ImpossibleNullComparison
This value cannot be null, and comparing it to null may be misleading.

Alternate names: ProtoFieldNullComparison

The problem

This checker looks for comparisons of protocol buffer fields with null. If a proto field is not specified, its field accessor will return a non-null default value. Thus, the result of calling one of these accessors can never be null, and comparisons like these often indicate a nearby error.

If you need to distinguish between an unset optional value and a default value, you have two options. In most cases, you can simply use the hasField() method. proto3 however does not generate hasField() methods for primitive types (including string and bytes). In those cases you will need to wrap your field in google.protobuf.StringValue or similar.

NOTE: This check applies to normal (server) protos and Lite protos. The deprecated nano runtime does produce objects which use null values to indicate field absence.

void test(MyProto proto) {
  if (proto.getField() == null) {
    ...
  }
  if (proto.getRepeatedFieldList() != null) {
    ...
  }
  if (proto.getRepeatedField(1) != null) {
    ...
  }
}

void test(MyProto proto) {
  if (!proto.hasField()) {
    ...
  }
  if (!proto.getRepeatedFieldList().isEmpty()) {
    ...
  }
  if (proto.getRepeatedFieldCount() > 1) {
    ...
  }
}

If the presence of a field is required information in proto3, the field can be wrapped. For example,

message MyMessage {
  google.protobuf.StringValue my_string = 1;
}

Presence can then be tested using myMessage.hasMyString(), and the value retrieved using myMessage.getMyString().getValue().

Suppression

Suppress false positives by adding the suppression annotation @SuppressWarnings("ImpossibleNullComparison") to the enclosing element.