feat: Prevent migration of singletonMap with null value or null key (#772)

* feat: Prevent migration of singletonMap with null value or null key

* Apply suggestions from code review

* Update MigrateCollectionsSingletonMap.java

* Update MigrateCollectionsSingletonMap.java

* Update MigrateCollectionsSingletonMap.java

---------

Co-authored-by: Tim te Beek <[email protected]>
Co-authored-by: Tim te Beek <[email protected]>

Author: ZzetT

src/main/java/org/openrewrite/java/migrate/util/MigrateCollectionsSingletonMap.java

@@ -40,7 +40,7 @@ public String getDisplayName() {
 
     @Override
     public String getDescription() {
-        return "Prefer `Map.Of(..)` instead of using `Collections.singletonMap()` in Java 9 or higher.";
+        return "Prefer `Map.of(..)` instead of using `Collections.singletonMap()` in Java 9 or higher.";
     }
 
     @Override
@@ -49,7 +49,7 @@ public TreeVisitor<?, ExecutionContext> getVisitor() {
             @Override
             public J visitMethodInvocation(J.MethodInvocation method, ExecutionContext ctx) {
                 J.MethodInvocation m = (J.MethodInvocation) super.visitMethodInvocation(method, ctx);
-                if (SINGLETON_MAP.matches(method)) {
+                if (SINGLETON_MAP.matches(method) && isNotLiteralNull(m)) {
                     maybeRemoveImport("java.util.Collections");
                     maybeAddImport("java.util.Map");
                     StringJoiner mapOf = new StringJoiner(", ", "Map.of(", ")");
@@ -68,6 +68,11 @@ public J visitMethodInvocation(J.MethodInvocation method, ExecutionContext ctx)
 
                 return m;
             }
+
+            private boolean isNotLiteralNull(J.MethodInvocation m) {
+                return !(J.Literal.isLiteralValue(m.getArguments().get(0), null) ||
+                         J.Literal.isLiteralValue(m.getArguments().get(1), null));
+            }
         });
     }
 }

src/test/java/org/openrewrite/java/migrate/util/MigrateCollectionsSingletonMapTest.java

@@ -85,4 +85,25 @@ class Test {
           )
         );
     }
+
+    @Issue("https://github.com/openrewrite/rewrite-migrate-java/issues/571")
+    @Test
+    void shouldNotConvertLiteralNull() {
+        //language=java
+        rewriteRun(
+          version(
+            java(
+              """
+                import java.util.*;
+
+                class Test {
+                    Map<String, String> mapWithNullKey = Collections.singletonMap(null, "foo");
+                    Map<String, String> mapWithNullValue = Collections.singletonMap("bar", null);
+                }
+                """
+            ),
+            9
+          )
+        );
+    }    
 }