Java postfix calculator push/pop method with a String[] array [duplicate]

Answer 1

You are storing the string "22+" in String[] str = {"22+"}; and then you are trying to use it as number in a = Integer.parseInt(stack.peek());

So, if you want to use that notation, you will have to parse your stack.peek() result.

If you know that peek() will return 22+ so you can do:

stack.peek().split("\\+")[0]

Btw, the java.lang.NumberFormatException: For input string: "[22+]" is exactly that, you are trying to convert a number using the invalid string 22+ for an integer. The line which return this error is: Integer.parseInt(stack.peek());

A debugger can help you to find this issue and you can see it easily by dividing:

a = Integer.parseInt(stack.peek()); 

Into:

String elem = stack.peek();
a = Integer.parseInt(elem);