Stream a list of n-integers from a file to create n object array

Question

Assume each T object can be instantiated as

T tobj = new T(//int value);

So to create an array of T[ ] from integers in a file seperated by space I do the following:

 BufferedReader br;
 FileReader fr;
 int[] arr;
 try{               
        fr = new FileReader(fo); // assume "fo" file name
        br = new BufferedReader(fr);
        arr = Arrays.stream(br.readLine().split("\\s")).mapToInt(Integer::parseInt).toArray();

 }catch(SomeException e){//something else}

T[] tobjarr = new T[arr.length];
    for(int i=0; i<arr.length; ++i)){
        tobjarr[i] = new T(arr[i]);
    }

1.Is the above method efficient in terms of time and space usage?

2.Is there any other way? if so how does it compare to above method?

Answer 1

In general your approach is fine. However, you can do that with a single stream cascade. Compared to your original approach this saves you one iteration.

Also note that nowadays we read files using Javas new I/O API called NIO. One big advantage is that it offers Stream methods. For example the Files#lines method that returns a stream over all lines of the file, perfectly suited for your approach.

So all in all, here is the complete code:

String file = ...
Pattern separator = Pattern.compile("\\s");

try (Stream<String> lines = Files.lines(Paths.get(file))) {
    T[] values = lines                      // Stream<String> lines
        .flatMap(separator::splitAsStream)  // Stream<String> words
        .mapToInt(Integer::parseInt)        // IntStream values
        .mapToObj(T::new)                   // Stream<T> valuesAsT
        .toArray(T[]::new);
} catch (IOException e) {
    System.out.println("Something went wrong.");
}

Note that this code is slightly different to yours, as yours will only process one line and mine all lines. You may change that if you don't want it:

List<T[]> valuesPerLine = Files.lines(Paths.get(file))  // Stream<String> lines
    .map(separator::splitAsStream)  // Stream<Stream<String>> wordsPerLine
    .map(lineStream -> {
        return lineStream                 // Stream<String> words
            .mapToInt(Integer::parseInt)  // IntStream values
            .mapToObj(T::new)             // Stream<T> valuesAsT
            .toArray(T[]::new);
    })                              // Stream<T[]> valuesPerLine
    .collect(Collectors.toList());

The main difference to your original approach is that we can easily transform an IntStream into a Stream<T> by using mapToObj(T::new) (or just map if it's a regular stream and not IntStream) which passes the elements to the constructor. After that we collect Stream<T> into an array by using toArray(T[]::new).

Answer 2

T[] array = Arrays.stream(br.readLine().split("\\s"))
    .map(s -> new T(Integer.parseInt(s)))
    .toArray(T[]::new)

EDIT: noticed that you're using a different delimiter