Skip private method stubbing in PowerMockitoDoStubbingToMockito (#957)

PowerMock's string-based `when(instance, "methodName")` API supports
stubbing private methods, but standard Mockito cannot. The recipe was
incorrectly rewriting these to `when(instance).methodName()`, producing
uncompilable code. Now detects private methods and leaves the call
unchanged with a comment guiding the user to refactor.

Author: bmuschko

src/main/java/org/openrewrite/java/testing/mockito/PowerMockitoDoStubbingToMockito.java

@@ -25,14 +25,14 @@
 import org.openrewrite.java.JavaIsoVisitor;
 import org.openrewrite.java.MethodMatcher;
 import org.openrewrite.java.search.UsesMethod;
-import org.openrewrite.java.tree.Expression;
-import org.openrewrite.java.tree.J;
-import org.openrewrite.java.tree.JavaType;
-import org.openrewrite.java.tree.Space;
+import org.openrewrite.java.tree.*;
+import org.openrewrite.marker.Markers;
+
 import java.util.List;
 
 import static java.util.Collections.emptyList;
 import static java.util.Collections.singletonList;
+import static org.openrewrite.java.tree.Flag.Private;
 
 public class PowerMockitoDoStubbingToMockito extends Recipe {
 
@@ -80,6 +80,19 @@ public J.MethodInvocation visitMethodInvocation(J.MethodInvocation method, Execu
                         String targetMethodName = (String) ((J.Literal) args.get(1)).getValue();
                         List<Expression> extraArgs = args.subList(2, args.size());
 
+                        // Private methods cannot be stubbed with standard Mockito; leave unchanged and add a comment
+                        JavaType.Method targetMethod = resolveTargetMethod(
+                                firstArg.getType(), targetMethodName, extraArgs.size());
+                        if (targetMethod != null && targetMethod.getFlags().contains(Private)) {
+                            String comment = " PowerMock private method stubbing is not supported by Mockito. Refactor the private method to package-private or extract to a collaborator.";
+                            if (mi.getPrefix().getComments().stream().noneMatch(c -> c.printComment(getCursor()).endsWith(comment.trim()))) {
+                                return mi.withPrefix(mi.getPrefix().withComments(ListUtils.concat(
+                                        mi.getPrefix().getComments(),
+                                        new TextComment(false, comment, "\n" + mi.getPrefix().getIndent(), Markers.EMPTY))));
+                            }
+                            return mi;
+                        }
+
                         // Rewrite: doX().when(instance, "method", args...) → doX().when(instance).method(args...)
 
                         // 1. Create when(instance) - strip method name and extra args, keep just the instance

src/test/java/org/openrewrite/java/testing/mockito/PowerMockitoDoStubbingToMockitoTest.java

@@ -147,6 +147,48 @@ void test() throws Exception {
         );
     }
 
+    @Test
+    void addsCommentForPrivateMethodStubbing() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              import org.junit.jupiter.api.BeforeEach;
+              import org.powermock.api.mockito.PowerMockito;
+              import java.util.Calendar;
+
+              class MyTest {
+
+                  private Calendar calendarSpy;
+
+                  @BeforeEach
+                  void setUp() throws Exception {
+                      calendarSpy = PowerMockito.spy(Calendar.getInstance());
+                      PowerMockito.doNothing().when(calendarSpy, "updateTime");
+                  }
+              }
+              """,
+            """
+              import org.junit.jupiter.api.BeforeEach;
+              import org.powermock.api.mockito.PowerMockito;
+              import java.util.Calendar;
+
+              class MyTest {
+
+                  private Calendar calendarSpy;
+
+                  @BeforeEach
+                  void setUp() throws Exception {
+                      calendarSpy = PowerMockito.spy(Calendar.getInstance());
+                      // PowerMock private method stubbing is not supported by Mockito. Refactor the private method to package-private or extract to a collaborator.
+                      PowerMockito.doNothing().when(calendarSpy, "updateTime");
+                  }
+              }
+              """
+          )
+        );
+    }
+
     @Test
     void doStubbingWithStringMethodNameAndArgs() {
         //language=java