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);
    }
}

What is your question? Does the code not work? What errors does it give you? Does it work, but yields unexpected results? Please specify. — Sergey Kalinichenko
– Sergey Kalinichenko, Commented Feb 28, 2018 at 13:27
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". — Anton Troitsky
– Anton Troitsky, Commented Feb 28, 2018 at 14:02

Thomas · Accepted Answer · 2018-02-28 13:37:55Z

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.

edited Feb 28, 2018 at 13:37

answered Feb 28, 2018 at 13:32

Thomas

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4 Comments

daniu Over a year ago

...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 Over a year ago

@daniu yes, just added the usage part :)

Anton Troitsky Over a year ago

Soory, It was my bad. I wanted to print the list of files in the console.

user3458 Over a year ago

Another possibility is .map(f.andThen(Path::toString)).collect(...)

Anton Troitsky · Accepted Answer · 2018-02-28 14:10:08Z

0

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);
    }
}

answered Feb 28, 2018 at 14:10

Anton Troitsky

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1 Comment

Thomas Over a year ago

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

Anton Troitsky · Accepted Answer · 2018-02-28 14:14:45Z

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);
    }
}

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