Clarify ClassModifierShouldBe error message when dealing with package…

… private classes.

Instead of an empty description (as per the JDK) AssertJ will describes such classes as package-private
Fixes assertj#1692

src/main/java/org/assertj/core/error/ClassModifierShouldBe.java

@@ -12,6 +12,11 @@
  */
 package org.assertj.core.error;
 
+import static java.lang.String.format;
+import static java.lang.reflect.Modifier.isPrivate;
+import static java.lang.reflect.Modifier.isProtected;
+import static java.lang.reflect.Modifier.isPublic;
+
 import java.lang.reflect.Modifier;
 
 /**
@@ -21,9 +26,11 @@
  */
 public class ClassModifierShouldBe extends BasicErrorMessageFactory {
 
+  private static final String PACKAGE_PRIVATE = "package-private";
+
   private ClassModifierShouldBe(Class<?> actual, boolean positive, String modifier) {
     super("%nExpecting:%n  <%s>%n" + (positive ? "to" : "not to") + " be a %s class but was %s.",
-          actual, modifier, Modifier.toString(actual.getModifiers()));
+          actual, modifier, modifiers(actual));
   }
 
   /**
@@ -73,7 +80,14 @@ public static ErrorMessageFactory shouldBeProtected(Class<?> actual) {
    * @return the created {@code ErrorMessageFactory}.
    */
   public static ErrorMessageFactory shouldBePackagePrivate(Class<?> actual) {
-    return new ClassModifierShouldBe(actual, true, "package-private");
+    return new ClassModifierShouldBe(actual, true, PACKAGE_PRIVATE);
+  }
+
+  private static String modifiers(Class<?> actual) {
+    int modifiers = actual.getModifiers();
+    boolean isPackagePrivate = !isPublic(modifiers) && !isProtected(modifiers) && !isPrivate(modifiers);
+    String modifiersDescription = Modifier.toString(modifiers);
+    return isPackagePrivate ? format("%s %s", PACKAGE_PRIVATE, modifiersDescription) : modifiersDescription;
   }
 
 }

src/test/java/org/assertj/core/error/ClassModifierShouldBe_create_Test.java

@@ -13,33 +13,59 @@
 package org.assertj.core.error;
 
 import static java.lang.String.format;
-import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.BDDAssertions.then;
 import static org.assertj.core.error.ClassModifierShouldBe.shouldBeFinal;
+import static org.assertj.core.error.ClassModifierShouldBe.shouldBePublic;
 import static org.assertj.core.error.ClassModifierShouldBe.shouldNotBeFinal;
 
 import org.assertj.core.internal.TestDescription;
+import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
 
+@DisplayName("ClassModifierShouldBe create")
 class ClassModifierShouldBe_create_Test {
 
   @Test
   void should_create_error_message_for_is_final() {
-    String error = shouldBeFinal(Object.class).create(new TestDescription("TEST"));
-
-    assertThat(error).isEqualTo(format("[TEST] %n" +
-                                       "Expecting:%n" +
-                                       "  <java.lang.Object>%n" +
-                                       "to be a \"final\" class but was \"public\"."));
+    // GIVEN
+    Class<Object> nonFinalClass = Object.class;
+    // WHEN
+    String error = shouldBeFinal(nonFinalClass).create(new TestDescription("TEST"));
+    // THEN
+    then(error).isEqualTo(format("[TEST] %n" +
+                                 "Expecting:%n" +
+                                 "  <java.lang.Object>%n" +
+                                 "to be a \"final\" class but was \"public\"."));
   }
 
   @Test
   void should_create_error_message_for_is_not_final() {
-    String error = shouldNotBeFinal(String.class).create(new TestDescription("TEST"));
+    // GIVEN
+    Class<String> finalClass = String.class;
+    // WHEN
+    String error = shouldNotBeFinal(finalClass).create(new TestDescription("TEST"));
+    // THEN
+    then(error).isEqualTo(format("[TEST] %n" +
+                                 "Expecting:%n" +
+                                 "  <java.lang.String>%n" +
+                                 "not to be a \"final\" class but was \"public final\"."));
+  }
+
+  @Test
+  void should_create_clear_error_message_when_actual_is_package_private() {
+    // GIVEN
+    Class<PackagePrivate> packagePrivateClass = PackagePrivate.class;
+    // WHEN
+    String error = shouldBePublic(packagePrivateClass).create(new TestDescription("TEST"));
+    // THEN
+    then(error).isEqualTo(format("[TEST] %n" +
+                                 "Expecting:%n" +
+                                 "  <org.assertj.core.error.ClassModifierShouldBe_create_Test.PackagePrivate>%n" +
+                                 "to be a \"public\" class but was \"package-private static final\"."));
+  }
 
-    assertThat(error).isEqualTo(format("[TEST] %n" +
-                                       "Expecting:%n" +
-                                       "  <java.lang.String>%n" +
-                                       "not to be a \"final\" class but was \"public final\"."));
+  enum PackagePrivate {
+    MONITOR
   }
 
 }