Java Puzzlers Advent Calendar 2016 is in a regrettable situation that only one post is missing, so I will substitute the article for Day 5 I will.
class MyTest
{
private static class A {
String message = "hello";
Consumer<String> hello = s -> System.out.print(message + s);
void hello(String s) { this.hello.accept(s); }
}
private static class B extends A {
String message = "Hello";
void hello(String s) { this.hello.accept(s); }
}
public static void main(String[] args) {
A b = (A) new B();
b.hello(" world");
System.out.print("/");
b.hello.accept(" world");
}
}
The result of executing the above code is
-[] A. Hello world / hello world -[] B. Hello world / Hello world -[] C. hello world / hello world
The answer is
It may seem complicated at first glance, but in fact, if you can understand polymorphism properly, you should be able to solve it immediately. Let's take a look at the processing of the main method.
--The first line of the main method declares a variable b of the ʻA type Bimplementation. The upcast process called(A)is written, but in fact it is a work that the compiler does automatically, so it is just annoying and meaningless (although it does not cause an error, of course). --When using a method ofb, the compiler first looks to see if the implementation has that method (second line). There was a method called hello in class B. The override isn't specified, but you can see that the parent class's hellomethod has been overridden because the signatures are the same. The rules of polymorphism execute thehello method of class B. --The method called hello calls the field of the consumer function called hello, but unfortunately hello exists only in the ʻA class. Therefore, it is unavoidable to use the hello function in the ʻAclass. (By the way, duplicate method names and field names are allowed, so no compilation error will occur.) --The functionhello uses the field message. The message of class ʻA is hello, so the output will be hello world. There is also a field called message in class B, but it has nothing to do with the hello function you are using now.
――Furthermore, on the 4th line of main, it looks like you are using the hello function of the b object. But keep in mind that there are no field variable overrides. The hello function, like the hello method, defines the processing procedure, but is essentially a field variable like message. Therefore, which field is used depends on the reference type at the time of declaration. Since it was declared as ʻA, the hello function of class ʻA is used and it looks the same as the output result on the second line.