Java - Intellij IDEA在enum getter方法lambda中警告NPE

Question

I have following enum:

import com.google.common.collect.Maps;

public enum ServiceType {
    SOME_SERVICE (4, SomeServiceEntity.class);

    private int id;
    private Class<? extends ServiceEntity> entityClass;

    private static final Map<Integer, ServiceType> LOOKUP = Maps.uniqueIndex(
            Arrays.asList(ServiceType.values()),
            ServiceType::getId     <<=======
    );

    ServiceType(int id, Class<? extends ServiceEntity> entityClass) {
        this.id = id;
        this.entityClass = entityClass;
    }

    public int getId() {
        return id;
    }

    // and other methods....
}

This line of code marked by Intellij IDEA as:

Method reference invocation 'ServiceType::getId' may produce 'java.lang.NullPointerException'

How is this possible while I have the only constructor which include my id field and enum is static list of objects so they all suppose to have id?

How can I get rid of this warning?

UPD: Sticked with:

private static final Map<Integer, ServiceType> LOOKUP = Arrays.stream(
        ServiceType.values()).collect(Collectors.toMap(
                ServiceType::getId, Function.identity()
        )
);

Answer 1

As the comment says, you are using a lambda there, that gets a parameter. And when that one is null, that gives an NPE.

So rather try something like:

private static final Map<Integer, ServiceType> LOOKUP = 
  Arrays
    .stream(ServiceType.values())
    .Collectors.toMap(ServiceType::getId, Function. identity());

which ... err ... might give you the same kind of warning.

So, if you really want to use stream'ing here, you probably have to suppress that warning.