Added some VALID_NODES consants

src/jrr1g18/boring/nodes/BinaryOperationNode.java

 package jrr1g18.boring.nodes;
 
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
 /**
  * A class representing a binary operation on the AST.
  * 
@@ -10,6 +14,16 @@ public class BinaryOperationNode extends ASTNode {
     private ASTNode m_lhs;
     private ASTNode m_rhs;
 
+    private static final List<NodeType> VALID_NODES;
+    static {
+        List<NodeType> nodes = new ArrayList<>();
+        nodes.add(NodeType.ADD);
+        nodes.add(NodeType.DIV);
+        nodes.add(NodeType.MUL);
+        nodes.add(NodeType.SUB);
+        VALID_NODES = Collections.unmodifiableList(nodes);
+    }
+
     /**
      * Create a new BinaryOperationNode
      * 
@@ -34,6 +48,6 @@ public class BinaryOperationNode extends ASTNode {
 
     @Override
     public boolean isValidType(NodeType type) {
-
+        return VALID_NODES.contains(type);
     }
 }

src/jrr1g18/boring/nodes/statements/EmptyStatementNode.java

-/**
- * 
- */
 package jrr1g18.boring.nodes.statements;
 
 /**
@@ -20,7 +17,6 @@ public class EmptyStatementNode extends StatementNode {
 
     @Override
     public boolean isValidType(NodeType type) {
-
-        return super.isValidType(type);
+        return type == NodeType.EMPTY_STATEMENT;
     }
 }