使用扫描仪从文本文件中读取某一行

Question

I am reading values in a text file one by one without any problem in Java. The file has integer values as shown below:

2 3
5 6 7 8 9
12 13 15 18 21 26 55
16 24 45 58 97

I need to read just a single line values instead of reading all values and there is an example on How to get line number using scanner , but I am looking a solution without using LineNumberReader and using a counter by looping all lines. Instead, it would be good to get the line number via scanner method that I already use to read values. Is it possible?

Here is the method I try to create:

public static List<Integer> getInput(int lineIndex) throws FileNotFoundException{   
    List list = new ArrayList<Integer>();
    Scanner scan = new Scanner(new File(INPUT_FILE_NAME));
    int lineNum=0;
    //s = new Scanner(new BufferedReader(new FileReader("input.txt")));

    while(scan.hasNextLine()) {         
        if(lineNum == lineIndex) {
               list.add(scan.nextInt()); //but in this case I also loop all values in this line
        }
        lineNum++;      
    }
    scan.close();
    return list;
}

Answer 1

If you have a small file, where it is acceptable to load the whole content into memory:

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.regex.Pattern;
import java.util.stream.Collectors;
import java.util.stream.Stream;

....

public static List<Integer> getInput(int lineIndex) {
    List<Integer> list = new ArrayList<>();
    try {
        String line = Files.readAllLines(Paths.get("path to your file")).get(lineIndex);
        list = Pattern.compile("\\s+")
                .splitAsStream(line)
                .map(Integer::parseInt)
                .collect(Collectors.toList());
    } catch (IOException ex) {
        ex.printStackTrace();
    }
    return list;
}

The method Files.readAllLines returns a list of all lines of the file as strings, where each string corresponds to one line. with get(lineIndex) you get the desired nth line back and only have to parse the strings to integers.

Second approach:

If you have a large file make use of the lazy evaluation of streams:

public static List<Integer> getInputLargeFile(int lineIndex) {
    List<Integer> list = new ArrayList<>();
    try (Stream<String> lines = Files.lines(Paths.get("path to your file"))) {
        String line = lines.skip(lineIndex).findFirst().get();
        list = Pattern.compile("\\s+")
                .splitAsStream(line)
                .map(Integer::parseInt)
                .collect(Collectors.toList());
    } catch (IOException ex) {
        ex.printStackTrace();
    }
    return list;
}

With this approach you use the method lines.skip to jump n lines from the beginning of the file. And with findFirst().get() you get the next line after your jump. Then do the same as above to convert the numbers.

Answer 2

Not to iterate while reading the number in desired line, maybe you can read the line with scanner.nextLine() , then you can use split("\\s") to get the staring values of integers into an array, then you can simply cast and add them in your list:

public static void main(String[] args) throws FileNotFoundException {
    Scanner scan = new Scanner(new File("input.txt"));
    List<Integer> list = new ArrayList<>();
    int lineNum = 0;
    int lineIndex = 2;

    while(scan.hasNextLine()) {
        if(lineNum == lineIndex) {
            String[] nums = scan.nextLine().split("\\s");
            for (String s : nums){
                list.add(Integer.parseInt(s));
            }
            break;
        }
        lineNum++;
        scan.nextLine();
    }

    System.out.println(list); // [12, 13, 15, 18, 21, 26, 55]
}