Do not ReplaceLambdaWithMethodReference for class fields

src/main/java/org/openrewrite/staticanalysis/ReplaceLambdaWithMethodReference.java

@@ -159,6 +159,7 @@ public J visitLambda(J.Lambda lambda, ExecutionContext ctx) {
                 }
 
                 if (hasSelectWithPotentialSideEffects(method) ||
+                    hasSelectWhoseReferenceMightChange(method) ||
                     !methodArgumentsMatchLambdaParameters(method, lambda) ||
                     method instanceof J.MemberReference) {
                     return l;
@@ -212,6 +213,14 @@ private boolean hasSelectWithPotentialSideEffects(MethodCall method) {
                    ((J.MethodInvocation) method).getSelect() instanceof MethodCall;
         }
 
+        private boolean hasSelectWhoseReferenceMightChange(MethodCall method) {
+            if (method instanceof J.MethodInvocation && ((J.MethodInvocation) method).getSelect() instanceof J.Identifier) {
+                JavaType.Variable fieldType = ((J.Identifier) ((J.MethodInvocation) method).getSelect()).getFieldType();
+                return fieldType != null && fieldType.getOwner() instanceof JavaType.Class && !fieldType.hasFlags(Flag.Final);
+            }
+            return false;
+        }
+
         private boolean methodArgumentsMatchLambdaParameters(MethodCall method, J.Lambda lambda) {
             JavaType.Method methodType = method.getMethodType();
             if (methodType == null) {

src/test/java/org/openrewrite/staticanalysis/ReplaceLambdaWithMethodReferenceTest.java

@@ -63,7 +63,8 @@ void multipleMethodInvocations() {
             """
               import java.nio.file.Path;
               import java.nio.file.Paths;
-              import java.util.List;import java.util.stream.Collectors;
+              import java.util.List;
+              import java.util.stream.Collectors;
 
               class Test {
                   Path path = Paths.get("");
@@ -190,6 +191,7 @@ List<Object> method(List<Object> input) {
                   }
               }
               """,
+            //language=java
             """
               import java.util.List;
               import java.util.stream.Collectors;
@@ -237,6 +239,7 @@ List<Object> method(List<Object> input) {
                   }
               }
               """,
+            //language=java
             """
               import org.test.CheckType;
 
@@ -288,6 +291,7 @@ Stream<Integer> method(List<CheckType> input) {
                   }
               }
               """,
+            //language=java
             """
               import java.util.List;
               import java.util.stream.Stream;
@@ -409,8 +413,10 @@ public void run() {
                           Collections.singletonList(1).forEach(n -> run());
                       }
                   }
-                  Test t = new Test();
-                  Runnable r = () -> t.run();
+                  void foo() {
+                      Test t = new Test();
+                      Runnable r = () -> t.run();
+                  }
               }
               """,
             """
@@ -423,8 +429,10 @@ public void run() {
                           Collections.singletonList(1).forEach(n -> run());
                       }
                   }
-                  Test t = new Test();
-                  Runnable r = t::run;
+                  void foo() {
+                      Test t = new Test();
+                      Runnable r = t::run;
+                  }
               }
               """
           )
@@ -595,6 +603,7 @@ List<Object> filter(List<Object> l) {
                   }
               }
               """,
+            //language=java
             """
               import org.test.CheckType;
 
@@ -1293,8 +1302,8 @@ private Integer foo(String bar) {
 
     @Test
     void newClassSelector() {
+        //language=java
         rewriteRun(
-          //language=java
           java(
             """
               class A {
@@ -1345,4 +1354,22 @@ public void groupOnGetClass() {
         );
     }
 
+    @Test
+    void dontReplaceNullableFieldReferences() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              import java.util.function.Supplier;
+              class A {
+                  Object field;
+                  void foo() {
+                      // Runtime exception when replaced with field::toString
+                      Supplier<String> supplier = () -> field.toString();
+                  }
+              }
+              """
+          )
+        );
+    }
 }