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Java records with nullable components

I really like the addition of records in Java 14, at least as a preview feature, as it helps to reduce my need to use lombok for simple, immutable "data holders". But I'm having an issue with the implementation of nullable components. I'm trying to avoid returning null in my codebase to indicate that a value might not be present. Therefore I currently often use something like the following pattern with lombok.
#Value
public class MyClass {
String id;
#Nullable String value;
Optional<String> getValue() { // overwrite the generated getter
return Optional.ofNullable(this.value);
}
}
When I try the same pattern now with records, this is not allowed stating incorrect component accessor return type.
record MyRecord (String id, #Nullable String value){
Optional<String> value(){
return Optional.ofNullable(this.value);
}
}
Since I thought the usage of Optionals as return types is now preferred, I'm really wondering why this restriction is in place. Is my understanding of the usage wrong? How can I achieve the same, without adding another accessor with another signature which does not hide the default one? Should Optional not be used in this case at all?

A record comprises attributes that primarily define its state. The derivation of the accessors, constructors, etc. is completely based on this state of the records.
Now in your example, the state of the attribute value is null, hence the access using the default implementation ends up providing the true state. To provide customized access to this attribute you are instead looking for an overridden API that wraps the actual state and further provides an Optional return type.
Of course, as you mentioned one of the ways to deal with it would be to have a custom implementation included in the record definition itself
record MyClass(String id, String value) {
Optional<String> getValue() {
return Optional.ofNullable(value());
}
}
Alternatively, you could decouple the read and write APIs from the data carrier in a separate class and pass on the record instance to them for custom accesses.
The most relevant quote from JEP 384: Records that I found would be(formatting mine):
A record declares its state -- the group of variables -- and commits
to an API that matches that state. This means that records give up a
freedom that classes usually enjoy -- the ability to decouple a
class's API from its internal representation -- but in return, records
become significantly more concise.

Due to restrictions placed on records, namely that canonical constructor type needs to match accessor type, a pragmatic way to use Optional with records would be to define it as a property type:
record MyRecord (String id, Optional<String> value){
}
A point has been made that this is problematic due to the fact that null might be passed as a value to the constructor. This can be solved by forbidding such MyRecord invariants through canonical constructor:
record MyRecord(String id, Optional<String> value) {
MyRecord(String id, Optional<String> value) {
this.id = id;
this.value = Objects.requireNonNull(value);
}
}
In practice most common libraries or frameworks (e.g. Jackson, Spring) have support for recognizing Optional type and translating null into Optional.empty() automatically so whether this is an issue that needs to be tackled in your particular instance depends on context. I recommend researching support for Optional in your codebase before cluttering your code possibly unnecessary.

Credits go to Holger! I really like his proposed way of questioning the actual need of null. Thus with a short example, I wanted to give his approach a bit more space, even if a bit convoluted for this use-case.
interface ConversionResult<T> {
String raw();
default Optional<T> value(){
return Optional.empty();
}
default Optional<String> error(){
return Optional.empty();
}
default void ifOk(Consumer<T> okAction) {
value().ifPresent(okAction);
}
default void okOrError(Consumer<T> okAction, Consumer<String> errorAction){
value().ifPresent(okAction);
error().ifPresent(errorAction);
}
static ConversionResult<LocalDate> ofDate(String raw, String pattern){
try {
var value = LocalDate.parse(raw, DateTimeFormatter.ofPattern(pattern));
return new Ok<>(raw, value);
} catch (Exception e){
var error = String.format("Invalid date value '%s'. Expected pattern '%s'.", raw, pattern);
return new Error<>(raw, error);
}
}
// more conversion operations
}
record Ok<T>(String raw, T actualValue) implements ConversionResult<T> {
public Optional<T> value(){
return Optional.of(actualValue);
}
}
record Error<T>(String raw, String actualError) implements ConversionResult<T> {
public Optional<String> error(){
return Optional.of(actualError);
}
}
Usage would be something like
var okConv = ConversionResult.ofDate("12.03.2020", "dd.MM.yyyy");
okConv.okOrError(
v -> System.out.println("SUCCESS: "+v),
e -> System.err.println("FAILURE: "+e)
);
System.out.println(okConv);
System.out.println();
var failedConv = ConversionResult.ofDate("12.03.2020", "yyyy-MM-dd");
failedConv.okOrError(
v -> System.out.println("SUCCESS: "+v),
e -> System.err.println("FAILURE: "+e)
);
System.out.println(failedConv);
which leads to the following output...
SUCCESS: 2020-03-12
Ok[raw=12.03.2020, actualValue=2020-03-12]
FAILURE: Invalid date value '12.03.2020'. Expected pattern 'yyyy-MM-dd'.
Error[raw=12.03.2020, actualError=Invalid date value '12.03.2020'. Expected pattern 'yyyy-MM-dd'.]
The only minor issue is that the toString prints now the actual... variants. And of course we do not NEED to use records for this.

Don't have the rep to comment, but I just wanted to point out that you've essentially reinvented the Either datatype. https://hackage.haskell.org/package/base-4.14.0.0/docs/Data-Either.html or https://www.scala-lang.org/api/2.9.3/scala/Either.html. I find Try, Either, and Validation to be incredibly useful for parsing and there are a few java libraries with this functionality that I use: https://github.com/aol/cyclops/tree/master/cyclops and https://www.vavr.io/vavr-docs/#_either.
Unfortunately, I think your main question is still open (and I'd be interested in finding an answer).
doing something like
RecordA(String a)
RecordAandB(String a, Integer b)
to deal with an immutable data carrier with a null b seems bad, but wrapping recordA(String a, Integer b) to have an Optional getB somewhere else seems contra-productive. There's almost no point to the record class then and I think the lombok #Value is still the best answer. I'm just concerned that it won't play well with deconstruction for pattern matching.

Map null values to default using builder with MapStruct

I want to map field from Source to Target class, and if the source value is null, I would like to convert it to default value based on the data type ("" for strings, 0 for numeric types etc.). For setting the values, I am not using regular setters, but builder (with protobuf, so the names of the methods is newBuilder() and build()).
class Source {
private final String value; // getter
}
class Target {
private final String value;
public static Builder newBuilder() {return new Builder()}
public static class Builder {
public static setValue() {/*Set the field*/}
public static Target build() {/*Return the constructed instance*/}
}
My mapper looks like this:
#Mapper(
nullValuePropertyMappingStrategy = NullValuePropertyMappingStrategy.SET_TO_DEFAULT,
nullValueMappingStrategy = NullValueMappingStrategy.RETURN_DEFAULT
)
public interface TargetMapper {
Target map(Source source);
}
The generated mapper implementation with this code calls target.setValue(source.getValue()), instead of performing the null check and setting default value if source returns null. The interesting part is when I add the following annotation to the map method, the null check is present in the implementation.
#Mapping(source="value", target="value", nullValuePropertyMappingStrategy = NullValuePropertyMappingStrategy.SET_TO_DEFAULT)
Is this a bug in MapStruct with builders, or am I missing some configuration to be ably to set the null mapping as a default policy, instead of duplicating it on all field mappings?
EDIT: For some reason, adding nullValueCheckStrategy = NullValueCheckStrategy.ALWAYS to the class level #Mapper annotation adds the null check, but does not explicitly set the value, just skips the call to setValue. For protobuf, this is okay, since this functionality is in the library, but for other implementations the field would remain null.

#Mapping(source="value", target="value", nullValuePropertyMappingStrategy = NullValuePropertyMappingStrategy.SET_TO_DEFAULT)
applies to update method (so methods that have the #MappingTarget annotated parameter
There's no real counterpart for regular methods:
1. NullValueMappingStragegy applies to the bean argument itself.
2. NullValueCheckStragegy does perform a check on bean properties, but does not return a default.
Naming is not really brilliant and it has a long history. We still have the intention to align this one day.
A solution would be to use an Object factory creating the builder target object and pre-populate it with default values and then let MapStuct override these one day.
Perhaps you could do something like this:
#Mapper(
// to perform a null check
nullValueCheckStrategy = NullValueCheckStrategy.ALWAYS
)
public interface TargetMapper {
Target map(Source source);
}
// to create a pre-defined object (defaults set a-priori). Not sure
// whether this works with builders.. just try
#ObjectFactory
default Target.Builder create() {
Target.Builder builder = Target.newBuilder();
builder.setValueX( "someDefaultValue" );
return builder;
}

Mockito when checking for specific object property value

I have the following in a working test:
when(client.callApi(anyString(), isA(Office.class))).thenReturn(responseOne);
Note that client is a Mock of class Client.
I want to change "isA(Office.class)" to tell it to match where the "id" property of an Office instance is "123L". How can I specify that I want a specific argument value in the method of a mocked object?
Edit: Not a duplicate because I'm trying to use it on a "when" and the linked question (and other resources I've found) are using ArgumentCaptor and ArgumentMatcher on "verify" and "assert". I'm thinking I can't actually do what I'm trying and will try out another way. Of course, I'm willing to be shown otherwise.

Reopening as requested, but the solution (use an ArgumentMatcher) is identical to the one in the linked answer. Naturally, you can't use an ArgumentCaptor when stubbing, but everything else is the same.
class OfficeWithId implements ArgumentMatcher<Office> {
long id;
OfficeWithId(long id) {
this.id = id;
}
#Override public boolean matches(Office office) {
return office.id == id;
}
#Override public String toString() {
return "[Office with id " + id + "]";
}
}
when(client.callApi(anyString(), argThat(new IsOfficeWithId(123L)))
.thenReturn(responseOne);
Because ArgumentMatcher has a single method, you can even make it a lambda in Java 8:
when(client.callApi(anyString(), argThat(office -> office.id == 123L))
.thenReturn(responseOne);
If you're already using Hamcrest, you can adapt a Hamcrest matcher using MockitoHamcrest.argThat, or use the built-in hasProperty:
when(client.callApi(
anyString(),
MockitoHamcrest.argThat(
hasProperty("id", equalTo(123L)))))
.thenReturn(responseOne);

I ended up going with "eq". This was ok in this case because the objects are pretty simple. First I created an object that is the same as what I expect to get back.
Office officeExpected = new Office();
officeExpected.setId(22L);
Then my 'when' statement becomes:
when(client.callApi(anyString(), eq(officeExpected))).thenReturn(responseOne);
This allows me to have better checking than "isA(Office.class)".

adding an answer for anyone with a more complex object.
answer from OP uses eq which works for simple objects.
However, I had a more complex object with many more fields. Its quite painful to create Mock object and fill in all the fields
public class CreateTenantRequest {
#NotBlank private String id;
#NotBlank private String a;
#NotBlank private String b;
...
...
}
I was able to use refEq to achieve the same thing without setting a value of each field.
Office officeExpected = new Office();
officeExpected.setId(22L);
verify(demoMock, Mockito.atLeastOnce()).foobarMethod(refEq(officeExpected, "a", "b"));

How to mass annotate constructor arguments?

I've got a problem where I want to make a lot of classes in our project de-serializable via jackson. The problem is that most of classes look like this:
public class FinalFieds{
private final String field;
private final String secondField;
public FinalFieds(String field, String secondField)
{
this.field = field;
this.secondField = secondField;
}
public String getField()
{
return field;
}
public String getSecondField()
{
return secondField;
}
}
So what I found is that in jackson you can do something like this:
public FinalFieds(#JsonProperty("field") String field, #JsonProperty("secondField") String secondField)
And that works nice. The problem is that I cannot make structural replace in intellij to work for me. When I try:
All my matches are in "Unclassified matches" section.
Furthermore when I try to replace, Intellij just removes a constructor from the class.
Any idea on what I'm doing wrong or is it a known bug in intellij?
Even an overcomplicated regex that will help me replace this (for single argument constructors I can create it myself; the problem is that our constructors in those classes have multi-argument constructors).

It's a bug or a missing feature depending on how you look at it.
https://youtrack.jetbrains.com/issue/IDEA-141143
However, it is possible to do it in two steps. First search for the constructor parameters you want to annotate:
class $Class$ implements OurCommonInterface {
$Class$($Type$ $parameter$);
}
where $parameter$ min: 1 max: unlimited, This variable is target of the search checked.
Then replace the parameter with an annotated parameter in scope Previous Search Results:
$Type$ $parameter$
Replacement template:
#JsonProperty("$parameter$") $Type$ $parameter$

Simulate named parameters in Java

I write a little web API which should it make easy to create URIs. Each resource class should contain a method createURI which takes the needed parameters. This method should use a helper method, populateUriTemplate, in the background to create an URI string. populateUriTemplate needs key value pairs to populate an URI template. In another language like Scala or Python I would use named parameters, but Java doesn't support them. So the question is: How to simulate named parameters in Java?
The straight forward solution would be to create a map:
public String createUri(int id, String name){
Map<String, Object> params = new HashMap<String, Object>();
params.put("id", id);
params.put("name", name);
return populateUriTemplate(params);
}
But I don't like to create a map first and put each parameter to it.
Another idea is to use a static method, param, to create key value pairs:
public String createUri(int id, String name){
return populateUriTemplate(param("id", id), param("name", name));
}
Looks much better to me!
It could be refined a bit to make it more self-explanatory, even if a few more characters are needed:
public String createUri(int id, String name){
return populateUriTemplate(key("id").value(id), key("name").value(name));
}
I've also thought of the builder pattern, but this would force the user of my API to create an explicit builder for each resource class, what would be tedious without a benefit. The type of the parameter is not important, as long as a proper implemented toString method exists.
My favourite is one of the both approaches with the static methods above (param(key, value) or key(k).value(v)). Do you know a better way to simulate named parameters in this case?

For some ideas on the builder pattern, you could see this blog post by Stephan Schmidt.
You also just gave me the idea to do the following, with fluent interfaces, a Callable, and a static method:
createUri().id(5).name("dennetik").call();
Which would require createing a Callable class (CreateUri) with the static method:
public static final CreateUriFluentInterface createUri() {
return FluentInterface.of(new CreateUri(), CreateUriFluentInterface.class);
}
And a fluent interface, like this:
public interface CreateUriFluentInterface {
public CreateUriFluentInterface id(Integer id);
public CreateUriFluentInterface name(String name);
}
Which isn't that much boilerplate code, is it?
(Well, if you tone down that horribly named CreateUriFluentInterface a bit, it isn't.)
(You would probably have CreateUriFluentInterface extend Callable<String>, to be able to reroute the call to Callable#call())

populateUriTemplate("id",id, "name",name);
void populateUriTemplate(Object... nvs){
for(int i=0; i<nvs.length/2; i++)
....
}

Maybe you like this approach:
class Params {
private HashMap<String, Object> allParams = new HashMap<String,Object>();
public Params(ParamEntry...params) {
for( ParamEntry p : params ) {
allParams.put(p.name, p.value);
}
}
public getParam(String name) {
return allParams.get(name);
}
class ParamEntry {
public String name;
public Object value;
}
}
public String createUri(Params.ParamsEntry ... params){
return populateUriTemplate(new Params(params));
}
To call it use
createUri(new Param.ParamEntry("name", valueObject) );
Inside the populateUriTemplate...
just use params.get("name");

Spring MVC does exactly this. As well as being able to bind requests to specific methods in controller classes, you can bind request parameters to method parameters. You can have a look to see how it works, but basically it picks a strategy to map the right request parameter to the right method parameter.
You basically get something like:
public String createUri(#RequestParam int id, #RequestParam String name){
return populateUriTemplate(id, name);
}

This is almost silly and slightly off topic, but using Lombok's #Builder annotation takes this closer to the desired result.
Furthermore if the builder, builder method and build method names are changed to _ they almost disappear:
import static foo.Template._;
class Resource {
String createURI(String id, String name) {
return populateURITemplate(_.id(id).name(name)._());
}
String populateURITemplate(Template t ){
return t.id+"="+t.name;
}
}
#Builder(builderClassName = "_", builderMethodName = "_", buildMethodName = "_" )
class Template {
static _ _ = _();
String id;
String name;
}

Named parameters are not the way:
Named parameters do not make your code any cleaner in this case. I would argue that they make things more complex and error prone in Java because you lose type safety and you lose compiler warnings about identifiers that do not exist.
TypeSafe Immutable Fluent Builders:
I wrote an article on a UrlBuilder implementation earlier this year, it shows a type safe fluent interface that enforces order of construction for mandatory input and allows for optional parts with sane defaults as well.
Now I will be the first to admit that the approach I use is fairly verbose, but it is extremely productive once that initial price is paid. It works with dependency injection and is easily unit testable and most importantly is composable for specialization.
final URL url1 = new UrlBuilder().scheme("http").host("www.google.com").build();
System.out.println("url1 = " + url1);
final URL url2 = new UrlBuilder().scheme("https").userInfo("xkcd", "correcthorsebatterystaple").host("admin.xkcd.com").build();
System.out.println("url2 = " + url2);
Produces:
url1 = http://www.google.com
url2 = https://xkcd:correcthorsebatterystaple#admin.xkcd.com
I am addressing the verbosity of the anonymous inner class implementations of the interfaces with another approach I am experimenting with; type safe implementations of value objects from interfaces using dynamic proxies.
This will do away with the boilerplate value objects and replace them with Map<String,?> but put a dynamically generated type safe immutable Interface wrapper around them.
I encourage you to read about both of these and see how combining them gives you a better solution than named properties ever would.
When I get time to refactor my UrlBuilder with the dynamic proxies I will post another blog post about it as well.
Named Parameters via Guice
If you are dead set on named parameters then I would recommend looking at Guice #Named bindings. You still lose the compile type checks and safety but at least you get some validations from Guice.
public class RealBillingService implements BillingService {
#Inject
public RealBillingService(#Named("Checkout") CreditCardProcessor processor,
TransactionLog transactionLog) {
...
}