Fix for issue #6.

src/main/java/de/platon42/intellij/plugins/cajon/CommonMatchers.kt

@@ -50,9 +50,21 @@ val MORE_EXTENSION_POINTS = CallMatcher.instanceCall(
     "singleElement", "hasOnlyOneElementSatisfying", "anyMatch", "noneMatch", "anySatisfy", "noneSatisfy"
 )!!
 
+val FILTERED_ON = CallMatcher.instanceCall(AssertJClassNames.ABSTRACT_ITERABLE_ASSERT_CLASSNAME, "filteredOn", "filteredOnNull", "filteredOnAssertions")!!
+
 val COMPLEX_CALLS_THAT_MAKES_STUFF_TRICKY = CallMatcher.anyOf(
+    EXTRACTING_CALL_MATCHERS,
     DESCRIBED_AS,
     WITH_REPRESENTATION_AND_SUCH,
+    FILTERED_ON,
+    USING_COMPARATOR,
+    IN_HEXADECIMAL_OR_BINARY
+)!!
+
+val COMPLEX_STUFF_THAT_MAKES_JOINING_IMPOSSIBLE = CallMatcher.anyOf(
+    EXTRACTING_CALL_MATCHERS,
+    WITH_REPRESENTATION_AND_SUCH,
+    FILTERED_ON,
     USING_COMPARATOR,
     IN_HEXADECIMAL_OR_BINARY
 )!!

src/main/java/de/platon42/intellij/plugins/cajon/inspections/JoinAssertThatStatementsInspection.kt

@@ -72,7 +72,7 @@ class JoinAssertThatStatementsInspection : AbstractAssertJInspection() {
                     if (!statement.hasAssertThat()) return null
 
                     val assertThatCall = PsiTreeUtil.findChildrenOfType(statement, PsiMethodCallExpression::class.java).find { ALL_ASSERT_THAT_MATCHERS.test(it) }
-                    return assertThatCall?.takeIf { it.findFluentCallTo(EXTRACTING_CALL_MATCHERS) == null }
+                    return assertThatCall?.takeIf { it.findFluentCallTo(COMPLEX_STUFF_THAT_MAKES_JOINING_IMPOSSIBLE) == null }
                 }
                 return null
             }
@@ -83,6 +83,7 @@ class JoinAssertThatStatementsInspection : AbstractAssertJInspection() {
                     val matched = when (element) {
                         is PsiUnaryExpression -> (element.operationTokenType == JavaTokenType.PLUSPLUS)
                                 || (element.operationTokenType == JavaTokenType.MINUSMINUS)
+
                         is PsiMethodCallExpression -> KNOWN_METHODS_WITH_SIDE_EFFECTS.test(element)
                         else -> false
                     }