Cleanup some pmd-scala methods

* Remove ScalaNode#childrenAccept: this only makes stack traces longer.
Besides, the node doesn't know how to accumulate values, so it's useless.
* Specialize return type of some methods of the interface.

pmd-scala/src/main/java/net/sourceforge/pmd/lang/scala/ast/AbstractScalaNode.java

@@ -39,18 +39,18 @@ abstract class AbstractScalaNode<T extends Tree> extends AbstractNode implements
     public abstract <D, R> R accept(ScalaParserVisitor<D, R> visitor, D data);
 
     @Override
-    public <D, R> R childrenAccept(ScalaParserVisitor<D, R> visitor, D data) {
-        int numChildren = jjtGetNumChildren();
-        R returnValue = null;
-        for (int i = 0; i < numChildren; ++i) {
-            returnValue = ((ScalaNode<?>) jjtGetChild(i)).accept(visitor, data);
-        }
-        return returnValue;
+    public T getNode() {
+        return node;
     }
 
     @Override
-    public T getNode() {
-        return node;
+    public ScalaNode<?> jjtGetChild(int index) {
+        return (ScalaNode<?>) super.jjtGetChild(index);
+    }
+
+    @Override
+    public ScalaNode<?> jjtGetParent() {
+        return (ScalaNode<?>) super.jjtGetParent();
     }
 
     @Override

pmd-scala/src/main/java/net/sourceforge/pmd/lang/scala/ast/ScalaNode.java

@@ -31,21 +31,6 @@ public interface ScalaNode<T extends Tree> extends Node {
      */
     <D, R> R accept(ScalaParserVisitor<D, R> visitor, D data);
 
-    /**
-     * Accept the visitor against all children of this node if there are any and
-     * return.
-     * 
-     * @param <D>
-     *            The type of the data input
-     * @param <R>
-     *            The type of the returned data
-     * @param visitor
-     *            the visitor to visit this node's children with
-     * @param data
-     *            context-specific data to pass along
-     * @return context-specific data for this Visitor pattern
-     */
-    <D, R> R childrenAccept(ScalaParserVisitor<D, R> visitor, D data);
 
     /**
      * Get the underlying Scala Node.
@@ -53,4 +38,12 @@ public interface ScalaNode<T extends Tree> extends Node {
      * @return the Scala Node for this node
      */
     T getNode();
+
+
+    @Override
+    ScalaNode<?> jjtGetChild(int idx);
+
+
+    @Override
+    ScalaNode<?> jjtGetParent();
 }

pmd-scala/src/main/java/net/sourceforge/pmd/lang/scala/ast/ScalaParserVisitorAdapter.java

@@ -35,9 +35,23 @@ import scala.meta.Type;
  */
 public class ScalaParserVisitorAdapter<D, R> implements ScalaParserVisitor<D, R> {
 
+    /** Initial value when combining values returned by children. */
+    protected R zero() {
+        return null;
+    }
+
+    /** Merge two values of type R, used to combine values returned by children. */
+    protected R combine(R acc, R r) {
+        return r;
+    }
+
     @Override
     public R visit(ScalaNode<?> node, D data) {
-        return node.childrenAccept(this, data);
+        R returnValue = zero();
+        for (int i = 0; i < node.jjtGetNumChildren(); ++i) {
+            returnValue = combine(returnValue, node.jjtGetChild(i).accept(this, data));
+        }
+        return returnValue;
     }
 
     @Override

pmd-scala/src/main/java/net/sourceforge/pmd/lang/scala/rule/ScalaRule.java

@@ -186,7 +186,10 @@ public class ScalaRule extends AbstractRule implements ScalaParserVisitor<RuleCo
 
     @Override
     public RuleContext visit(ScalaNode<?> node, RuleContext data) {
-        return node.childrenAccept(this, data);
+        for (int i = 0; i < node.jjtGetNumChildren(); ++i) {
+            node.jjtGetChild(i).accept(this, data);
+        }
+        return data;
     }
 
     @Override