How to convert a Java 8 Stream into a two dimensional array?

Question

I’m trying to convert a map based Stream into a two-dimensional array. I have figured out how to store it in a one dimensional array. Here is working code snippet:

Float[] floatArray = map.entrySet()
                        .stream()
                        .map(key -> key.getKey().getPrice())
                        .toArray(size -> new Float[size]);

When I execute the above code, I get my Float array populated as expected. Now I need to extend this to a two-dimensional array where I need to store the result in first dimension of a 2d array along these lines:

Float[][1] floatArray = map.entrySet()
                           .stream()
                           .map(key -> key.getKey().getPrice())
                           .toArray(size -> new Float[size][1]);

The code above does not work. Can you please let me know how to accomplish this task with Java 8 streams? Thanks in advance!

"convert and map based stream to a two-dimensional array" - please post a (small) example of such map and how is the 2D array supposed to look like (expected output). — Nir Alfasi
– Nir Alfasi, Commented Nov 20, 2017 at 17:17

Eran · Accepted Answer · 2017-11-20 17:19:12Z

12

If you look at <A> A[] toArray(IntFunction<A[]> generator), you see that it converts a Stream<A> to a A[], which is a 1D array of A elements. So in order for it to create a 2D array, the elements of the Stream must be arrays.

Therefore you can create a 2D array if you first map the elements of your Stream to a 1D array and then call toArray:

Float[][] floatArray = 
    map.entrySet()
       .stream()
       .map(key -> new Float[]{key.getKey().getPrice()})
       .toArray(size -> new Float[size][1]);

answered Nov 20, 2017 at 17:19

Eran

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6 Comments

MazS Over a year ago

Thank you so much, that works! My next question is if I want to populate three dimensions of my floatArray from the map for instance: Float[][] floatArray = map.entrySet() .stream() .map(key -> new Float[]{key.getKey().getPrice()},{key.getKey().getOpen()},{key.getKey().getClose()}) .toArray(size -> new Float[size][3]); How can this be accomplished?

MazS Over a year ago

I want to be able to first populate all three dimensions/columns of my 2D floatArray from map entries and then loop through the floatArray later in the code to perform downstream calculations using floatArray like so: for (int i=0; i<floatArray.rows; i++) { for (int j=0; j<floatArray.cols; j++) { //do some calcs here someCalc = floatArray[i][j] * 2; }

MazS Over a year ago

One last request, in the third column of floatArray, I want to store the counter for example if there are 10 entries in the map, I want to store value 1...10 in third column. Thanks

MazS Over a year ago

I was able to figure out how to populate multiple dimensions: Float[][] floatArray = map.entrySet() .stream() .map(key -> new Float[]{(float) 0.00,key.getKey().getPrice(),(float) 0.00}) .toArray(size -> new Float[size][3]); The only thing left is to add a counter in the first position or how to replace (float) 0.00 in position 1 with a counter. Thanks

Eran Over a year ago

@MazS that's still a 2D array, not 3D. If that's what you wanted, your comment about "three dimensions" was confusing.

|

Sourabh Bhat · Accepted Answer · 2019-08-13 09:29:35Z

8

You can make use of the following:

Float[][] array = map.entrySet()
    .stream()
    .map(Map.Entry::getKey)
    .map(YourKeyClass::getPrice) // 1)
    .map(price -> new Float[]{ price })
    .toArray(Float[][]::new);

Which creates a 2D array just like you expected.

Note: By the comment 1) you have to replace YourKeyClass with the class containing the method getPrice() returning a Float object.

An alternative is also to use .keySet() instead of .entrySet():

Float[][] array = map.keySet().stream()
    .map(YourKeyClass::getPrice)
    .map(price -> new Float[]{price})
    .toArray(Float[][]::new);

edited Aug 13, 2019 at 9:29

Sourabh Bhat

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answered Nov 20, 2017 at 17:52

Lino

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