The calculator above doesn't seem to work for me.
However, I can make a couple of suggestions that think will help you a long the way.
A) Please don't take this the wrong way. Part of programming is resolving the issues that you are having -- that is the fun of it. Gets more fun when your under pressure of deadlines.
C) Finally, use div tags to write the validated key press to read from that separating the values by operators.
calculation_parts = calculation.split('+');
answer = parseInt(calculation_part) + parseInt(calculation_part).
$('#output_div_id').html($('#output_div_id').html()+ "=" + answer);
Hope that helps - good luck with learning java