如何清除jtextarea中的文本，然后继续附加其他文本（使用相同的按钮）？

Question

I am trying to develop a simple calculator, where when the user keeps on clicking numbers, they appear on the textarea. If they click an operator ("+", "-", "*", etc), the number that was on the textarea, and the operator they click, they get saved on a number and operator variables respectively. When the user clicks another number after clicking the operator, it should remove the number that was at the jtextarea, and write the new number on it on it, so that I can store it on a second number variable and finally make an operation with both numbers.

The problem I'm facing, is that when the user clicks a number for the second time, ie, after clicking the operator, I can't find a way to clear what was on the textarea, and also keep appending the numbers being clicked. If I put a code to clear the textarea and the append a number, all it does, is clear the previous number, and write ONLY ONE NUMBER , for instance, if I wanted to write "857" It can only write "8", and if I click "5", "8" gets cleared and "5" is written.

I understand and get why it doesn't work, because the clear and the append codes happen each time a user clicks a number. So I wanted to know if there is a way around that, and also if it is possible to: clear the text, and then keep appending a text, ie, the second time a user clicks a number, it should append the number on the screen, not clear it.

Answer 1

I want to approach my answer to this problem with two concepts in program design: the first is the design dependence on an external feature, and the second is a state machine.

I'm making some guesses and assumptions here, but from your description it appears that you have fastened onto the "append" feature of a text area and are now programming so that you can use it in a couple of situations: adding a number to the number on the display, and putting a new number in the display ('appending' to an empty display).

That is a nice feature of textarea, it might be useful for your program, but you don't have to use it here.

However you solve this problem, you have a 'state' to save -- when the user presses a key, the thing the program does depends on its state. You have to be able to discern what state it's in so that you can program the response to the keypress correctly.

So let me suggest this: have a variable that stores what you want in the display -- all of it. When someone presses a key, discern your state in whatever fashion and update that variable with what is supposed to be in the display. When you're done processing, set the text area to that variable's value.

Note that this allows the program flow -- receive input, process, display -- to be the same in all cases. No part of the program has to figure out whether to 'clear' the display. Because of that, there is less complexity in the state you are saving.

If your state is "accumulating numbers for first operand", and a number is pressed, then you append the number pressed to the string being displayed.

If your state is "accumulating..." and an operator is pressed, append the operator.

If your state is "operator just pressed" and another number is pressed, set the variable to the number pressed.

and so forth.

These may or may not be the correct states to use, I just have them here as simple examples. But done in some similar way, I think you'll find your program gets simpler to write and to understand -- you remove the code that interacts between the 'append' feature and the program's state, which was making your code more complicated than it needed to be.

It is very common, I've found, for programmers (including myself) to get attached to some implementation that we then work around -- in fact, we are always working around implementations to some extent, that's what we do. But sometimes if we take a step back, we can see that we are doing more programming working around some implementation detail that we can eliminate, and by eliminating it we can simplify our solution.