Null-Restricted and Nullable Types(bugs.openjdk.org)

  String? id(String! arg) { return arg; }
  
  String s = null;
  Object! o1 = s; // NPE
  Object o2 = id(s); // NPE
  Object o3 = (String!) s; // NPE

  String s = ...
  if (s != null) {
    # Here the compiler knows that the effective type is String!
    String! ss = s; # OK
  }

    String labels[] = null;

    String![]! labels;

    vec<String> $foo; // neither foo nor the elements are null
    ?vec<string> $foo; // the elements are non-null but foo can be null
    vec<?string? $foo; // foo cannot be null, elements can be
    ?vec<?string> $foo; // either can be null

    String? id(String! arg) { return arg; }

    String s = null;
    Object! o1 = s; // NPE
    Object o2 = id(s); // NPE
    Object o3 = (String!) s; // NPE

    class A {}
    class extends B {}
    void foo(B $b) {}
    ?B $b = new B();
    foo($b); // compiler error
    A $a = $b as A; // runtime error if $b is null
    foo($a as B); // runtime error if $a is not a B
    ?B $b2 = $a as ?B; // $b2 is cast to B if it is one, otherwise null is returned    

    private String? foo;
    private String! bar;
    // or
    private nullable String foo;
    private nonnull String bar;