Convert String array to Map using Java 8 Lambda expressions

Question

Is there a better functional way of converting an array of Strings in the form of "key:value" to a Map using the Java 8 lambda syntax?

Arrays.asList("a:1.0", "b:2.0", "c:3.0")
        .stream()
        .map(elem -> elem.split(":")
        .collect(Collectors.toMap(keyMapper?, valueMapper?));

The solution I have right now does not seem really functional:

Map<String, Double> kvs = new HashMap<>();
Arrays.asList("a:1.0", "b:2.0", "c:3.0")
        .stream()
        .map(elem -> elem.split(":"))
        .forEach(elem -> kvs.put(elem[0], Double.parseDouble(elem[1])));

Eran · Accepted Answer · 2020-03-26 09:12:25Z

97

You can modify your solution to collect the Stream of String arrays into a Map (instead of using forEach) :

Map<String, Double> kvs =
    Arrays.asList("a:1.0", "b:2.0", "c:3.0")
        .stream()
        .map(elem -> elem.split(":"))
        .collect(Collectors.toMap(e -> e[0], e -> Double.parseDouble(e[1])));

Of course this solution has no protection against invalid input. Perhaps you should add a filter just in case the split String has no separator :

Map<String, Double> kvs =
    Arrays.asList("a:1.0", "b:2.0", "c:3.0")
        .stream()
        .map(elem -> elem.split(":"))
        .filter(elem -> elem.length==2)
        .collect(Collectors.toMap(e -> e[0], e -> Double.parseDouble(e[1])));

This still doesn't protect you against all invalid inputs (for example "c:3r" would cause NumberFormatException to be thrown by parseDouble).

edited Mar 26, 2020 at 9:12

answered Jul 29, 2015 at 7:24

Eran

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9 Comments

Tagir Valeev Over a year ago

I don't think that silently ignoring erroneous input is a good advice and this was not requested by OP. I'd better use elem -> elem.split(":", 2) instead of adding the filter step. This way erroneous input will be manifested either by ArrayIndexOutOfBoundsException (if no colon was found) or by NumberFormatException (if more than one colon was found).

bsam Over a year ago

Thanks for all the comments and additional input on the error checking. This is what I needed.

Vlad.Bachurin Over a year ago

If task is to convert a real array to Map, then the expression should start with Arrays.stream(<arrayVariable>)

Eran Over a year ago

@Vlad.Bachurin It also works with the current code - Arrays.asList(<arrayVariable>).stream().... The Arrays.asList("a:1.0", "b:2.0", "c:3.0") part was taken as is from the question.

Vlad.Bachurin Over a year ago

@Eran I got your point, but Arrays.stream(<array>) is shorter anyway. And fits better the subject of the question. Peace. :)

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