Java: how to pass Class to lambda

Question

I try to make create multi dirs and files more easier with follwoing code:

package ro.ex;

class File {

    public interface Lamb {
        public void call(Class c);
    }

    public static void tap(Lamb l) {
        l.call(File.class);
    }

    public static void mkdir(String path) {
    }

    public static void main(String[] args) {
        File.tap((f) -> {
            f.mkdir("dir");
            f.mkdir("dir2");
        });
    }
}

but in f.mkdir("dir"), intellij idea raise can't resolve method 'mkdir'

my question is: how to change code to make code in main block work well

Why do your mkdir calls all pass a String but your mkdir method doesn't take any parameters? — David Conrad
– David Conrad, Commented Nov 10, 2014 at 22:56

Konstantin Yovkov · Accepted Answer · 2014-11-09 09:04:35Z

3

According to the method contract, File.tap() accepts a Lamb parameter.

Lamb is a functional interface (contains only one abstract method) and the body of your lambda is the anonymous implementation of its abstract method.

The abstract method has a definition public void call(Class c) and this is why your code fails to compile. You're trying to pass a File object, instead of a Class one.

In the mean time, mkdir is a static method and can be invoked like this:

public static void main(String[] args) {
    File.tap((f) -> {
        File.mkdir("dir");
        File.mkdir("dir2");
    });
}

edited Nov 9, 2014 at 9:04

answered Nov 9, 2014 at 8:54

Konstantin Yovkov

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2 Comments

zizhuo64062kw Over a year ago

How to change "Class" to "File.class", i try "File.class" but syntax is wrong

Konstantin Yovkov Over a year ago

Just make it: public void call(File f);

Seelenvirtuose · Accepted Answer · 2014-11-09 08:53:41Z

1

You defined a (functional) interface Lamb whose single method takes a Class as an argument.

So the f in your lambda expression (f) -> { ... } is of type Class. But this class does not know the method mkdir.

You have a static method mkdir in your custom class File. Static methods are called like this:

File.tap((f) -> {
    File.mkdir("dir");
    File.mkdir("dir2");
});

This makes the Class argument in method Lamb.call(Class) rather useless. Maybe you want to reconsider the design.

answered Nov 9, 2014 at 8:53

Seelenvirtuose

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2 Comments

zizhuo64062kw Over a year ago

How to change "Class" to "File.class", i try "File.class" but syntax is wrong

Seelenvirtuose Over a year ago

If you want to call a file, simply declare your method: void call(File f) and make sure to not mix it up with java.io.File.

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