Получить результат из потока в главный поток

ExecutorService[] executors = new ExecutorService[matrix.length];
for(int i = 0; i < executors.length; i++){
     executors[i] = Executors.newSingleThreadExecutor();
     executoes[i].execute(yourRunnable);
}

import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.stream.IntStream;
 
public class Solution {
 
    private static final int MATRIX_SIZE = 500;
    
    private static Random random = new Random();
 
    public static void main(String[] args) throws Exception {
        int[][] matrix = generateMatrix(MATRIX_SIZE, MATRIX_SIZE);
 
        List<CompletableFuture<Integer>> rowFutures = new ArrayList<>();
        IntStream.range(0, matrix.length).forEach(i -> rowFutures.add(CompletableFuture.supplyAsync(() -> IntStream.of(matrix[i]).max().getAsInt())));
        int max = rowFutures.stream().max(Solution::compareForMaxValue).get().get();
        
        System.out.println("Max: " + max);
 
    }
 
    private static int compareForMaxValue(CompletableFuture<Integer> o1, CompletableFuture<Integer> o2) {
        try {
            return o1.get().compareTo(o2.get());
        } catch (InterruptedException | ExecutionException e) {
            e.printStackTrace();
            return -1;
        }
    }
 
    private static int[][] generateMatrix(int h, int w) {
        int[][] matrix = new int[h][w];
        IntStream.range(0, h).forEach(i -> matrix[i] = random.ints(w, 0, Integer.MAX_VALUE).toArray());
        return matrix;
    }
}