1 files changed, 87 insertions, 0 deletions
diff --git a/src/main/java/com/google/devtools/build/lib/collect/nestedset/EmptyNestedSet.java b/src/main/java/com/google/devtools/build/lib/collect/nestedset/EmptyNestedSet.java
new file mode 100644
index 0000000000..5889ba8bf1
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/collect/nestedset/EmptyNestedSet.java
@@ -0,0 +1,87 @@
+// Copyright 2014 Google Inc. All rights reserved.
+//
+// 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 com.google.devtools.build.lib.collect.nestedset;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
+
+import java.util.Iterator;
+import java.util.List;
+import java.util.Objects;
+import java.util.Set;
+
+import javax.annotation.Nullable;
+
+/**
+ * An empty nested set.
+ */
+final class EmptyNestedSet<E> extends NestedSet<E> {
+  private static final NestedSet[] EMPTY_NESTED_SET = new NestedSet[0];
+  private static final Object[] EMPTY_ELEMENTS = new Object[0];
+  private final Order order;
+
+  EmptyNestedSet(Order type) {
+    this.order = type;
+  }
+
+  @Override
+  public Iterator<E> iterator() {
+    return ImmutableList.<E>of().iterator();
+  }
+
+  @Override
+  Object[] directMembers() {
+    return EMPTY_ELEMENTS;
+  }
+
+  @Override
+  NestedSet[] transitiveSets() {
+    return EMPTY_NESTED_SET;
+  }
+
+  @Override
+  public boolean isEmpty() {
+    return true;
+  }
+
+  @Override
+  public List<E> toList() {
+    return ImmutableList.of();
+  }
+
+  @Override
+  public Set<E> toSet() {
+    return ImmutableSet.of();
+  }
+
+  @Override
+  public String toString() {
+    return "{}";
+  }
+
+  @Override
+  public Order getOrder() {
+    return order;
+  }
+
+  @Override
+  public boolean shallowEquals(@Nullable NestedSet<? extends E> other) {
+    return other != null && getOrder() == other.getOrder() && other.isEmpty();
+  }
+
+  @Override
+  public int shallowHashCode() {
+    return Objects.hash(getOrder());
+  }
+}