Try this code, pastit as private in your clase or as a static method in other one.
private int matrixMult(int m1, int m2)
int m3 = new int[m1.length][m2.length];
m3[i][j] += m1[i][k]*m2[k][j];
the null happens when the sizes of the matrix are not compatible.
n m n m
as you can see if m in a has the same size of n in b.
the calculation can proceed.
n in a and m in b define the new dimensions of the future matrix.
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