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What do you see when you add this print statement to the code just before the while() test?
System.out.println("cond=" + (a != 'y' || a != 'Y' || a != 'n' || a != 'N') + " for a=" + a);

An easy way to see what the results of a large condition like you are using is to print it:
System.out.println("cond=" + (a != 'y' || a != 'Y' || a != 'n' || a != 'N'));

Put the above just before...