Add FilterExpressionVisitor SPI with accept() on FilterExpression

Introduces a visitor interface for traversing the sealed FilterExpression
hierarchy. Since FilterExpression permits exactly 5 subtypes, visitor
implementations are compile-time complete — all expression types must
be handled.

Adds accept(FilterExpressionVisitor<R>) default method on
FilterExpression that dispatches to the appropriate visit() method.
This replaces the instanceof chain pattern needed by non-in-memory
backends (LDAP, SQL, etc.) for filter-to-query translation.

Generated-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bdemers

scim-spec/scim-spec-schema/src/main/java/org/apache/directory/scim/spec/filter/FilterExpression.java

@@ -34,4 +34,21 @@ public sealed interface FilterExpression extends Serializable
   default <U> U map(Function<? super FilterExpression, U> mapper) {
     return mapper.apply(this);
   }
+
+  /**
+   * Dispatches to the appropriate {@link FilterExpressionVisitor} method based on
+   * this expression's concrete type.
+   *
+   * @param visitor the visitor to dispatch to
+   * @param <R>     the result type
+   * @return the result of visiting this expression
+   */
+  default <R> R accept(FilterExpressionVisitor<R> visitor) {
+    if (this instanceof AttributeComparisonExpression e) return visitor.visit(e);
+    if (this instanceof AttributePresentExpression e) return visitor.visit(e);
+    if (this instanceof LogicalExpression e) return visitor.visit(e);
+    if (this instanceof GroupExpression e) return visitor.visit(e);
+    if (this instanceof ValuePathExpression e) return visitor.visit(e);
+    throw new IllegalStateException("Unknown FilterExpression type: " + getClass().getName());
+  }
 }

scim-spec/scim-spec-schema/src/main/java/org/apache/directory/scim/spec/filter/FilterExpressionVisitor.java

@@ -0,0 +1,87 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+
+ * http://www.apache.org/licenses/LICENSE-2.0
+
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.directory.scim.spec.filter;
+
+/**
+ * Visitor interface for traversing a SCIM {@link FilterExpression} tree.
+ *
+ * <p>Since {@link FilterExpression} is a sealed interface with exactly five permitted
+ * subtypes, implementations of this visitor are guaranteed to handle all possible
+ * expression types at compile time.</p>
+ *
+ * <p>Usage: call {@link FilterExpression#accept(FilterExpressionVisitor)} to dispatch
+ * to the appropriate {@code visit} method based on the expression's concrete type.</p>
+ *
+ * <p>Example — translating SCIM filters to SQL WHERE clauses:</p>
+ * <pre>
+ * class SqlFilterVisitor implements FilterExpressionVisitor&lt;String&gt; {
+ *   &#64;Override
+ *   public String visit(AttributeComparisonExpression expr) {
+ *     return expr.getAttributePath().getAttributeName() + " = ?";
+ *   }
+ *   // ... other visit methods
+ * }
+ * String sql = filter.getExpression().accept(new SqlFilterVisitor());
+ * </pre>
+ *
+ * @param <R> the result type produced by visiting each expression node
+ */
+public interface FilterExpressionVisitor<R> {
+
+  /**
+   * Visits an attribute comparison expression (eq, ne, co, sw, ew, gt, ge, lt, le).
+   *
+   * @param expr the comparison expression
+   * @return the visitor result
+   */
+  R visit(AttributeComparisonExpression expr);
+
+  /**
+   * Visits an attribute presence expression (pr).
+   *
+   * @param expr the presence expression
+   * @return the visitor result
+   */
+  R visit(AttributePresentExpression expr);
+
+  /**
+   * Visits a logical expression (and, or).
+   *
+   * @param expr the logical expression containing left and right operands
+   * @return the visitor result
+   */
+  R visit(LogicalExpression expr);
+
+  /**
+   * Visits a group expression (parenthesized sub-expression, optionally negated with not).
+   *
+   * @param expr the group expression
+   * @return the visitor result
+   */
+  R visit(GroupExpression expr);
+
+  /**
+   * Visits a value path expression (e.g., {@code emails[type eq "work"].value}).
+   *
+   * @param expr the value path expression
+   * @return the visitor result
+   */
+  R visit(ValuePathExpression expr);
+}

scim-spec/scim-spec-schema/src/test/java/org/apache/directory/scim/spec/filter/FilterExpressionVisitorTest.java

@@ -0,0 +1,133 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+
+ * http://www.apache.org/licenses/LICENSE-2.0
+
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.directory.scim.spec.filter;
+
+import org.apache.directory.scim.spec.filter.attribute.AttributeReference;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class FilterExpressionVisitorTest {
+
+  /**
+   * A tracking visitor that records which visit overload was called
+   * and returns the simple class name of the expression type.
+   */
+  static class TrackingVisitor implements FilterExpressionVisitor<String> {
+    String visitedType;
+
+    @Override
+    public String visit(AttributeComparisonExpression expr) {
+      visitedType = "AttributeComparisonExpression";
+      return visitedType;
+    }
+
+    @Override
+    public String visit(AttributePresentExpression expr) {
+      visitedType = "AttributePresentExpression";
+      return visitedType;
+    }
+
+    @Override
+    public String visit(LogicalExpression expr) {
+      visitedType = "LogicalExpression";
+      return visitedType;
+    }
+
+    @Override
+    public String visit(GroupExpression expr) {
+      visitedType = "GroupExpression";
+      return visitedType;
+    }
+
+    @Override
+    public String visit(ValuePathExpression expr) {
+      visitedType = "ValuePathExpression";
+      return visitedType;
+    }
+  }
+
+  @Test
+  void accept_withAttributeComparisonExpression_callsCorrectVisitOverload() {
+    TrackingVisitor visitor = new TrackingVisitor();
+    AttributeReference attrRef = new AttributeReference("userName");
+    // Declare as FilterExpression to test polymorphic dispatch
+    FilterExpression expr = new AttributeComparisonExpression(attrRef, CompareOperator.EQ, "john");
+
+    String result = expr.accept(visitor);
+
+    assertThat(result).isEqualTo("AttributeComparisonExpression");
+    assertThat(visitor.visitedType).isEqualTo("AttributeComparisonExpression");
+  }
+
+  @Test
+  void accept_withAttributePresentExpression_callsCorrectVisitOverload() {
+    TrackingVisitor visitor = new TrackingVisitor();
+    AttributeReference attrRef = new AttributeReference("emails");
+    FilterExpression expr = new AttributePresentExpression(attrRef);
+
+    String result = expr.accept(visitor);
+
+    assertThat(result).isEqualTo("AttributePresentExpression");
+    assertThat(visitor.visitedType).isEqualTo("AttributePresentExpression");
+  }
+
+  @Test
+  void accept_withLogicalExpression_callsCorrectVisitOverload() {
+    TrackingVisitor visitor = new TrackingVisitor();
+    AttributeReference leftRef = new AttributeReference("userName");
+    AttributeReference rightRef = new AttributeReference("displayName");
+    FilterExpression left = new AttributeComparisonExpression(leftRef, CompareOperator.EQ, "john");
+    FilterExpression right = new AttributeComparisonExpression(rightRef, CompareOperator.EQ, "doe");
+    FilterExpression expr = new LogicalExpression(left, LogicalOperator.AND, right);
+
+    String result = expr.accept(visitor);
+
+    assertThat(result).isEqualTo("LogicalExpression");
+    assertThat(visitor.visitedType).isEqualTo("LogicalExpression");
+  }
+
+  @Test
+  void accept_withGroupExpression_callsCorrectVisitOverload() {
+    TrackingVisitor visitor = new TrackingVisitor();
+    AttributeReference attrRef = new AttributeReference("userName");
+    FilterExpression inner = new AttributeComparisonExpression(attrRef, CompareOperator.EQ, "john");
+    FilterExpression expr = new GroupExpression(false, inner);
+
+    String result = expr.accept(visitor);
+
+    assertThat(result).isEqualTo("GroupExpression");
+    assertThat(visitor.visitedType).isEqualTo("GroupExpression");
+  }
+
+  @Test
+  void accept_withValuePathExpression_callsCorrectVisitOverload() {
+    TrackingVisitor visitor = new TrackingVisitor();
+    AttributeReference attrRef = new AttributeReference("emails");
+    AttributeReference filterRef = new AttributeReference("type");
+    FilterExpression filterExpr = new AttributeComparisonExpression(filterRef, CompareOperator.EQ, "work");
+    FilterExpression expr = new ValuePathExpression(attrRef, filterExpr);
+
+    String result = expr.accept(visitor);
+
+    assertThat(result).isEqualTo("ValuePathExpression");
+    assertThat(visitor.visitedType).isEqualTo("ValuePathExpression");
+  }
+}