Method references with a parameter

Question

In Java 8, with the following class

 class Person { private boolean born; Person() { } public void setBornTrue() { born = true; } public void setBorn(boolean state) { born = state; } }

it is possible to call the setBornTrue method via a method reference:

 ArrayList<Person> people = new ArrayList<>(); people.add(new Person()); people.forEach(Person::setBornTrue);

but how would I use the forEach method and use the setBorn using a method reference? Trying:

 people.forEach(Person::setBorn);

results in an error, "Cannot resolve method setBorn".

In addition, how would I pass in the value of True?

What are you trying to achieve ? This is wrong - people.add(new Person(null));people.forEach(Person::setBornTrue); — Erran Morad
– Erran Morad, Commented Aug 24, 2014 at 5:14
people.add(new Person(null)); is this line right ? why null ? your code works for me but if you removed null. — user1935024
– user1935024, Commented Aug 24, 2014 at 5:14
Take a look at MethodHandle, but it so complicated, nothing like std::bind in C++ — Martin Meeser
– Martin Meeser, Commented Jan 30, 2020 at 9:58

Volune · Accepted Answer · 2014-08-24 05:46:44Z

27

With lambda:

people.forEach((p) -> p.setBorn(true));

Found no other ways only using the java 8 API.

With this custom function:

public static <T, U> Consumer<T> bind2(BiConsumer<? super T, U> c, U arg2) { return (arg1) -> c.accept(arg1, arg2); }

You can do:

people.forEach(bind2(Person::setBorn, true));

If this kind of utility methods is available in the java API or in a library, please let us know.

answered Aug 24, 2014 at 5:18

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3 Comments

Yes ... while the answer above is how to set this without a method reference, is there no way to code this with a method reference?

@user465342 Updated with an alternative solution, which is not plain java, but uses a method reference.

Very impressive! Is there any reference you used to create that? Been looking for a good resource.