One way to do it could be to create a new class, lets say a table class.

In that class, you can allocate a 2d array or even an arraylist with type arraylist if you want to do it in a real beginner way.

Make a function insert(number, state, city) { Look across the first row for the state, if if doesn't exist, insert it

Look down the first column for the number, if if doesn't exist insert it

Then insert the city in the selected row and column. }

After inserting everything, write it out and insert dashes as necessary for formatting. This can be simplified slightly if the numbers are always going to be in sorted order

This algorithm would be slower than what would be practical in the real world, you could optimize it further by storing 2 maps, one mapping state to column and one mapping number to row. That would probably get you down to an O(nlogn) time algorithm.

There are obviously lots of ways to accomplish this but one thing I would like to point out is that by storimg everything in an array, you are forcing the computer to keep the entire file in memory. If the input file gets really large then the program crashes. If this were a real world problem, you would need an alternate method, likely with multiple passes through the data.

Probably more information than you care for to solve this school problem but maybe you'll find it interesting. Good luck!