Task: Kronecker product
Go: package main

import (
    "fmt"
    "strings"
)

type uintMatrix [][]uint

func (m uintMatrix) String() string {
    var max uint
    for _, r := range m {
        for _, e := range r {
            if e > max {
                max = e
            }
        }
    }
    w := len(fmt.Sprint(max))
    b := &strings.Builder{}
    for _, r := range m {
        fmt.Fprintf(b, "|%*d", w, r[0])
        for _, e := range r[1:] {
            fmt.Fprintf(b, " %*d", w, e)
        }
        fmt.Fprintln(b, "|")
    }
    return b.String()
}

func kronecker(m1, m2 uintMatrix) uintMatrix {
    p := make(uintMatrix, len(m1)*len(m2))
    for r1i, r1 := range m1 {
        for r2i, r2 := range m2 {
            rp := make([]uint, len(r1)*len(r2))
            for c1i, e1 := range r1 {
                for c2i, e2 := range r2 {
                    rp[c1i*len(r2)+c2i] = e1 * e2
                }
            }
            p[r1i*len(m2)+r2i] = rp
        }
    }
    return p
}

func sample(m1, m2 uintMatrix) {
    fmt.Println("m1:")
    fmt.Print(m1)
    fmt.Println("m2:")
    fmt.Print(m2)
    fmt.Println("m1 ⊗ m2:")
    fmt.Print(kronecker(m1, m2))
}

func main() {
    sample(uintMatrix{
        {1, 2},
        {3, 4},
    }, uintMatrix{
        {0, 5},
        {6, 7},
    })
    sample(uintMatrix{
        {0, 1, 0},
        {1, 1, 1},
        {0, 1, 0},
    }, uintMatrix{
        {1, 1, 1, 1},
        {1, 0, 0, 1},
        {1, 1, 1, 1},
    })
}

Java: package kronecker;


public class Product {
  
  public static int[][] product(final int[][] a, final int[][] b) {
    
    
    final int[][] c = new int[a.length*b.length][];
    
    
    for (int ix = 0; ix < c.length; ix++) {
      final int num_cols = a[0].length*b[0].length;
      c[ix] = new int[num_cols];
    }
    
    
    for (int ia = 0; ia < a.length; ia++) {
      for (int ja = 0; ja < a[ia].length; ja++) {
        
        for (int ib = 0; ib < b.length; ib++) {
          for (int jb = 0; jb < b[ib].length; jb++) {
             c[b.length*ia+ib][b[ib].length*ja+jb] = a[ia][ja] * b[ib][jb];
          }
        }
      }
    }

    
    return c;
  }

  
  public static void print_matrix(final int[][] m) {
    
    
    
    
    final String[][] sts = new String[m.length][];
    int max_length = 0;  
    for (int im = 0; im < m.length; im++) {
      sts[im] = new String[m[im].length];
      for (int jm = 0; jm < m[im].length; jm++) {
        final String st = String.valueOf(m[im][jm]);
        if (st.length() > max_length) {
          max_length = st.length();
        }
        sts[im][jm] = st;
      }
    }

    
    
    
    final String format = String.format("%%%ds", max_length);
    for (int im = 0; im < m.length; im++) {
      System.out.print("|");
      
      for (int jm = 0; jm < m[im].length - 1; jm++) {
        System.out.format(format, m[im][jm]);
        System.out.print(" ");
      }
      System.out.format(format, m[im][m[im].length - 1]);
      System.out.println("|");
    }
  }

  
  private static void test(final int[][] a, final int[][] b) {
    
    System.out.println("Testing Kronecker product");
    System.out.println("Size of matrix a: " + a.length + " by " + a[0].length);
    System.out.println("Matrix a:");
    print_matrix(a);
    System.out.println("Size of matrix b: " + b.length + " by " + b[0].length);
    System.out.println("Matrix b:");
    print_matrix(b);
    System.out.println("Calculating matrix c as Kronecker product");
    final int[][] c = product(a, b);
    System.out.println("Size of matrix c: " + c.length + " by " + c[0].length);
    System.out.println("Matrix c:");
    print_matrix(c);
  }

  
  private static void test1() {
    
    final int[][] a = new int[2][];  
    a[0] = new int[]{1, 2};
    a[1] = new int[]{3, 4};
    final int[][] b = new int[2][];  
    b[0] = new int[]{0, 5};
    b[1] = new int[]{6, 7};
    
    test(a, b);
  }

  
  private static void test2() {
    
    final int[][] a = new int[3][];  
    a[0] = new int[]{0, 1, 0};
    a[1] = new int[]{1, 1, 1};
    a[2] = new int[]{0, 1, 0};
    final int[][] b = new int[3][];  
    b[0] = new int[]{1, 1, 1, 1};
    b[1] = new int[]{1, 0, 0, 1};
    b[2] = new int[]{1, 1, 1, 1};
    
    test(a, b);
  }

  
  public static void main(final String[] args) {
    
    test1();
    test2();
  }

}