快速Matlab矩阵到Java列表转换

Question

I must massage data in a matrix of type 131072x1 int32 into a Java List<Integer> in Matlab. So far, the only working conversion I've come up with is to roll through the values and directly add them to a LinkedList.

count = size(data_flattened, 1);
ll = java.util.LinkedList;
for i = 1:count
    ll.add(data_flattened(i));
end

Which is slow in the extreme (5 seconds). I've tried several formulations of converting first to a Java array and then to a List but I always end up with an array with 1 column and 131072 rows.

I need a way of quickly assigning an N-by-1 Matlab matrix of int32s to a Java List<Integer> type.

Answer 1

Convert to a cell

I found one way of getting Matlab to behave the way I want is to convert the matrix to cells.

    cells = num2cell(data_flattened);        
    the_list = java.util.Arrays.asList(cells)

It is faster than rolling through the array and appending to the list, but it is still too slow. On average 0.25 seconds per conversion which is better but still too high.

Java 8 Stream

After some research and testing implementing a function in Java to handle the conversion to from an int[] to a List<Integer> in reasonable time (0.001 seconds).

public static List<Integer> flatten(int[] arr) {
    return IntStream.of(arr).parallel().boxed().collect(Collectors.toList());
}

To use Java 8 you'll need to point your MATLAB_JAVA environment variable to the newer JRE. The location of your JRE can be found using java_home on a Mac.

/usr/libexec/java_home

Then in .bashrc or similar

export MATLAB_JAVA="$(/usr/libexec/java_home)/jre"

Launching MATLAB from the terminal will now correctly pick up the new JRE.

In Matlab you can check your Java version

version -java

and then in Matlab

matlab_data_flattened = matlab_data(:);
java_list = com.my.package.ClassName.flatten(matlab_data_flattened);