Java execute method on all objects in a List

Question

I've got such an interface:

public interface Listener {
    void onA();
    void onB();
    void onC();
}

And there is a list of listeners/observers:

List<Listener> listeners = new ArrayList<Listener>();

How can I easily inform all listeners, that A, B, C occurred via Listener.onA(), Listener.onB(), Listener.onC()?

Do I have to copy-paste iteration over all listeners at least three times?

In C++ I would create such a function:

void Notify(const std::function<void(Listener *listener)> &command) {
  for(auto &listener : listeners) {
    command(listener);
  }
}

And pass lambda for each of methods:

Notify([](Listener *listener) {listener->onA();});

or

Notify([](Listener *listener) {listener->onB();});

or

Notify([](Listener *listener) {listener->onC();});

Is there a similar approach in Java?

Aasmund Eldhuset · Accepted Answer · 2016-04-20 19:32:32Z

6

In Java 8, there is:

listeners.forEach(listener -> listener.onA());

or:

listeners.forEach(Listener::onA);

Note that there's a host of other functional-programming-style operations that can be performed with lambda functions, but most of them will require that you first create a stream from your collection by calling .stream() on it. forEach(), as I learned from the comments, is an exception to this.

edited Apr 20, 2016 at 19:32

answered Apr 20, 2016 at 19:26

Aasmund Eldhuset

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5 Comments

tddmonkey Over a year ago

You don't need 'stream'. The List has a forEach method directly on it

Yassin Hajaj Over a year ago

Simply foreach without stream

Aasmund Eldhuset Over a year ago

@tddmonkey and Yassin Hajaj: Thanks; I wasn't aware of that.

logger Over a year ago

where did you find those information? I know java 8 support lamda but how do you know it can be used like this?

Aasmund Eldhuset Over a year ago

@user3659052: Java 8 added not only lambda function but the stream concept and a bunch of methods for manipulating them with lambdas, so you might want to read up on streams.

ΦXocę 웃 Пepeúpa ツ · Accepted Answer · 2016-04-20 19:38:53Z

1

If your java version is not allowing a Lambdas then do:

List<Listener> l = ...
for (Listener myListeners : listenersList) {
    myListeners.onA();
    myListeners.onB();
    myListeners.onC();
}

answered Apr 20, 2016 at 19:38

ΦXocę 웃 Пepeúpa ツ

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