将二维数组转换为 Java 中的列表列表

Question

So, this might be a simple question, but I wasn't able to find any easy or elegant way to do this. Converting an array to a list is trivial in Java

Double[] old = new Double[size];
List<Double> cast = Arrays.asList(old);

But I'm dealing with images currently and I would like the ability to extend this functionality to a 2d array without having to iterate through one dimension of the array appending to a list.

Double[][] -> List<List<Double>>

Is basically what I would like to achieve. I have a solution along the lines of:

Double[][] old= new Double[width][height];
List<List<Double>> new= new ArrayList<List<Double>>();
for (int i=0;i<old.length();i++){
    new.add(Arrays.asList(old[i]));
}

I would like something better and potentially faster than this.

Answer 1

The only faster way to do this would be with a fancier view; you could do this with Guava like so:

Double[][] array;
List<List<Double>> list = Lists.transform(Arrays.asList(array),
  new Function<Double[], List<Double>>() {
    @Override public List<Double> apply(Double[] row) {
      return Arrays.asList(row);
    }
  }
}

That returns a view in constant time.

Short of that, you already have the best solution.

(FWIW, if you do end up using Guava, you could use Doubles.asList(double[]) so you could use a primitive double[][] instead of a boxed Double[][] .)

Answer 2

After JAVA 8 stream APIs we can get the list of lists from 2d array in a much faster and cleaner way.

Double[][] old= new Double[width][height];
List<List<Double>> listOfLists = Arrays.stream(Objects.requireNonNull(old)).map(row -> {
        return Arrays.asList((row != null) ? row : new Double[0]);
    }).collect(Collectors.toList());

Answer 3

作为 Java8，你做Arrays.stream(array).map(Arrays::asList).collect(Collectors.toList()) 。