Convert a List<String> to Map<String, String> using the list values in Java 8

Question

I have a List that I am trying to convert to a Map<String, String> using Java 8.

The list is as below

List<String> data = Arrays.asList("Name", "Sam", "Class", "Five", "Medium", "English")

I want a Map<String, String> such that the key value pair will be like

{{"Name", "Sam"}, {"Class", "Five"}, {"Medium", "English"}}

I am trying to achieve this in Java 8 and tried using Instream.range() but did not get the exact result.

IntStream.range(0, data.size() - 1).boxed()
                .collect(Collectors.toMap(i -> data.get(i), i -> data.get(i + 1)));

The issue with the above code is the result also gives an output as {"Sam", "Class"}

WJS · Accepted Answer · 2022-03-25 14:29:54Z

1

Here is one way.

generate a range of values, i, from 0 to size/2.
then use i*2 and i*2 + 1 as the indices into the list.

List<String> data = Arrays.asList("Name", "Sam", "Class",
        "Five", "Medium", "English");

Map<String, String> map = IntStream
        .range(0,data.size()/2).boxed()
        .collect(Collectors.toMap(i -> data.get(i*2),
                i -> data.get(i*2+1)));

map.entrySet().forEach(System.out::println);

prints

Medium=English
Class=Five
Name=Sam

answered Mar 25, 2022 at 14:29

WJS

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Comments

Michael Berry · Accepted Answer · 2022-03-25 10:52:34Z

1

The issue here is that you're stepping through every index, whereas you just want to step through every other index. You could either filter the odd values, adding .filter(i->i%2==0) to your stream - or use IntStream.iterate() to get the numbers you want directly:

IntStream.iterate(0, i->i<data.size()-1,i->i+2)

answered Mar 25, 2022 at 10:52

Michael Berry

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Comments

frascu · Accepted Answer · 2022-03-25 14:33:26Z

1

You have to iterate every 2 elements, therefore the steps to do are 3 equal to 6 (list size) / 2 (step size).

So iterate by steps and not by elements and you can find the two elements of each step in the collect phase. Here is the example:

IntStream.range(0, data.size() / 2)
         .boxed()
         .collect(Collectors.toMap(i -> data.get(i * 2), i -> data.get(i * 2 + 1)));

answered Mar 25, 2022 at 14:33

frascu

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2 Comments

WJS Over a year ago

How is that different from my answer?

frascu Over a year ago

The code is equal, the explanation is different. When I was writing the solution, I did not see that meanwhile you wrote a similar answer.

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