I am reading Java 8 book, and it comes with a sample I reproduce:
@FunctionalInterface
public interface Action {
public void perform();
}
An Implementor:
public final class ActionImpl implements Action {
public ActionImpl() {
System.out.println("constructor[ActionIMPL]");
}
@Override
public void perform() {
System.out.println("perform method is called..");
}
}
A caller:
public final class MethodReferences {
private final Action action;
public MethodReferences(Action action) {
this.action = action;
}
public void execute() {
System.out.println("execute->called");
action.perform();
System.out.println("execute->exist");
}
public static void main(String[] args) {
MethodReferences clazz = new MethodReferences(new ActionImpl());
clazz.execute();
}
}
If this is called the following is print into the output:
constructor[ActionIMPL]
execute->called
perform method is called..
execute->exist
Everything is all right but if I use method references not perform message
method is printed! Why is this, am I missing something?
If I use this code:
MethodReferences clazz = new MethodReferences(() -> new ActionImpl());
clazz.execute();
Or this code:
final MethodReferences clazz = new MethodReferences(ActionImpl::new);
This is printed:
execute->called
constructor[ActionIMPL]
execute->exist
No exception message or anything else is printed. I am using Java 8 1.8.25 64bit.
For readers that are studying like me, this is the right running code.
I have created a class the caller.
Because I need to implement a empty method "perform from the Action functional interface" which I need to pass as parameter to class constructor MethodReference
I reference the "constructor of the MethodReferenceCall which is a empty constructor" and I can use it.
public class MethodReferenceCall {
public MethodReferenceCall() {
System.out.println("MethodReferenceCall class constructor called");
}
public static void main(String[] args) {
MethodReferenceCall clazz = new MethodReferenceCall();
MethodReferences constructorCaller = new MethodReferences(MethodReferenceCall::new);
constructorCaller.execute();
}
}
This
MethodReferences clazz = new MethodReferences(() -> new ActionImpl());
does not use method reference, it uses a lambda expression. The functional interface is Action
's
public void perform();
So
() -> new ActionImpl()
gets translated into something similar to
new Action() {
public void perform() {
new ActionImpl();
}
}
Similarly, in
MethodReferences clazz = new MethodReferences(ActionImpl::new);
the ActionImpl::new
, which does use a constructor reference, is translated into something like
new Action() {
public void perform() {
new ActionImpl();
}
}
This ActionImpl::new
does not invoke new ActionImpl()
. It resolves to an instance of the expected type whose functional interface method is implemented as invoking that constructor.