Java. Reference to method without parameters

Question

I have one problem in understanding of Java 8 associated with reference to methods as a parameter to static methods. There is my code where I can't find how to send a reference to a method which doesn't have any parameters and must be a method for an object of the definite class.

So, I want that the method which I send to static function may be used for the object in the static method.

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.stream.Stream;
interface FuncForPath<T> {
 T func(T path);
}
class MethodsForFolder {
 public static void printFilesInFolder(Path path, FuncForPath<Path> funcPath) {
 Stream<Path> paths = null;
 try {
 paths = Files.list(path);
 } catch (IOException e) {
 System.out.println(
 "The problems have been appeared during reading folder: " + path.toAbsolutePath().toString());
 e.printStackTrace();
 }
 System.out.println("Files list in folder:" + path.toAbsolutePath().toString());
 paths.forEach(p -> funcPath.func(p).toString()); // I don't know to how to write this code to perform
 }
}
public class TestRefToIntance {
 public static String testWindowsFloder = "C://Logs";
 public static void main(String[] args) {
 Path path = Paths.get(testWindowsFloder);
 // I want that this 2 methods are performed depending on transfered methods
 // reference
 MethodsForFolder.printFilesInFolder(path, Path::toAbsolutePath);
 MethodsForFolder.printFilesInFolder(path, Path::getFileName);
 }
}

• Sergey Kalinichenko: What is your question? Does the code not work? What errors does it give you? Does it work, but yields unexpected results? Please specify.
• Anton Troitsky: Sorry I have posted before the code with a static method from another class. I have edited the code it works now like a previous version. I see a message "Files list in folder:C:\Logs".

Answer 1

Try Function<Path, Path> instead of FuncForPath. This will require a method taking a Path parameter and returning a Path. Note that instance methods always have an "invisible" this parameter, hence Path::getFileName matches that signature.

You'd then call it like this: paths.forEach( p -> funcPath.apply( p ).toString() ); (although you're not doing anything with the returned string, so you probably want to call paths.map( f ).map( Path::toString ).collect( someCollector ); instead.

• daniu: ...and you can do paths.map(funcPath).map(Path::toString) so it transforms from your path to the path your function returns, then into the string representation. Also note you might want to actually print them by .forEach(System.out::println) or something.
• Thomas: @daniu yes, just added the usage part :)
• Anton Troitsky: Soory, It was my bad. I wanted to print the list of files in the console.
• user3458: Another possibility is .map(f.andThen(Path::toString)).collect(...)

Answer 2

How I understand I have resolved this problem. There is code below.

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.stream.Stream;
interface FuncForPath<T> {
 T func(T path);
}
class MethodsForFolder {
 public static void printFilesInFolder(Path path, FuncForPath<Path> funcPath) {
 Stream<Path> paths = null;
 try {
 paths = Files.list(path);
 } catch (IOException e) {
 System.out.println(
 "The problems have been appeared during reading folder: " + path.toAbsolutePath().toString());
 e.printStackTrace();
 }
 System.out.println("Files list in folder:" + path.toAbsolutePath().toString());
 paths.forEach(p -> System.out.println(funcPath.func(p).toString())); // I don't know to how to write this code to perform
 }
}
public class TestRefToIntance {
 public static String testWindowsFloder = "C://Logs";
 public static void main(String[] args) {
 Path path = Paths.get(testWindowsFloder);
 // I want that this 2 methods are performed depending on transfered methods
 // reference
 MethodsForFolder.printFilesInFolder(path, Path::toAbsolutePath);
 MethodsForFolder.printFilesInFolder(path, Path::getFileName);
 }
}

• Thomas: You can do it that way but there's normally no need for the FuncForPath interface. Just use Function<Path, Path>.

Answer 3

Also, I have deleted the generic specification in interface description.

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.stream.Stream;
interface FuncForPath { // there is interface without type <T>
 Path func(Path path);
}
class MethodsForFolder {
 public static void printFilesInFolder(Path path, FuncForPath funcPath) {
 Stream<Path> paths = null;
 try {
 paths = Files.list(path);
 } catch (IOException e) {
 System.out.println(
 "The problems have been appeared during reading folder: " + path.toAbsolutePath().toString());
 e.printStackTrace();
 }
 System.out.println("Files list in folder:" + path.toAbsolutePath().toString());
 paths.forEach(p -> System.out.println(funcPath.func(p).toString())); // I don't know to how to write this code
 // to perform
 }
}
public class TestRefToIntance {
 public static String testWindowsFloder = "C://Logs";
 public static void main(String[] args) {
 Path path = Paths.get(testWindowsFloder);
 // I want that this 2 methods are performed depending on transfered methods
 // reference
 MethodsForFolder.printFilesInFolder(path, Path::toAbsolutePath);
 MethodsForFolder.printFilesInFolder(path, Path::getFileName);
 }
}