Only catch certain word lengths

Question

If I got a .txt file named words.txt and I want to catch the words to input them into an arraylist how do I do that. I know buffered reader exists but I dont quite get how to use it. All words are seperated by a space or an enter key. It has to then for example filter out words that are not 4 characters long and place the 4 long words in an arraylist to use later. For example I got this txt file :

one > gets ignored
two > gets ignored
three > gets ignored
four >  caught and put into for example arraylist
five > 4 long so gets caught and put into arraylist
six > ignored
seven > ignored
eight >  ignored
nine >  caught because its 4 char long
ten > ignored

Answer 1

You can do it using streams and NIO.2

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Main {

    public static void main(String[] args) {
        Path path = Paths.get("words.txt");
        try (Stream<String> lines = Files.lines(path)) {
            List<String> list = lines.filter(word -> word.length() == 4)
                                     .collect(Collectors.toList());
            System.out.println(list);
        }
        catch (IOException xIo) {
            xIo.printStackTrace();
        }
    }
}

Here is my words.txt file:

one
two
three
four
five
six
seven
eight
nine
ten

And running the above code, using the above file, prints the following:

[four, five, nine]

Alternatively, you can use a Scanner

import java.io.IOException;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Path source = Paths.get("words.txt");
        try (Scanner scanner = new Scanner(source)) {
            List<String> list = new ArrayList<>();
            while (scanner.hasNextLine()) {
                String word = scanner.nextLine();
                if (word.length() == 4) {
                    list.add(word);
                }
            }
            System.out.println(list);
        }
        catch (IOException xIo) {
            xIo.printStackTrace();
        }
    }
}

Note that both the above versions of class Main use try-with-resources.

Yet another way is to use class java.io.BufferedReader (since you mentioned it in your question).

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) {
        File f = new File("words.txt");
        try (FileReader fr = new FileReader(f);
             BufferedReader br = new BufferedReader(fr)) {
            List<String> list = new ArrayList<>();
            String line = br.readLine();
            while (line != null) {
                if (line.length() == 4) {
                    list.add(line);
                }
                line = br.readLine();
            }
            System.out.println(list);
        }
        catch (IOException xIo) {
            xIo.printStackTrace();
        }
    }
}