parse("subcommand", "help")).isInstanceOf(Help.class);
+    }
+}