Java 8: Map instead of simple loop

Question

I am new in functional programming in java 1.8. I have simple loop like the below code:

File folder = new File("./src/renamer/Newaudio");
File[] listOfFiles = folder.listFiles();

for (int i = 0; i < listOfFiles.length; i++) {
    System.out.println("Here is file number: " + i);
}

I am going to change the upper for loop to the functional format using the arrow function (like a lambda).

listOfFiles.map((file) = > {
        System.out.println("Here is file number: " + i);
});

Unfortunately, it complains with:

Cannot invoke map((<no type> file) -> {}) on the array type File[]

Why i get this error? how i can resolve it?

I like to learn about Lambda, so i am not interested to the foreach

Answer 1

You can use IntStream.range to print the file numbers only

    IntStream.range(0,listOfFiles.length).forEach(System.out::println);

or To print Content of array you can use

    Arrays.stream(listOfFiles).forEach(System.out::println);

Compiler will convert your method reference to lambda expression at compile time so this

    Arrays.stream(listOfFiles).forEach(System.out::println);

will be converted into this

=>  Arrays.stream(listOfFiles).forEach(i->System.out.println(i));

With lambda you can use the expended lambda {} as well

=> Arrays.stream(listOfFiles).forEach(i->{
       if (!i.isHidden()) { // don't print hidden files
           System.out.println(i);               
       }
   });

So listFiles returns null is file is not a directory and for simplicity you can simply put a check at the first place

    File folder = new File("./src/renamer/Newaudio");
    if (folder.isDirectory()) {
        File[] listOfFiles = folder.listFiles();
        Arrays.stream(listOfFiles).forEach(i->System.out.println(i));           
    }

Reference

Lambda and Method reference concise details

Answer 2

folder.listFiles() should be wrapped with the Optional.ofNullable to prevent from a NullPointerException. It can return null if the given pathname does not denote a directory, or if an I/O error occurs.

Stream.of(ofNullable(folder.listFiles()).
          orElseThrow(() -> new IllegalArgumentException("It's not a directory!")))
      .forEach(f -> {});

where the forEach method could take these arguments:

(1) an anonymous class

new Consumer<File>() {
    public @Override void accept(File file) {
        System.out.println(file);
    }
}

(2) expended lambda expressions

(File f) -> { System.out.println(f); }
(f) -> { System.out.println(f); }

(3) simplified lambda expressions

f -> { System.out.println(f); }
f -> System.out.println(f)

(4) a method reference

System.out::println

Answer 3

IntStream.range(0, listOfFiles.length).forEach(file -> {
    System.out.println(file);
});