fix: #337 NPE thrown on empty command tree (#358)

Fixes #337 by making an empty command three throw NoSuchCommandException instead of NullPointerException.

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - Fix missing caption registration for the regex caption ([#351](https://github.com/Incendo/cloud/pull/351))
+- Fix NPE thrown on empty command tree ([#337](https://github.com/Incendo/cloud/issues/337))
 
 ### Changed
 - Minecraft: Support sender-aware description decorators in MinecraftHelp ([#354](https://github.com/Incendo/cloud/pull/354))

cloud-core/src/main/java/cloud/commandframework/CommandTree.java

@@ -132,6 +132,19 @@ public final class CommandTree<C> {
             final @NonNull CommandContext<C> commandContext,
             final @NonNull Queue<@NonNull String> args
     ) {
+        // Special case for empty command trees.
+        if (this.internalTree.isLeaf() && this.internalTree.value == null) {
+            return Pair.of(
+                    null,
+                    new NoSuchCommandException(
+                        commandContext.getSender(),
+                        new ArrayList<>(),
+                        this.stringOrEmpty(args.peek()
+                    )
+                )
+            );
+        }
+
         final Pair<@Nullable Command<C>, @Nullable Exception> pair = this.parseCommand(
                 new ArrayList<>(),
                 commandContext,

cloud-core/src/test/java/cloud/commandframework/issue/Issue337.java

@@ -0,0 +1,64 @@
+//
+// MIT License
+//
+// Copyright (c) 2021 Alexander Söderberg & Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+//
+package cloud.commandframework.issue;
+
+import cloud.commandframework.CommandManager;
+import cloud.commandframework.CommandTree;
+import cloud.commandframework.TestCommandSender;
+import cloud.commandframework.context.CommandContext;
+import cloud.commandframework.exceptions.NoSuchCommandException;
+import cloud.commandframework.types.tuples.Pair;
+import java.util.Arrays;
+import java.util.LinkedList;
+import java.util.Queue;
+import org.junit.jupiter.api.Test;
+
+import static cloud.commandframework.util.TestUtils.createManager;
+import static com.google.common.truth.Truth.assertThat;
+
+/**
+ * Test for https://github.com/Incendo/cloud/issues/337.
+ */
+class Issue337 {
+
+    @Test
+    void emptyCommandThreeThrowsNoSuchCommandException() {
+        // Arrange
+        final TestCommandSender commandSender = new TestCommandSender();
+        final CommandManager<TestCommandSender> commandManager = createManager();
+        final CommandTree<TestCommandSender> commandTree = CommandTree.newTree(commandManager);
+        final Queue<String> strings = new LinkedList<>(Arrays.asList("test", "this", "thing"));
+
+        // Act
+        final Pair<?, Exception> result = commandTree.parse(new CommandContext<>(commandSender, commandManager), strings);
+
+        // Assert
+        assertThat(result.getFirst()).isNull();
+        assertThat(result.getSecond()).isInstanceOf(NoSuchCommandException.class);
+
+        final NoSuchCommandException noSuchCommandException = (NoSuchCommandException) result.getSecond();
+        assertThat(noSuchCommandException.getSuppliedCommand()).isEqualTo("test");
+        assertThat(noSuchCommandException).hasCauseThat().isNull();
+    }
+}