Make container groups list collections rather than iterables.

java-compiler-testing/src/main/java/io/github/ascopes/jct/compilers/impl/JctCompilationFactoryImpl.java

@@ -30,7 +30,7 @@
 import java.io.IOException;
 import java.io.OutputStreamWriter;
 import java.util.Collection;
-import java.util.LinkedHashSet;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
 import javax.annotation.Nullable;
@@ -158,13 +158,10 @@ private Set<JavaFileObject> findCompilationUnits(JctFileManager fileManager) thr
       );
     }
 
-    var objects = new LinkedHashSet<JavaFileObject>();
+    var objects = new HashSet<JavaFileObject>();
 
     for (var location : locations) {
-      var items = fileManager.list(location, "", Set.of(Kind.SOURCE), true);
-      for (var fileObject : items) {
-        objects.add(fileObject);
-      }
+      objects.addAll(fileManager.list(location, "", Set.of(Kind.SOURCE), true));
     }
 
     if (objects.isEmpty()) {

java-compiler-testing/src/main/java/io/github/ascopes/jct/containers/PackageContainerGroup.java

@@ -193,13 +193,11 @@ public interface PackageContainerGroup extends ContainerGroup {
    * @param kinds       the kinds of file to look for.
    * @param recurse     {@code true} to recurse subpackages, {@code false} to only consider the
    *                    given package.
-   * @param collection  the collection to fill.
    * @throws IOException if the file lookup fails due to an IO exception.
    */
-  void listFileObjects(
+  Set<JavaFileObject> listFileObjects(
       String packageName,
       Set<? extends Kind> kinds,
-      boolean recurse,
-      Collection<JavaFileObject> collection
+      boolean recurse
   ) throws IOException;
 }

java-compiler-testing/src/main/java/io/github/ascopes/jct/containers/impl/AbstractPackageContainerGroup.java

@@ -28,7 +28,7 @@
 import java.io.IOException;
 import java.nio.file.Path;
 import java.util.ArrayList;
-import java.util.Collection;
+import java.util.HashSet;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Locale;
@@ -259,15 +259,17 @@ public boolean isEmpty() {
   }
 
   @Override
-  public void listFileObjects(
+  public Set<JavaFileObject> listFileObjects(
       String packageName,
       Set<? extends Kind> kinds,
-      boolean recurse,
-      Collection<JavaFileObject> collection
+      boolean recurse
   ) throws IOException {
+    // XXX: could this be run in parallel?
+    var collection = new HashSet<JavaFileObject>();
     for (var container : containers) {
       container.listFileObjects(packageName, kinds, recurse, collection);
     }
+    return collection;
   }
 
   /**

java-compiler-testing/src/main/java/io/github/ascopes/jct/filemanagers/JctFileManager.java

@@ -19,10 +19,14 @@
 import io.github.ascopes.jct.containers.OutputContainerGroup;
 import io.github.ascopes.jct.containers.PackageContainerGroup;
 import io.github.ascopes.jct.workspaces.PathRoot;
+import java.io.IOException;
 import java.util.Collection;
+import java.util.Set;
 import javax.annotation.Nullable;
 import javax.annotation.concurrent.ThreadSafe;
 import javax.tools.JavaFileManager;
+import javax.tools.JavaFileObject;
+import javax.tools.JavaFileObject.Kind;
 import org.apiguardian.api.API;
 import org.apiguardian.api.API.Status;
 
@@ -120,4 +124,25 @@ public interface JctFileManager extends JavaFileManager {
    * @return the output container impl.
    */
   Collection<OutputContainerGroup> getOutputContainerGroups();
+
+  /**
+   * List all file objects in the given location matching the given criteria.
+   *
+   * @param location    the location to search in.
+   * @param packageName the package name to search in, or {@code ""} to search in the root
+   *                    location.
+   * @param kinds       the kinds of file to return, or
+   *                    {@link Set#of} {@code (} {@link Kind#OTHER} {@code )} to find all files.
+   * @param recurse     {@code true} to recurse into subpackages, {@code false} to only check the
+   *                    current package.
+   * @return a collection of unique file objects that were found.
+   * @throws IOException if an IO error occurs reading the file system.
+   */
+  @Override
+  Set<JavaFileObject> list(
+      Location location,
+      String packageName,
+      Set<Kind> kinds,
+      boolean recurse
+  ) throws IOException;
 }

java-compiler-testing/src/main/java/io/github/ascopes/jct/filemanagers/impl/JctFileManagerImpl.java

@@ -34,6 +34,7 @@
 import java.lang.module.ModuleFinder;
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.HashSet;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
@@ -440,7 +441,7 @@ public int isSupportedOption(String option) {
   }
 
   @Override
-  public Iterable<JavaFileObject> list(
+  public Set<JavaFileObject> list(
       Location location,
       String packageName,
       Set<Kind> kinds,
@@ -449,13 +450,9 @@ public Iterable<JavaFileObject> list(
 
     var group = getExistingPackageOrientedOrOutputGroup(location);
 
-    if (group == null) {
-      return List.of();
-    }
-
-    var files = new ArrayList<JavaFileObject>();
-    group.listFileObjects(packageName, kinds, recurse, files);
-    return files;
+    return group == null
+        ? Set.of()
+        : group.listFileObjects(packageName, kinds, recurse);
   }
 
   @Override