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I used paper and pencil a lot for arrays and linked lists. It's good to see all the details visually before trying to write any code.

To get an idea of how the indexes would go, take a piece of paper and draw the slots for the third array to receive the contents of the other two. Then beneath it, draw the slots for the two shorter...

If the data types are compatible, copy the two arrays into the third array.
You could use one of the Arrays class's copy methods for one and a loop for the other.