[java] SwitchDensity false positive with pattern matching

Fixes #5030

docs/pages/release_notes.md

@@ -32,6 +32,8 @@ The old rule names still work but are deprecated.
 ### 🐛 Fixed Issues
 * java
   * [#4532](https://github.com/pmd/pmd/issues/4532): \[java] Rule misnomer for JUnit* rules
+* java-design
+  * [#5030](https://github.com/pmd/pmd/issues/5030): \[java] SwitchDensity false positive with pattern matching
 * java-errorprone
   * [#5067](https://github.com/pmd/pmd/issues/5067): \[java] CloseResource: False positive for FileSystems.getDefault()
 

pmd-java/src/main/java/net/sourceforge/pmd/lang/java/ast/ASTSwitchLabel.java

@@ -50,8 +50,9 @@ public final class ASTSwitchLabel extends AbstractJavaNode implements Iterable<A
 
     /**
      * Returns the expressions of this label, or an empty list if this
-     * is the default label. This may contain {@linkplain  ASTPatternExpression pattern expressions}
+     * is the default label. This does neither contain {@linkplain  ASTTypePattern TypePatterns}
-     * to represent patterns.
+     * nor {@linkplain ASTRecordPattern RecordPatterns}. To check for this,
+     * use {@link #isPatternLabel()}.
      */
     public NodeStream<ASTExpression> getExprList() {
         return children(ASTExpression.class);
@@ -66,4 +67,12 @@ public final class ASTSwitchLabel extends AbstractJavaNode implements Iterable<A
     public Iterator<ASTExpression> iterator() {
         return children(ASTExpression.class).iterator();
     }
+
+    /**
+     * Checks whether this label tests a {@link ASTTypePattern} or a {@link ASTRecordPattern}.
+     * @since 7.7.0
+     */
+    public boolean isPatternLabel() {
+        return children(ASTPattern.class).nonEmpty();
+    }
 }

pmd-java/src/main/java/net/sourceforge/pmd/lang/java/rule/design/SwitchDensityRule.java

@@ -55,7 +55,7 @@ public class SwitchDensityRule extends AbstractJavaRulechainRule {
         int stmtCount = node.descendants(ASTStatement.class).count();
         int labelCount = node.getBranches()
                 .map(ASTSwitchBranch::getLabel)
-                .sumBy(label -> label.isDefault() ? 1 : label.getExprList().count());
+                .sumBy(label -> label.isDefault() || label.isPatternLabel() ? 1 : label.getExprList().count());
 
         // note: if labelCount is zero, double division will produce +Infinity or NaN, not ArithmeticException
         double density = stmtCount / (double) labelCount;

pmd-java/src/test/resources/net/sourceforge/pmd/lang/java/rule/design/xml/SwitchDensity.xml

@@ -152,6 +152,32 @@ public class SwitchWithDefault
         {
         }
     }
+}
+        ]]></code>
+    </test-code>
+
+    <test-code>
+        <description>[java] SwitchDensity false positive with pattern matching #5030</description>
+        <rule-property name="minimum">4</rule-property>
+        <expected-problems>0</expected-problems>
+        <code><![CDATA[
+public class SwitchDensityPattern {
+    public void foo(Object o) {
+        switch (o) {
+            case Integer i when i > 0 ->
+            {
+                System.err.println("I am a fish.");
+            }
+            case Integer i ->
+            {
+                System.err.println("I am not a fish.");
+            }
+            default ->
+            {
+                System.err.println("default");
+            }
+        }
+    }
 }
         ]]></code>
     </test-code>