Create a separate interface for validating required fields in a builder

policy/src/main/java/dev/cel/policy/BUILD.bazel

@@ -38,6 +38,7 @@ java_library(
     tags = [
     ],
     deps = [
+        ":required_fields_checker",
         ":source",
         "//:auto_value",
         "@maven//:com_google_errorprone_error_prone_annotations",
@@ -89,6 +90,18 @@ java_library(
     ],
 )
 
+java_library(
+    name = "required_fields_checker",
+    srcs = [
+        "RequiredFieldsChecker.java",
+    ],
+    visibility = ["//visibility:private"],
+    deps = [
+        "//:auto_value",
+        "@maven//:com_google_guava_guava",
+    ],
+)
+
 java_library(
     name = "parser_context",
     srcs = [

policy/src/main/java/dev/cel/policy/CelPolicyConfig.java

@@ -15,15 +15,12 @@
 package dev.cel.policy;
 
 import static com.google.common.base.Preconditions.checkNotNull;
-import static com.google.common.collect.ImmutableList.toImmutableList;
-import static java.util.Arrays.stream;
 
 import com.google.auto.value.AutoValue;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.errorprone.annotations.CheckReturnValue;
-import java.util.AbstractMap;
 import java.util.Arrays;
 import java.util.Optional;
 
@@ -108,7 +105,7 @@ public abstract static class VariableDecl {
 
     /** Builder for {@link VariableDecl}. */
     @AutoValue.Builder
-    public abstract static class Builder {
+    public abstract static class Builder implements RequiredFieldsChecker {
 
       abstract Optional<String> name();
 
@@ -118,10 +115,10 @@ public abstract static class Builder {
 
       public abstract Builder setType(TypeDecl typeDecl);
 
-      ImmutableList<String> getMissingRequiredFieldNames() {
-        return getMissingRequiredFields(
-            new AbstractMap.SimpleEntry<>("name", name()),
-            new AbstractMap.SimpleEntry<>("type", type()));
+      @Override
+      public ImmutableList<RequiredField> requiredFields() {
+        return ImmutableList.of(
+            RequiredField.of("name", this::name), RequiredField.of("type", this::type));
       }
 
       /** Builds a new instance of {@link VariableDecl}. */
@@ -148,7 +145,7 @@ public abstract static class FunctionDecl {
 
     /** Builder for {@link FunctionDecl}. */
     @AutoValue.Builder
-    public abstract static class Builder {
+    public abstract static class Builder implements RequiredFieldsChecker {
 
       abstract Optional<String> name();
 
@@ -158,10 +155,10 @@ public abstract static class Builder {
 
       public abstract Builder setOverloads(ImmutableSet<OverloadDecl> overloads);
 
-      ImmutableList<String> getMissingRequiredFieldNames() {
-        return getMissingRequiredFields(
-            new AbstractMap.SimpleEntry<>("name", name()),
-            new AbstractMap.SimpleEntry<>("overloads", overloads()));
+      @Override
+      public ImmutableList<RequiredField> requiredFields() {
+        return ImmutableList.of(
+            RequiredField.of("name", this::name), RequiredField.of("overloads", this::overloads));
       }
 
       /** Builds a new instance of {@link FunctionDecl}. */
@@ -200,7 +197,7 @@ public abstract static class OverloadDecl {
 
     /** Builder for {@link OverloadDecl}. */
     @AutoValue.Builder
-    public abstract static class Builder {
+    public abstract static class Builder implements RequiredFieldsChecker {
 
       abstract Optional<String> id();
 
@@ -228,10 +225,10 @@ public Builder addArguments(TypeDecl... args) {
 
       public abstract Builder setReturnType(TypeDecl returnType);
 
-      ImmutableList<String> getMissingRequiredFieldNames() {
-        return getMissingRequiredFields(
-            new AbstractMap.SimpleEntry<>("id", id()),
-            new AbstractMap.SimpleEntry<>("return", returnType()));
+      @Override
+      public ImmutableList<RequiredField> requiredFields() {
+        return ImmutableList.of(
+            RequiredField.of("id", this::id), RequiredField.of("return", this::returnType));
       }
 
       /** Builds a new instance of {@link OverloadDecl}. */
@@ -259,7 +256,7 @@ public abstract static class TypeDecl {
 
     /** Builder for {@link TypeDecl}. */
     @AutoValue.Builder
-    public abstract static class Builder {
+    public abstract static class Builder implements RequiredFieldsChecker {
 
       abstract Optional<String> name();
 
@@ -283,8 +280,9 @@ public Builder addParams(Iterable<TypeDecl> params) {
 
       public abstract Builder setIsTypeParam(boolean isTypeParam);
 
-      ImmutableList<String> getMissingRequiredFieldNames() {
-        return getMissingRequiredFields(new AbstractMap.SimpleEntry<>("type_name", name()));
+      @Override
+      public ImmutableList<RequiredField> requiredFields() {
+        return ImmutableList.of(RequiredField.of("type_name", this::name));
       }
 
       @CheckReturnValue
@@ -319,7 +317,7 @@ public abstract static class ExtensionConfig {
 
     /** Builder for {@link ExtensionConfig}. */
     @AutoValue.Builder
-    public abstract static class Builder {
+    public abstract static class Builder implements RequiredFieldsChecker {
 
       abstract Optional<String> name();
 
@@ -329,8 +327,9 @@ public abstract static class Builder {
 
       public abstract Builder setVersion(Integer version);
 
-      ImmutableList<String> getMissingRequiredFieldNames() {
-        return getMissingRequiredFields(new AbstractMap.SimpleEntry<>("name", name()));
+      @Override
+      public ImmutableList<RequiredField> requiredFields() {
+        return ImmutableList.of(RequiredField.of("name", this::name));
       }
 
       /** Builds a new instance of {@link ExtensionConfig}. */
@@ -352,13 +351,4 @@ public static ExtensionConfig of(String name, int version) {
       return newBuilder().setName(name).setVersion(version).build();
     }
   }
-
-  @SafeVarargs
-  private static ImmutableList<String> getMissingRequiredFields(
-      AbstractMap.SimpleEntry<String, Optional<?>>... requiredFields) {
-    return stream(requiredFields)
-        .filter(entry -> !entry.getValue().isPresent())
-        .map(AbstractMap.SimpleEntry::getKey)
-        .collect(toImmutableList());
-  }
 }

policy/src/main/java/dev/cel/policy/RequiredFieldsChecker.java

@@ -0,0 +1,49 @@
+// Copyright 2024 Google LLC
+//
+// 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
+//
+//      https://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 dev.cel.policy;
+
+import static com.google.common.collect.ImmutableList.toImmutableList;
+
+import com.google.auto.value.AutoValue;
+import com.google.common.collect.ImmutableList;
+import java.util.Optional;
+import java.util.function.Supplier;
+
+/**
+ * Interface to be implemented on a builder that can be used to verify all required fields being
+ * set.
+ */
+interface RequiredFieldsChecker {
+
+  ImmutableList<RequiredField> requiredFields();
+
+  default ImmutableList<String> getMissingRequiredFieldNames() {
+    return requiredFields().stream()
+        .filter(entry -> !entry.fieldValue().get().isPresent())
+        .map(RequiredField::displayName)
+        .collect(toImmutableList());
+  }
+
+  @AutoValue
+  abstract class RequiredField {
+    abstract String displayName();
+
+    abstract Supplier<Optional<?>> fieldValue();
+
+    static RequiredField of(String displayName, Supplier<Optional<?>> fieldValue) {
+      return new AutoValue_RequiredFieldsChecker_RequiredField(displayName, fieldValue);
+    }
+  }
+}