Java 8 Filter Array Using Lambda

Question

I have a double[] and I want to filter out (create a new array without) negative values in one line without adding for loops. Is this possible using Java 8 lambda expressions?

In python it would be this using generators:

[i for i in x if i > 0]

Is it possible to do something similarly concise in Java 8?

What do you mean by _filter out_? Do you mean create a new array without those values? — Sotirios Delimanolis, Jun 09 '14 at 01:53
Not sure if possible with a `double[]`, but easy with a `List`: http://www.dreamsyssoft.com/java-8-lambda-tutorial/filter-tutorial.php — Avery, Jun 09 '14 at 01:54
FYI that's called a list comprehension. Generators use parens, like this: `(i for i in x if i > 0)` — djsmith, Oct 04 '16 at 11:10

Alex - GlassEditor.com · Accepted Answer · 2016-05-17T16:47:02.787

149

Yes, you can do this by creating a DoubleStream from the array, filtering out the negatives, and converting the stream back to an array. Here is an example:

double[] d = {8, 7, -6, 5, -4};
d = Arrays.stream(d).filter(x -> x > 0).toArray();
//d => [8, 7, 5]

If you want to filter a reference array that is not an Object[] you will need to use the toArray method which takes an IntFunction to get an array of the original type as the result:

String[] a = { "s", "", "1", "", "" };
a = Arrays.stream(a).filter(s -> !s.isEmpty()).toArray(String[]::new);

edited May 17 '16 at 16:47

answered Jun 09 '14 at 01:57

Alex - GlassEditor.com

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I think that is about as concise as Java will get :D – Makoto Jun 09 '14 at 02:12
I think it should be double[] toArray = Arrays.stream(d).filter(x -> x > 0).toArray(); – Raj Jun 09 '14 at 02:24
@Raj that's valid also if you want to assign the returned array to a separate reference. – Alex - GlassEditor.com Jun 09 '14 at 02:27
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I think the call to `toArray()` will return an `Object []` instead, so that second line might fail with a cast exception. Thanks! – Jesús Zazueta May 17 '16 at 16:37
1

@JesúsZazueta I have edited the answer, does that solve your problem? – Alex - GlassEditor.com May 17 '16 at 16:48

score 3 · Answer 2 · answered Mar 12 '18 at 03:00

even simpler, adding up to String[],

use built-in filter filter(StringUtils::isNotEmpty) of org.apache.commons.lang3

import org.apache.commons.lang3.StringUtils;

    String test = "a\nb\n\nc\n";
    String[] lines = test.split("\\n", -1);


    String[]  result = Arrays.stream(lines).filter(StringUtils::isNotEmpty).toArray(String[]::new);
    System.out.println(Arrays.toString(lines));
    System.out.println(Arrays.toString(result));

and output: [a, b, , c, ] [a, b, c]

Java 8 Filter Array Using Lambda

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