Limitations of forEach with instance method references in Java 8

Question

Assume I have the following functional interface:

public interface TemperatureObserver {     void react(BigDecimal t); } 

and then in another class an already filled-in ArrayList of objects of type TemperatureObserver. Assuming that temp is a BigDecimal, I can invoke react in a loop using:

observers.forEach(item -> item.react(temp)); 

My question: can I use a method reference for the code above?

The following does not work:

observers.forEach(TemperatureObserver::react); 

The error message is telling me that

1. forEach in the Arraylist observers is not applicable to the type TemperatureObserver::react
2. TemperatureObserver does not define a method react(TemperatureObserver)

Fair enough, as forEach expects as an argument a Consumer<? super TemperatureObserver>, and my interface, although functional, does not comply to Consumer because of the different argument of react (a BigDecimal in my case).

So can this be solved, or it is a case in which a lambda does not have a corresponding method reference?

Answer 1

There are three kinds of method references that can be used when a single value is available from the stream:

1. A parameter-less method of the streamed object.

   class Observer {     public void act() {         // code here     } }  observers.forEach(Observer::act);  observers.forEach(obs -> obs.act()); // equivalent lambda 

   The streamed object becomes the this object of the method.

2. A static method with the streamed object as parameter.

   class Other {     public static void act(Observer o) {         // code here     } }  observers.forEach(Other::act);  observers.forEach(obs -> Other.act(obs)); // equivalent lambda 

3. A non-static method with the streamed object as parameter.

   class Other {     void act(Observer o); }  Other other = new Other(); observers.forEach(other::act);  observers.forEach(obs -> other.act(obs)); // equivalent lambda 

There is also a constructor reference, but that is not really relevant to this question.

Since you have an external value temp, and you want to use a method reference, you can do the third option:

class Temp {     private final BigDecimal temp;     public Temp(BigDecimal temp) {         this.temp = temp;     }     public void apply(TemperatureObserver observer) {         observer.react(this.temp);     } }  Temp tempObj = new Temp(temp);  observers.forEach(tempObj::apply);