Summary

Upgrade existing one-arg visit methods on the visitor interfaces to be default methods.

Problem

When visitModule as added to ElementVisitor (JDK-8142968), it was added as a normal abstract interface method. This approach was necessary in analogous cases back in Java SE 8 since the package in question is built under the previously JDK during the bootstrap process. For 9, better source compatibility would come from using a default method here.

Likewise, several convenience methods in the visitor interfaces can be upgraded to default methods as a small convenience for those directly implementing the interfaces.

Solution

Use default methods where appropriate.

Specification

--- old/src/java.compiler/share/classes/javax/lang/model/element/AnnotationValueVisitor.java    2017-01-18 14:27:09.971342501 -0800
+++ new/src/java.compiler/share/classes/javax/lang/model/element/AnnotationValueVisitor.java    2017-01-18 14:27:09.875342503 -0800
@@ -68,8 +68,8 @@
  * javax.lang.model.*} packages bundled in Java SE 8 were required to
  * also be runnable on Java SE 7.  Therefore, default methods
  * were <em>not</em> used when extending {@code javax.lang.model.*}
- * to cover Java SE 8 language features.  However, default methods may
- * be used in subsequent revisions of the {@code javax.lang.model.*}
+ * to cover Java SE 8 language features.  However, default methods
+ * are used in subsequent revisions of the {@code javax.lang.model.*}
  * packages that are only required to run on Java SE 8 and higher
  * platform versions.
  *
@@ -90,11 +90,16 @@
     R visit(AnnotationValue av, P p);

     /**
-     * A convenience method equivalent to {@code v.visit(av, null)}.
+     * A convenience method equivalent to {@code visit(av, null)}.
+     *
+     * @implSpec The default implementation is {@code visit(av, null)}.
+     *
      * @param av the value to visit
      * @return  a visitor-specified result
      */
-    R visit(AnnotationValue av);
+    default R visit(AnnotationValue av) {
+        return visit(av, null);
+    }

     /**
      * Visits a {@code boolean} value in an annotation.
--- old/src/java.compiler/share/classes/javax/lang/model/element/ElementVisitor.java    2017-01-18 14:27:10.371342494 -0800
+++ new/src/java.compiler/share/classes/javax/lang/model/element/ElementVisitor.java    2017-01-18 14:27:10.235342496 -0800
@@ -59,8 +59,8 @@
  * javax.lang.model.*} packages bundled in Java SE 8 were required to
  * also be runnable on Java SE 7.  Therefore, default methods
  * were <em>not</em> used when extending {@code javax.lang.model.*}
- * to cover Java SE 8 language features.  However, default methods may
- * be used in subsequent revisions of the {@code javax.lang.model.*}
+ * to cover Java SE 8 language features.  However, default methods
+ * are used in subsequent revisions of the {@code javax.lang.model.*}
  * packages that are only required to run on Java SE 8 and higher
  * platform versions.
  *
@@ -85,11 +85,16 @@
     R visit(Element e, P p);

     /**
-     * A convenience method equivalent to {@code v.visit(e, null)}.
+     * A convenience method equivalent to {@code visit(e, null)}.
+     *
+     * @implSpec The default implementation is {@code visit(e, null)}.
+     *
      * @param e  the element to visit
      * @return a visitor-specified result
      */
-    R visit(Element e);
+    default R visit(Element e) {
+        return visit(e, null);
+    }

     /**
      * Visits a package element.
@@ -146,10 +151,16 @@

     /**
      * Visits a module element.
+     *
+     * @implSpec Visits a {@code ModuleElement} by calling {@code
+     * visitUnknown(e, p)}.
+     *
      * @param e  the element to visit
      * @param p  a visitor-specified parameter
      * @return a visitor-specified result
      * @since 9
      */
-    R visitModule(ModuleElement e, P p);
+    default R visitModule(ModuleElement e, P p) {
+        return visitUnknown(e, p);
+    }
 }
--- old/src/java.compiler/share/classes/javax/lang/model/type/TypeVisitor.java  2017-01-18 14:27:10.719342487 -0800
+++ new/src/java.compiler/share/classes/javax/lang/model/type/TypeVisitor.java  2017-01-18 14:27:10.603342489 -0800
@@ -59,8 +59,8 @@
  * javax.lang.model.*} packages bundled in Java SE 8 were required to
  * also be runnable on Java SE 7.  Therefore, default methods
  * were <em>not</em> used when extending {@code javax.lang.model.*}
- * to cover Java SE 8 language features.  However, default methods may
- * be used in subsequent revisions of the {@code javax.lang.model.*}
+ * to cover Java SE 8 language features.  However, default methods
+ * are used in subsequent revisions of the {@code javax.lang.model.*}
  * packages that are only required to run on Java SE 8 and higher
  * platform versions.
  *
@@ -85,11 +85,16 @@
     R visit(TypeMirror t, P p);

     /**
-     * A convenience method equivalent to {@code v.visit(t, null)}.
+     * A convenience method equivalent to {@code visit(t, null)}.
+     *
+     * @implSpec The default implementation is {@code visit(t, null)}.
+     *
      * @param t the element to visit
      * @return  a visitor-specified result
      */
-    R visit(TypeMirror t);
+    default R visit(TypeMirror t) {
+        return visit(t, null);
+    }

     /**
      * Visits a primitive type.
--- old/src/java.compiler/share/classes/javax/lang/model/util/AbstractElementVisitor6.java  2017-01-18 14:27:11.059342480 -0800
+++ new/src/java.compiler/share/classes/javax/lang/model/util/AbstractElementVisitor6.java  2017-01-18 14:27:10.963342482 -0800
@@ -139,6 +139,7 @@
      */
     @Override
     public R visitModule(ModuleElement e, P p) {
-        return visitUnknown(e, p);
+        // Use implementation from interface default method
+        return ElementVisitor.super.visitModule(e, p);
     }
 }