Interleaved Matrix

First, you are reading n twice, but you should read n , m and p independently.

Then the most problematic part is this:

    for(int i=0; i<n; i++){
        for(int j=0; j<(n+n); ){
            q[i][j] = m[i][a];
            a++;
            j++;

            q[i][j] = p[i][b];
            b++;
            j++;                
        }
    }

Look at a++; and b++; , they will each go from 0 to 2n² and do not think that's what you want, since when accessing m[i][a] or p[i][b] you will have a column index outside the valid range, and therefore the exception:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 5
        at Ex22.main(Ex22.java:46)

There is probably a lack of a = 0; and a b = 0; somewhere.