Java 8: replace anonymous class with lambda

Question

I'm having problem in replacing this particular example:

Consumer consumer = new DefaultConsumer(channel) {
    @Override
    public void handleDelivery(String consumerTag, Envelope envelope, AMQP.BasicProperties properties, byte[] body)
            throws IOException {
        String message = new String(body, "UTF-8");
        System.out.println(" [x] Received '" + message + "'");
    }
};

Is it possible to replace that with lambda as it uses non-default constructor for DefaultConsumer?

It's from rabbitMQ java tutorial -> LINK to whole class

It depends on DefaultConsumer. Is there more than one abstract method ? How are you using Consumer consumer ? — AxelH
– AxelH, Commented Nov 14, 2017 at 13:35
@AxelH rabbitmq.com/releases/rabbitmq-java-client/current-javadoc/com/… — slim
– slim, Commented Nov 14, 2017 at 13:36
@slim, thanks, that's answer my questions, nope it can't be used for it! — AxelH
– AxelH, Commented Nov 14, 2017 at 13:40
If you do this for multiple DefaultConsumers though, you could create a helper method, taking a lambda as input and returning an anonymous subclass of Defaultconsumer. — tobias_k
– tobias_k, Commented Nov 14, 2017 at 13:43

shmosel · Accepted Answer · 2017-11-28 19:08:13Z

7

No you can't. DefaultConsumer is not a FunctionalInterface (and can't be: more info here) hence is not a lambda target.

Explanation:

Can every anonymous class be replaced by a lambda expression?

The answer is no. You can create an anonymous class for non-final classes and interfaces. Is not the same for lambda expressions. These can be only used where a SAM interface is expected, i.e. interfaces with only a Single Abstract Method (before Java 8 every interface method was abstract but since Java 8 interfaces can also have default and static methods which aren't abstract because they have implementation).

So which anonymous classes can be replaced with a lambda expression?

Only anonymous classes which are implementations of a SAM interface (like Runnable, ActionListener, Comparator, Predicate) can be replaced by a lambda expression. DefaultConsumer cannot be a lambda target because is not even an Interface.

And what about Consumer?

Even though Consumer is an Interface, it is not a SAM Interface because it has more than 1 abstract method so it can't be a lambda target either.

edited Nov 28, 2017 at 19:08

shmosel

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answered Nov 14, 2017 at 13:36

Juan Carlos Mendoza

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5 Comments

AxelH Over a year ago

Doesn't change much on the result here but FunctionalInterface is not mandatory .

slim Over a year ago

An interface can be a "functional interface" without being annotated @FunctionInterface -- but DefaultConsumer is not a functional interface for a number of reasons -- not an interface; multiple methods none of which is abstract...

Juan Carlos Mendoza Over a year ago

@AxelH I know but it needs to be a SAM type anyway to be a lamda target

AxelH Over a year ago

I know, that's why this "doesn't change much on the result" ;) but the explanation is a bit broad, I prefer your comment as an explanation. Because your answer don't do the difference between "is not a FunctionnalInterface" and "can not be a FunctionnalInterface" since the annotation itself is optional.

Juan Carlos Mendoza Over a year ago

@AxelH understood. Edited my answer.

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