[ggj][ast][engx] fix: add node validator to refactor/centralize null element checks

src/main/java/com/google/api/generator/engine/ast/MethodInvocationExpr.java

@@ -19,7 +19,6 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
-import java.util.Objects;
 import javax.annotation.Nullable;
 
 @AutoValue
@@ -117,17 +116,15 @@ public MethodInvocationExpr build() {
               || methodInvocationExpr.staticReferenceType() == null,
           "Only the expression reference or the static reference can be set, not both");
 
-      Preconditions.checkState(
-          methodInvocationExpr.arguments().stream().allMatch(e -> !Objects.isNull(e)),
-          String.format(
-              "Found null expression in arguments %s for %s",
-              methodInvocationExpr.arguments(), methodInvocationExpr.methodIdentifier().name()));
+      NodeValidator.checkNoNullElements(
+          methodInvocationExpr.arguments(),
+          "arguments",
+          String.format("method invocation of %s", methodInvocationExpr.methodIdentifier().name()));
 
-      Preconditions.checkState(
-          methodInvocationExpr.generics().stream().allMatch(e -> !Objects.isNull(e)),
-          String.format(
-              "Found null expression in generics %s for %s",
-              methodInvocationExpr.generics(), methodInvocationExpr.methodIdentifier().name()));
+      NodeValidator.checkNoNullElements(
+          methodInvocationExpr.generics(),
+          "generics",
+          String.format("method invocation of %s", methodInvocationExpr.methodIdentifier().name()));
 
       return methodInvocationExpr;
     }

src/main/java/com/google/api/generator/engine/ast/NewObjectExpr.java

@@ -20,7 +20,6 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
-import java.util.Objects;
 
 @AutoValue
 public abstract class NewObjectExpr implements Expr {
@@ -70,10 +69,8 @@ public NewObjectExpr build() {
       Preconditions.checkState(
           TypeNode.isReferenceType(type()), "New object expression should be reference types.");
 
-      Preconditions.checkState(
-          arguments().stream().allMatch(e -> !Objects.isNull(e)),
-          String.format(
-              "Found null argument for the \"new\" constructor of %s", type().reference().name()));
+      NodeValidator.checkNoNullElements(
+          arguments(), "the \"new\" constructor", type().reference().name());
 
       // Only the case where generics() is empty and isGeneric() is false, we set isGeneric() to
       // false. Otherwise, isGeneric() should be true.

src/main/java/com/google/api/generator/engine/ast/NodeValidator.java

@@ -0,0 +1,29 @@
+// Copyright 2020 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
+//
+//      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.api.generator.engine.ast;
+
+import com.google.common.base.Preconditions;
+import java.util.Collection;
+import java.util.Objects;
+
+public class NodeValidator {
+  public static void checkNoNullElements(
+      Collection<?> collection, String fieldTypeName, String nodeContextInfo) {
+    Preconditions.checkState(
+        collection.stream().allMatch(e -> !Objects.isNull(e)),
+        String.format(
+            "Found null expression in %s %s for %s", fieldTypeName, collection, nodeContextInfo));
+  }
+}

src/main/java/com/google/api/generator/engine/ast/ReferenceConstructorExpr.java

@@ -20,7 +20,6 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
-import java.util.Objects;
 
 @AutoValue
 public abstract class ReferenceConstructorExpr implements Expr {
@@ -77,11 +76,8 @@ public ReferenceConstructorExpr build() {
           referenceConstructorExpr.type().isReferenceType(referenceConstructorExpr.type()),
           "A this/super constructor must have a reference type.");
 
-      Preconditions.checkState(
-          arguments().stream().allMatch(e -> !Objects.isNull(e)),
-          String.format(
-              "Found null argumentfor in the \"this/super\" initialization of %s",
-              type().reference().name()));
+      NodeValidator.checkNoNullElements(
+          arguments(), "the \"this\" or \"super\" initialization", type().reference().name());
 
       return referenceConstructorExpr;
     }