java using function as parameter

Question

public static <E extends EntityBase, FE>
FE findForeignEntity(E entities, Function<E, Long> fnForeignId, Function<Long, FE> fnFindIdIn) {
    if (entities == null) return null;
    Long fid = fnForeignId.apply(entities);
    return fnFindIdIn.apply(fid);
}

City city = Utils.findForeignEntity(candidate,
            c -> c.getAddress() != null ? c.getAddress().getCity() : null,
            fid -> cityRepo.findOne(fid));

what is "fid" stand for ? I am not familiar with this kind of usage in java. Can anybody give me a clue.

score 1 · Answer 1 · edited May 23 '17 at 11:55

In the method findForeignEntity you are passing 3 arguments, first one is entities which is the argument to method fnForeignId. The result of the method fnForeignId is fid and fid is the input to your method fnFindIdIn which returns you FE(Foreign Entity).

City city = Utils.findForeignEntity(candidate,
            c -> c.getAddress() != null ? c.getAddress().getCity() : null,
            fid -> cityRepo.findOne(fid));

In this code what confuses you might be the lambdas. I will try to make it simple. The 2 arguments of your method findForeignEntity are Function, which is a functional interface, which requires a definition.

The definition of the method fnForeignId is

c -> c.getAddress() != null ? c.getAddress().getCity() : null

which is something like

return c.getAddress() != null ? c.getAddress().getCity() : null

And definition of the method fnFindIdIn is

fid -> cityRepo.findOne(fid)

which can be read as something like

return cityRepo.findOne(fid)

These are all lambdas which was introduced in Java8

As @GhostCat has told fid stands for foreign id.

Thank you, just what I need. – Green Baylee May 22 '17 at 07:48 — Green Baylee, May 22 '17 at 07:48

GhostCat · Answer 2 · 2017-05-22T07:51:03.083

0

Obviously, fid abbreviates "foreign id".

If you are asking about the -> syntax: that is a lambda expression. Their main goal is:

Lambda expressions let you express instances of single-method classes more compactly.

But obviously there is a lot "behind" that concept.

edited May 22 '17 at 07:51

answered May 22 '17 at 03:12

GhostCat

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That will be very helpful. – Green Baylee May 22 '17 at 07:49

score 0 · Accepted Answer · answered May 22 '17 at 03:19

0

Are you looking for a lambda expression? Look at java's documetation on it. Lambdas are a great way of using a function as a parameter, but it will only ever be a one line function. A quick little way of getting around things.

answered May 22 '17 at 03:19

Spencer Pollock

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java using function as parameter

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