Java 8 method reference

Question

In Java 8 is it possible to call 2 reference methods in a row?

customer::new::getName

Can be concatenated 2 methods reference of the form:

a::b::c

• azurefrog: No, but see stackoverflow.com/questions/26920866/… for ideas for alternatives.
• Michael Peyper: Possible duplicate of Chain of Map method references
• Holger: What’s wrong with ()->new customer().getName() or ()->a.b().c()?

Answer 1

How to inline two method reference expression ?

Let's decomposing Customer::new::getName expression;

T1 customer = Customer::new;
T2 name = customer::getName; 

according JLS described Method Reference Expression,so T1 and T2 must are FunctionalInterface.

A method reference expression is compatible in an assignment context, invocation context, or casting context with a target type T if T is a functional interface type (§9.8) and the expression is congruent with the function type of the ground target type derived from T.

when you use Supplier<Customer> with T1 then the compiler reports error:can't resolve method getName defined on Supplier. but you can define your own FunctionalInterface, e.g: CustomerSupplier.

interface CustomerSupplier {
 Customer get();
 default String getName() {
 return get().getName();
 }
}

then you can refer Customer::new to CustomerSupplier:

CustomerSupplier customer = Customer::new;
Supplier<String> name = customer::getName;

Indeed, you can inline two expression together in further, but each method reference expression must be a FunctionalInterface, so you must cast Customer::new to CustomerSupplier.

Supplier<String> customerName = ((CustomerSupplier) Customer::new)::getName;

However, the simplest one is access directly that provided by @Holger.

Supplier<String> customerName = () -> new Customer().getName();

How to chain two method reference expression ?

Due to Supplier has no methods for composing FunctionInterface, but you can chain the method reference expression like this:

interface Reference<T> extends Supplier<T> {
 static <T> Reference<T> of(Supplier<T> reference) {
 return reference::get;
 }
 default <R> Reference<R> of(Function<T, R> after) {
 return () -> after.apply(get());
 }
}
//NOTE:Reference is derived from Supplier, so it is compatible with Supplier. 
Supplier<String> customerName = Reference.of(Customer::new).of(Customer::getName);

However, you can introduce a method to avoid introduce a type for composing Function Interfaces that provided by @Holger.

private <T, R> Supplier<R> map(Supplier<T> target, Function<T, R> mapper) {
 return () -> mapper.apply(target.get());
}
Supplier<String> customerName = map(Customer::new, Customer::getName);

Answer 2

In Java 8 is it possible to call 2 reference methods in a row?

No, it's not possible to chain methods when using method references because a method reference is essentially a shorthand syntax for a lambda expression that executes just ONE method. Hence, once you start chaining methods it would no longer be valid syntax for a method reference.

syntax

Object::methodName

Answer 3

No, you can't chain two method references.

Basically, a method reference is a syntactic sugar which will allow you to pass the definition of an existing method to a Lambda expression (which implements single abstract method).