Postfix expressions evaluation [closed]

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As a newbie to Java .. How to evaluate postfix expression of multi-digit numbers including negative ones (ie. "12 -12 +")

My snippet gives "java.lang.NullPointerException" !

I used "StringTokenizer" , however it didn't work efficiently

Anyone could appreciately help improving code ?

Thanks in advance

Answer 1

The algorithm is:

1. read the string from the last element to the first and put each element in a stack of Strings (each element of your stack will be a single char operator or a word convertible to a number).
2. pop 3 elements convert the first two to a number and depending on the operator calculate the result
3. push the result on the stack
4. repeat 2) and 3) till the stack is empty.