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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.
```java
void test(MyProto proto) {
if (proto.getField() == null) {
...
}
if (proto.getRepeatedFieldList() != null) {
...
}
if (proto.getRepeatedField(1) != null) {
...
}
}
```
```java
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,
```java
message MyMessage {
google.protobuf.StringValue my_string = 1;
}
```
Presence can then be tested using `myMessage.hasMyString()`, and the value
retrieved using `myMessage.getMyString().getValue()`.
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