How to rewrite code to optionals?

Question

In my current job we are rewriting some code to Java 8. If you have code like this:

if(getApi() != null && getApi().getUser() != null 
     && getApi().getUser().getCurrentTask() != null)  
{
   getApi().getUser().getCurrentTask().pause();
}

you can simply rewrite it to

Optional.ofNullable(this.getApi())
.map(Api::getUser)
.map(User::getCurrentTask)
.ifPresent(Task::pause);

without changing code behaviour. but what if something in the middle can throw NPE because it is not checked to null?

for example:

if(getApi() != null && getApi().getUser() != null 
     && getApi().hasTasks())  
{
   getApi().getMasterUser(getApi().getUser()) //<- npe can be here
     .getCurrentTask().pause();
}

what is the best way to rewrite code like this using optionals?(it should work exactly the same and throw npe when getMasterUser(...) returns null)

UPD second example:

if(getApi()!=null && getApi.getUser() != null)
{
   if(getApi().getUser().getDepartment().getBoss() != null)// <- nre if department is null
     {
        getApi().getUser().getDepartment().getBoss().somefunc();
     }
 }

it has nullchecks for api, user, boss, but not department. how can it be made using optionals?

Answer 1

if(getApi() != null && getApi().getUser() != null) {
    if(getApi().getUser().getDepartment().getBoss() != null) {
        getApi().getUser().getDepartment().getBoss().somefunc();
    }
}

One way of writing this with optionals is:

Optional.ofNullable(this.getApi())
    .map(Api::getUser)
    .map(user -> Objects.requireNonNull(user.getDepartment()))
    .map(Department::getBoss)
    .ifPresent(Boss::somefunc);

But this is error-prone because it requires the client to keep track of what is and isn't optional. A better way would be to make the api itself return an optional instead of a nullable value. Then the client code is:

this.getApi()
    .flatMap(Api::getUser)
    .map(user -> user.getDepartment().getBoss())
    .ifPresent(Boss::somefunc));

This would make it clearer in the api which values should be optional and make it a compile-time error to not handle them.

if(getApi() != null && getApi().getUser() != null && getApi().hasTasks()) {
    getApi().getMasterUser(getApi().getUser()).getCurrentTask().pause();
}

Here, you need access to api and user at the same time so you probably need to nest the lambdas:

getApi().filter(Api::hasTasks).ifPresent(api -> {
    api.getUser().ifPresent(user -> {
        api.getMasterUser(user).getCurrentTask().ifPresent(Task::pause);
    });
});