Creating a Map<String, ArrayList<String[]>> from ArrayList<String[]> in Java

Question

I have an ArrayList of String Array and the 2nd element of every String Array contains a name. I want to create a Map where it's Key is the name and Value is an ArrayList of all String Array which has the name.

How can I achive this in Java?

Input: ArrayList<String[]>

{"1", "NameABC", "somestring"}
{"2", "NameDEF", "somestring"}
{"3", "NameDEF", "somestring"}
{"4", "NameABC", "somestring"}
{"5", "NameXYZ", "somestring"}

Output: Map<String, ArrayList<String[]>

Key = NameABC, Value = ArrayList of String[] where every String[] has NameABC

Key = NameXYZ, Value = ArrayList of String[] where every String[] has NameXYZ

I've tried using stream.collect(Collectors.toMap()) but I can't figure out how to achieve the proper output.

Why did you try stream.collect(Collectors.toMap())? What was wrong with your attempt? — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Jan 31, 2023 at 17:22
I cannot figure out a way to map the 2nd element of the String[] — Devang Patel
– Devang Patel, Commented Jan 31, 2023 at 17:59
By the way, in real work it would likely make more sense to define a record to hold the elements of each array as fields in an object. A record can be defined locally, within a method. — Basil Bourque
– Basil Bourque, Commented Jan 31, 2023 at 20:23

Bohemian · Accepted Answer · 2023-01-31 21:11:36Z

1

Streams and collectors are not always the best option:

Map<String, List<String[]>> map = new HashMap<>();
myListOfArrays.forEach(arr -> map.computeIfAbsent(arr[1], x -> new ArrayList<>()).add(arr));

If the key can be null and you want to store it under a blank, use the expression arr[1] == null ? "" : arr[1] instead:

myListOfArrays.forEach(arr -> map.computeIfAbsent(arr[1] == null ? "" : arr[1], x -> new ArrayList<>()).add(arr));

edited Jan 31, 2023 at 21:11

answered Jan 31, 2023 at 17:15

Bohemian♦

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1 Comment

shmosel Over a year ago

Streams are not always the best option, but a stream with Collectors.groupingBy() seems perfectly appropriate here.

Madhuri Kadam · Accepted Answer · 2023-02-01 12:13:38Z

1

Try this -

List<String[]> l1 = new ArrayList<>();
    l1.add(new String[]{"1", "NameABC", "somestring"});
    l1.add(new String[]{"2", "NameDEF", "somestring"});
    l1.add(new String[]{"3", "NameDEF", "somestring"});
    l1.add(new String[]{"4", "NameABC", "somestring"});
    l1.add(new String[]{"5", "NameXYZ", "somestring"});
    
    Map<String, List<String[]>> map = l1.stream().collect(Collectors.groupingBy(arr -> arr[1]));
    
    for (Map.Entry<String, List<String[]>> entry : map.entrySet()) {
        String key = entry.getKey();
        List<String[]> val = entry.getValue();
        
        System.out.println(key + " -- "+ val);
    }

edited Feb 1, 2023 at 12:13

answered Jan 31, 2023 at 18:18

Madhuri Kadam

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3 Comments

Devang Patel Over a year ago

Thank you. Your solution worked. I got an NullPointerException when Key was null but that won't be an issue if I replace null values with empty String.

shmosel Over a year ago

What's the point of new HashMap<>()? Just assign the stream result straight to your variable. Also, you don't need to work so hard to print a map. Just do System.out.println(map);. Otherwise, good answer.

Holger Over a year ago

Or, to get your specific output format, use map.forEach((key, val) -> System.out.println(key + " -- "+ val));

Oleg Cherednik · Accepted Answer · 2023-01-31 21:17:37Z

0

You can use Stream with groupingBy():

public static Map<String, List<String[]>> convert(List<String[]> data) {
    return data.stream().collect(Collectors.groupingBy(arr -> arr[1]));
}

answered Jan 31, 2023 at 21:17

Oleg Cherednik

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