Require a public no args constructor

Fixes #933

Author: timtebeek

src/main/java/org/openrewrite/java/migrate/lang/MigrateMainMethodToInstanceMain.java

@@ -106,15 +106,16 @@ private boolean hasNoArgConstructor(J.ClassDeclaration classDecl) {
                         .filter(J.MethodDeclaration::isConstructor)
                         .collect(toList());
 
-                // If no constructors are declared, the class has an implicit no-arg constructor
+                // If no constructors are declared, the class has an implicit public no-arg constructor
                 if (constructors.isEmpty()) {
                     return true;
                 }
 
-                // Check if any explicit constructor is a no-arg constructor
+                // Instance main requires a public no args constructor
                 return constructors.stream()
-                        .anyMatch(ctor -> ctor.getParameters().isEmpty() ||
-                                (ctor.getParameters().size() == 1 && ctor.getParameters().get(0) instanceof J.Empty));
+                        .anyMatch(ctor -> (ctor.getParameters().isEmpty() ||
+                                (ctor.getParameters().size() == 1 && ctor.getParameters().get(0) instanceof J.Empty)) &&
+                                ctor.hasModifier(J.Modifier.Type.Public));
             }
 
             private boolean isMainMethodReferenced(J.MethodDeclaration mainMethod) {

src/test/java/org/openrewrite/java/migrate/lang/MigrateMainMethodToInstanceMainTest.java

@@ -373,4 +373,46 @@ public static void main(String[] args) {
           )
         );
     }
+
+    @Test
+    void doNotMigrateMainWithPrivateNoArgConstructor() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              class Foo {
+                  private Foo() {}
+                  public static void main(String[] args) {
+                      System.out.println("Hello, World!");
+                  }
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void migrateMainWithPublicNoArgConstructor() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              class Application {
+                  public Application() {}
+                  public static void main(String[] args) {
+                      System.out.println("Hello, World!");
+                  }
+              }
+              """,
+            """
+              class Application {
+                  public Application() {}
+                  void main() {
+                      System.out.println("Hello, World!");
+                  }
+              }
+              """
+          )
+        );
+    }
 }