Map to 2d array with Streams

Question

I am trying to create a 2d array of String using Streams:

String[] fruit1DArray;
String[][] fruit2DArray;

Map<String, String> fruitMap = new HashMap<>();
fruitMap.put("apple", "red");
fruitMap.put("pear", "green");
fruitMap.put("orange", "orange");

fruit1DArray = fruitMap.entrySet()
    .stream()
    .map(key -> key.getKey())
    .toArray(size -> new String[size]);

fruit2DArray = fruitMap.entrySet()
    .stream()
    .map(entry-> new String[]{entry.getKey()})
    .toArray(size -> new String[size][1]);

System.out.println(Arrays.deepToString(fruit1DArray));
System.out.println(Arrays.deepToString(fruit2DArray));

The output is:

[orange, apple, pear]
[[orange], [apple], [pear]]

The output I am after is:

[orange, apple, pear]
[[orange, orange], [apple, red], [pear, green]]

I am referring https://stackoverflow.com/a/47397601/887235

Eran · Accepted Answer · 2018-07-09 07:00:12Z

6

You forgot to grab the value from your input Map:

fruit2DArray = fruitMap.entrySet()
                       .stream()
                       .map(e -> new String[]{e.getKey(),e.getValue()})
                       .toArray(String[][]::new);

Output:

[[orange, orange], [apple, red], [pear, green]]

edited Jul 9, 2018 at 7:00

answered Jul 9, 2018 at 6:58

Eran

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1 Comment

Holger Over a year ago

Time to fix the already linked answer to similarly use .toArray(Float[][]::new); instead of .toArray(size -> new Float[size][1]); which wastes performance populating the new array with subarrays which are overwritten afterwards.

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