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

Use .forEach instead of .map. Note that you can use Files.list(Paths.get("./src/renamer/Newaudio")).forEach(...) to jump into the Stream API directly. — aioobe
– aioobe, Commented Jan 16, 2017 at 14:03
If you're using Java 8, Don't use the File class. It's outdated and should be laid to rest. Use Path and Files. — RealSkeptic
– RealSkeptic, Commented Jan 16, 2017 at 14:04

Community · Accepted Answer · 2017-05-23 12:09:57Z

4

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

edited May 23, 2017 at 12:09

CommunityBot

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answered Jan 16, 2017 at 14:06

Pavneet_Singh

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2 Comments

Sal-laS Over a year ago

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

Andy Turner Over a year ago

@Salman you seem to misunderstand what "Lambda" is. The lambda is the parameter of the map or forEach method, not the map method itself.

Andrew · Accepted Answer · 2017-01-16 18:40:46Z

3

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

edited Jan 16, 2017 at 18:40

answered Jan 16, 2017 at 14:19

Andrew

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3 Comments

Sal-laS Over a year ago

when i used the multiple line format of lambda (f->{system.out.pringlin()}) it complains that Syntax error, insert "AssignmentOperator Expression" to complete Expression - The left-hand side of an assignment must be a variable

Holger Over a year ago

@Salman: The System class starts with an upper case S, the method is println, not pringlin, and there has to be a semi-colon after the statement.

Holger Over a year ago

@Andrew Tobilko: forEach (not foerEach) can take more than these four kind of arguments. A named class implementing Consumer works as well, as any other expression leading to an instance of that type, like reading a variable containing an existing instance or invoking a factory method. And you can pass null, the compiler won’t object, though the result is predictably of little use…

hoat4 · Accepted Answer · 2017-07-30 08:01:33Z

3

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

edited Jul 30, 2017 at 8:01

answered Jan 16, 2017 at 14:04

hoat4

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1 Comment

tobias_k Over a year ago

Note: OP's original code did not print the file names but just the numbers.

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