Java - Input from file

[Tohrchur]

So I'm trying to get ahead in my labs for my programming class. I've got this one lab almost done but she added a part where I have to take integers from a txt file "input.txt" and have them in an array in the program, then methods with take over.  I've done a whole bunch of Googling but couldn't quite figure out how to get input from a file.  Also it needs input validation to make sure that the only thing being passed to the array are integers.

Any help is greatly appreciated.

[afyeung]

Look at the Scanner API https://docs.oracle.com/javase/7/docs/api/java/util/Scanner.html you need to instantiate a new scanner object as a new file with your "input.txt". from then on just look at the methods in the API. Good luck

[Imabigmac]

So I would use a scanner that accesses the .txt. A counter to count .nextInt to get the length of .txt. Then use that for the "for loop" to use the scanner which will grab only ints as that will be required in an "if" statement. 

 

But I think I may be making it harder, as you can just scan   "scanner.nextInt" and grab the int. Like this page explains. 

http://stackoverflow.com/questions/13185727/reading-a-txt-file-using-scanner-class-in-java

 

 

[Philosobyte]

27 minutes ago, Imabigmac said:

So I would use a scanner that accesses the .txt. A counter to count .nextInt to get the length of .txt. Then use that for the "for loop" to use the scanner which will grab only ints as that will be required in an "if" statement. 

 

But I think I may be making it harder, as you can just scan   "scanner.nextInt" and grab the int. Like this page explains. 

http://stackoverflow.com/questions/13185727/reading-a-txt-file-using-scanner-class-in-java

 

 

Yes, the #nextInt method is the most practical way to get the next integer. Scanner not only has #nextInt, but also the boolean-returning method #hasNextInt. If you look at Scanner's JavaDoc, you can see that #hasNextInt checks if the next token is an integer. 

 

A lot of classes have this format with both a "getter" method to get a certain next value and a condition-checking method to make sure that what you're looking for exists. 

[Tohrchur]

I always get an exception error message with the 

Scanner sc = new Scanner(file);

do i absolutely have to do a catch and try to work with files? thats what Ive seen from stackoverflow but just seems weird to me that the only way to read from a file is with catch and try.

[Blade of Grass]

58 minutes ago, Tohrchur said:

I always get an exception error message with the 

Scanner sc = new Scanner(file);

do i absolutely have to do a catch and try to work with files? thats what Ive seen from stackoverflow but just seems weird to me that the only way to read from a file is with catch and try.

Unless you want to make that method throw IOExceptions (and any other method that calls this method, you really shouldn't do this), then yes, you need to use a try and catch. This is how Java handles possible errors (such as the file not existing) and is the proper way to do things. 

You should actually be using a try-with-resources though so that it handles the opening and closing of the scanner. 

[Koolwaterstof]

I've always found BufferedReader the easiest to use when reading files.

 

BufferedReader br = new BufferedReader(new FileReader(new File("FILENAME.txt")));

You will need to try-catch this statement. Use .readLine() to get the next line of the file. If you have 1 integer per line, you could use the following method:

 

LinkedList<Integer> data = new LinkedList<>();

String line;

while((line = br.readLine()) != null)
{
  data.add(Integer.parseInt(line));
}

This uses a Linked List instead of an array because you don't know how long the file is going to be, so you can't declare the length of the array. If you want to convert the Linked List to an array afterwards, you can do by doing this:

 

int[] arr = new int[data.size()];

for(int i=0;i<data.size();i++)
{
  arr[i] = data.get(i);
}

And of course, don't forget to close your Buffered Reader at the end using

br.close();

Feel free to ask if there's anything here that you don't understand.

[Blade of Grass]

4 hours ago, Koolwaterstof said:

I've always found BufferedReader the easiest to use when reading files.

 

BufferedReader br = new BufferedReader(new FileReader(new File("FILENAME.txt")));

You will need to try-catch this statement. Use .readLine() to get the next line of the file. If you have 1 integer per line, you could use the following method:

 

LinkedList<Integer> data = new LinkedList<>();

String line;

while((line = br.readLine()) != null)
{
  data.add(Integer.parseInt(line));
}

This uses a Linked List instead of an array because you don't know how long the file is going to be, so you can't declare the length of the array. If you want to convert the Linked List to an array afterwards, you can do by doing this:

 

int[] arr = new int[data.size()];

for(int i=0;i<data.size();i++)
{
  arr[i] = data.get(i);
}

And of course, don't forget to close your Buffered Reader at the end using

br.close();

Feel free to ask if there's anything here that you don't understand.

Why would you suggest the use of a LinkedList over an ArrayList? What factors would you consider when making this decision?

[Koolwaterstof]

17 hours ago, Blade of Grass said:

Why would you suggest the use of a LinkedList over an ArrayList? What factors would you consider when making this decision?

Purely because I've never used an ArrayList before, so that's not something that crossed my mind. I said that a LinkedList would be more appropriate than an array, not an ArrayList - you could use an ArrayList instead if you prefer.