Read in file and store into a dynamically allocated string array

Question

Beginner Java programmer here. I have searched for a while on the interwebs without much success.

I need to read in a text file and store each line into a string array. However I do not know how big the text file will be thus I was trying to figure out a easy way to dynamically allocate the size of the string array. I didn't know if there was a handy tool already in the Java library I could use. I was thinking maybe counting the total # of lines in the file first, then allocating the string array, but I also didn't know the best way to do that.

Thank you for any input!

Use an array-list. To read in the entire file, use streams. Use a buffered reader. The reading of the file should be done in a while loop, using ReadLine, and looping until the next ReadLine is not null. You can then determine the size of the array-list, and define an array of that length and then just transfer the contents of the array-list into the array. — user6911980
– user6911980, Commented Jun 3, 2017 at 0:04

Heath Raftery · Accepted Answer · 2017-06-03 00:07:28Z

1

You could use an ArrayList and not worry about sizing:

List<String> fileLines = new ArrayList<String>();

try (BufferedReader br = new BufferedReader(new FileReader(file)))
{
    String line;
    while ((line = br.readLine()) != null)
        fileLines.add(line);
}

fileLines could get pretty big, but if you're okay with that then this is an easy way to get started.

answered Jun 3, 2017 at 0:07

Heath Raftery

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2 Comments

eg04lt3r Over a year ago

Also just ensure that file size do not exceed your heap memory allocated for your jvm process.

Chris Corley Over a year ago

Excellent, thank you for your help. This is far more efficient than the code I was writing on my own!

user6911980 · Accepted Answer · 2017-06-03 00:23:41Z

1

Define an array-list which does not require a fixed length, as you can add or remove as many elements as you wish:

    List<String> fileList = new ArrayList<String>();
    //Declare a file at a set location:
    File file = new File("C:\\Users\\YourPC\\Desktop\\test.txt");
    //Create a buffered reader that reads a file at the location specified:
    try (BufferedReader br = new BufferedReader(new FileReader(file)))
    {
        String line;
        //While there is something left to read, read it:
        while ((line = br.readLine()) != null)
            //Add the line to the array-list:
            fileList.add(line);
    }catch(Exception e){
        //If something goes wrong:
        e.printStackTrace();
    }

    //Determine the length of the array-list:
    int listTotal = fileList.size();
    //Define an array of the length of the array-list:
    String[] fileSpan = new String[listTotal];

    //Set each element index as its counterpart from the array-list to the array:
    for(int i=0; i<listTotal; i++){

        fileSpan[i] = fileList.get(i);
    }

answered Jun 3, 2017 at 0:23

user6911980

1 Comment

Chris Corley Over a year ago

Excellent, thank you for your help. This is far more efficient than the code I was writing on my own! Also, this code snipet really helps me get started. Thanks!

dave_thompson_085 · Accepted Answer · 2017-06-03 03:43:05Z

0

If you only want a working program (and not practice in coding and debugging) in Java8+:

 String[] ary = java.nio.file.Files.readAllLines(Paths.get(filename)).toArray(new String[0]);
 // substitute the (Path,Charset) overload if your data isn't compatible with UTF8
 // if a List<String> is sufficient for your needs omit the .toArray part

answered Jun 3, 2017 at 3:43

dave_thompson_085

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