[ggj][ast][engx] fix: validate non-null elements for MethodDefinition (…

…#463)

* fix: swap assertEquals args in JavaWriterVisitorTest to match (expected, actusl) order

* fix: swap assertEquals args in ImportWriterVisitorTest to match (expected, actusl) order

* fix: add node validator to refactor/centralize null element checks

* fix: validate non-null elements for IfStatement

* fix: validate non-null elements for AnonymousClassExpr

* fix: validate non-null elements for BlockStatement

* fix: validate non-null elements for ClassDefinition

* fix: validate non-null elements for ConcreteReference

* fix: validate non-null elements for ForStatement

* fix: validate non-null elements for GeneralForStatement

* fix: validate non-null elements for MethodDefinition

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

@@ -167,11 +167,14 @@ public Builder setReturnExpr(Expr expr) {
     abstract Builder setMethodIdentifier(IdentifierNode methodIdentifier);
 
     // Private accessors.
-
     abstract String name();
 
+    abstract ImmutableList<CommentStatement> headerCommentStatements();
+
     abstract ImmutableList<AnnotationNode> annotations();
 
+    abstract ImmutableList<TypeNode> throwsExceptions();
+
     abstract TypeNode returnType();
 
     abstract boolean isOverride();
@@ -195,6 +198,8 @@ public Builder setReturnExpr(Expr expr) {
     abstract ImmutableList<String> returnTemplateNames();
 
     public MethodDefinition build() {
+      performNullChecks();
+
       // Handle templates.
       setTemplateIdentifiers(
           templateNames().stream()
@@ -329,5 +334,16 @@ public MethodDefinition build() {
 
       return method;
     }
+
+    void performNullChecks() {
+      String contextInfo = String.format("method definition of %s", name());
+      NodeValidator.checkNoNullElements(headerCommentStatements(), "header comments", contextInfo);
+      NodeValidator.checkNoNullElements(annotations(), "annotations", contextInfo);
+      NodeValidator.checkNoNullElements(throwsExceptions(), "declared exceptions", contextInfo);
+      NodeValidator.checkNoNullElements(body(), "body", contextInfo);
+      NodeValidator.checkNoNullElements(templateNames(), "template names", contextInfo);
+      NodeValidator.checkNoNullElements(
+          returnTemplateNames(), "return template names", contextInfo);
+    }
   }
 }