Fixes #2061: ArgumentMatcher error messages use lambda class names

src/main/java/org/mockito/internal/matchers/text/MatcherToString.java

@@ -5,7 +5,7 @@
 package org.mockito.internal.matchers.text;
 
 import static org.mockito.internal.util.ObjectMethodsGuru.isToStringMethod;
-import static org.mockito.internal.util.StringUtil.decamelizeMatcher;
+import static org.mockito.internal.util.StringUtil.decamelizeMatcherName;
 
 import java.lang.reflect.Method;
 
@@ -37,6 +37,17 @@ static String toString(ArgumentMatcher<?> matcher) {
             }
             cls = cls.getSuperclass();
         }
-        return decamelizeMatcher(matcher.getClass().getSimpleName());
+
+        String matcherName;
+        Class<?> matcherClass = matcher.getClass();
+        // Lambdas have non-empty getSimpleName() (despite being synthetic)
+        // but that name is not useful for user
+        if (matcherClass.isSynthetic()) {
+            matcherName = "";
+        } else {
+            matcherName = matcherClass.getSimpleName();
+        }
+
+        return decamelizeMatcherName(matcherName);
     }
 }

src/main/java/org/mockito/internal/util/StringUtil.java

@@ -60,7 +60,7 @@ public static String join(String start, String linePrefix, Collection<?> lines)
         return out.substring(0, out.length() - 1); // lose last EOL
     }
 
-    public static String decamelizeMatcher(String className) {
+    public static String decamelizeMatcherName(String className) {
         if (className.length() == 0) {
             return "<custom argument matcher>";
         }

src/test/java/org/mockito/internal/matchers/text/MatcherToStringTest.java

@@ -13,22 +13,26 @@
 public class MatcherToStringTest extends TestBase {
 
     static class MatcherWithoutDescription implements ArgumentMatcher<Object> {
+        @Override
         public boolean matches(Object argument) {
             return false;
         }
     }
 
     static class MatcherWithDescription implements ArgumentMatcher<Object> {
+        @Override
         public boolean matches(Object argument) {
             return false;
         }
 
+        @Override
         public String toString() {
             return "*my custom description*";
         }
     }
 
     static class MatcherWithInheritedDescription extends MatcherWithDescription {
+        @Override
         public boolean matches(Object argument) {
             return false;
         }
@@ -45,4 +49,35 @@ public void better_toString_for_matchers() {
                 "*my custom description*",
                 MatcherToString.toString(new MatcherWithInheritedDescription()));
     }
+
+    @Test
+    public void default_name_for_anonymous_matchers() {
+        ArgumentMatcher<Object> anonymousMatcher =
+                new ArgumentMatcher<Object>() {
+                    @Override
+                    public boolean matches(Object argument) {
+                        return false;
+                    }
+                };
+        assertEquals("<custom argument matcher>", MatcherToString.toString(anonymousMatcher));
+
+        ArgumentMatcher<Object> anonymousDescriptiveMatcher =
+                new MatcherWithDescription() {
+                    @Override
+                    public boolean matches(Object argument) {
+                        return false;
+                    }
+                };
+        assertEquals(
+                "*my custom description*", MatcherToString.toString(anonymousDescriptiveMatcher));
+    }
+
+    @Test
+    public void default_name_for_synthetic_matchers() {
+        ArgumentMatcher<Object> lambdaMatcher = argument -> true;
+        assertEquals("<custom argument matcher>", MatcherToString.toString(lambdaMatcher));
+
+        ArgumentMatcher<Object> methodRefMatcher = lambdaMatcher::matches;
+        assertEquals("<custom argument matcher>", MatcherToString.toString(methodRefMatcher));
+    }
 }

src/test/java/org/mockito/internal/util/StringUtilTest.java

@@ -15,15 +15,16 @@
 public class StringUtilTest {
 
     @Test
-    public void decamelizes_matcher() throws Exception {
+    public void decamelizes_matcher_name() throws Exception {
         assertEquals(
                 "<Sentence with strong language>",
-                StringUtil.decamelizeMatcher("SentenceWithStrongLanguage"));
-        assertEquals("<W e i r d o 1>", StringUtil.decamelizeMatcher("WEIRDO1"));
-        assertEquals("<_>", StringUtil.decamelizeMatcher("_"));
+                StringUtil.decamelizeMatcherName("SentenceWithStrongLanguage"));
+        assertEquals("<W e i r d o 1>", StringUtil.decamelizeMatcherName("WEIRDO1"));
+        assertEquals("<_>", StringUtil.decamelizeMatcherName("_"));
         assertEquals(
-                "<Has exactly 3 elements>", StringUtil.decamelizeMatcher("HasExactly3Elements"));
-        assertEquals("<custom argument matcher>", StringUtil.decamelizeMatcher(""));
+                "<Has exactly 3 elements>",
+                StringUtil.decamelizeMatcherName("HasExactly3Elements"));
+        assertEquals("<custom argument matcher>", StringUtil.decamelizeMatcherName(""));
     }
 
     @Test