How to create a two dimensional array from a stream in Java 8?

Question

I have a text file like this:

ids.txt

1000
999
745
123
...

I want to read this file and load it in a two dimension

Answer 1

Your last line should probably be size -> new Object[size], but you would need to provide arrays of Integers of size one and you would also need to parse the strings into Integers.

I suggest the following:

try (Stream<String> idsStream = Files.lines(idsFile.toPath(), StandardCharsets.US_ASCII)) {
    Object[][] ids = idsStream
       .map(String::trim)
       .filter(s -> !s.isEmpty())
       .map(Integer::valueOf)
       .map(i -> new Integer[] { i })
       .toArray(Object[][]::new);

    // Process ids array here...
}

Answer 2

Given a Stream<String> you can parse each item to an int and wrap it into an Object[] using:

 strings
        .filter(s -> s.trim().length() > 0)
        .map(Integer::parseInt)
        .map(i -> new Object[]{i})

Now to turn that result into a Object[][] you can simply do:

Object[][] result = strings
        .filter(s -> s.trim().length() > 0)
        .map(Integer::parseInt)
        .map(i -> new Object[]{i})
        .toArray(Object[][]::new);

For the input:

final Stream<String> strings = Stream.of("1000", "999", "745", "123");

Output:

[[1000], [999], [745], [123]]