Minor

Author: rubenada

core/src/main/java/org/apache/calcite/rel/rules/LoptOptimizeJoinRule.java

@@ -60,7 +60,6 @@
 import java.util.Map;
 import java.util.Set;
 import java.util.TreeSet;
-import java.util.function.BiFunction;
 
 import static java.util.Objects.requireNonNull;
 
@@ -955,10 +954,10 @@ private static boolean isJoinTree(RelNode rel) {
     RelOptCost costPushDown = null;
     RelOptCost costTop = null;
     if (pushDownTree != null) {
-      costPushDown = config.costFunction().apply(call, pushDownTree.getJoinTree());
+      costPushDown = config.costFunction().getCost(call, pushDownTree.getJoinTree());
     }
     if (topTree != null) {
-      costTop = config.costFunction().apply(call, topTree.getJoinTree());
+      costTop = config.costFunction().getCost(call, topTree.getJoinTree());
     }
 
     if (pushDownTree == null) {
@@ -2083,20 +2082,25 @@ private static boolean areSelfJoinKeysUnique(RelMetadataQuery mq,
         joinInfo.leftSet());
   }
 
+  /** Function to compute cost. */
+  @FunctionalInterface
+  public interface CostFunction {
+    @Nullable RelOptCost getCost(RelOptRuleCall call, RelNode relNode);
+  }
+
   /** Rule configuration. */
   @Value.Immutable
   public interface Config extends RelRule.Config {
     Config DEFAULT = ImmutableLoptOptimizeJoinRule.Config.of()
         .withOperandSupplier(b -> b.operand(MultiJoin.class).anyInputs());
 
-    /** Method to calculate intermediate cost computations. */
-    @SuppressWarnings("argument.type.incompatible")
-    @Value.Default default BiFunction<RelOptRuleCall, RelNode, RelOptCost> costFunction() {
+    /** Function to calculate intermediate cost computations. */
+    @Value.Default default CostFunction costFunction() {
       return (call, rel) -> call.getMetadataQuery().getCumulativeCost(rel);
     }
 
     /** Sets {@link #costFunction()}. */
-    Config withCostFunction(BiFunction<RelOptRuleCall, RelNode, RelOptCost> function);
+    Config withCostFunction(CostFunction function);
 
     @Override default LoptOptimizeJoinRule toRule() {
       return new LoptOptimizeJoinRule(this);