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

Question

I have a text file like this:

ids.txt

I want to read this file and load it in a two dimensional array. I expect to have an array similar to the one below:

Object[][] data = new Object[][] { //
     { new Integer(1000) }, //
     { new Integer(999) }, //
     { new Integer(745) }, //
     { new Integer(123) }, //
     ...
};

Here is the code I wrote:

File idsFile = ... ;
try (Stream<String> idsStream = Files.lines(idsFile.toPath(), StandardCharsets.US_ASCII)) {
    Object[][] ids = idsStream
       .filter(s -> s.trim().length() > 0)
       .toArray(size -> new Object[size][]);

    // Process ids array here...
}

When running this code, an exception is raised:

java.lang.ArrayStoreException: null
at java.lang.System.arraycopy(Native Method) ~[na:1.8.0_45]
at java.util.stream.SpinedBuffer.copyInto(Unknown Source) ~[na:1.8.0_45]
at java.util.stream.Nodes$SpinedNodeBuilder.copyInto(Unknown Source) ~[na:1.8.0_45]
at java.util.stream.SpinedBuffer.asArray(Unknown Source) ~[na:1.8.0_45]
at java.util.stream.Nodes$SpinedNodeBuilder.asArray(Unknown Source) ~[na:1.8.0_45]
at java.util.stream.ReferencePipeline.toArray(Unknown Source) ~[na:1.8.0_45]
...

How can resolve this exception?

Why new Integer(1000)? And you want an [][] where the inner array always has length == 1? — Boris the Spider
– Boris the Spider, Commented Jun 13, 2015 at 10:08
@BoristheSpider The ids are passed to a third party library that accept only Object[][] as input. — Stephan
– Stephan, Commented Jun 13, 2015 at 10:09

Boris the Spider · Accepted Answer · 2015-06-13 10:17:54Z

16

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]]

answered Jun 13, 2015 at 10:17

Boris the Spider

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5 Comments

Tagir Valeev Over a year ago

If you suspect that strings may sporadically have spaces before and after, then it's better to add the separate trim step: strings.map(String::trim).filter(s -> !s.isEmpty()). Also I would use Integer::valueOf instead of Integer::parseInt as it matches better the wanted type (Integer, not int). Though probably that's a matter of taste.

Stephan Over a year ago

@TagirValeev Why is it "better to add the separate trim step" ?

Tagir Valeev Over a year ago

@Stephan: this way you can parse files which have spaces after numbers (like "1 \n2\n3 \n"). Currently you will have NumberFormatException on such files.

Holger Over a year ago

Do you think there is a benefit in these two map operations instead of simply saying .map(s -> new Object[]{Integer.parseInt(s)})?

Boris the Spider Over a year ago

@Holger don't know really. I like to split each logical transformation into a separate map step - I think it makes to code more readable. I suppose it's just a matter of personal preference - I doubt there's much of a performance impact.

assylias · Accepted Answer · 2015-06-13 18:20:49Z

5

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...
}

edited Jun 13, 2015 at 18:20

answered Jun 13, 2015 at 10:16

assylias

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