在Java中评估数学表达式

Question

I'm trying to learn java in my spare time. Thus far I love it. I am currently working on a more advanced version of a simple 4-function calculator. In essence I would like to enter an expression, say "4 + 5" and then split it apart into a first term, a second term, and an operator, then calculate the answer based on the operator symbol detected and output the answer. My issue has been that I'm storing the expression in a char array and that works fine, unless the first term is a double-digit number, or the user adds in another operator. How can I keep this dynamic enough to allow for many forms of simple expressions?

Answer 1

I'd try working with the StringTokenizer class.

Capture the input as a string, then make the StringTokenizer like this:

String delimiter =" ";
StringTokenizer st = new StringTokenizer(inputString, delimiter);

The delimiter is string which you have between the interesting pieces of the string, so you can set it to be whitespace.

After that, you can extract parts of the input by calling nextToken method.

For example:

String firstTerm=st.nextToken();
String operator=st.nextToken();
String secondTerm=st.nextToken();

double ft=Double.parseDouble(firstTerm);
double st=Double.parseDouble(secondTerm);

Then you can make a decision which operation to do based on the input operator using for example a switch statement.

You may need to do some exception handling here, since StringTokenizer can throw exceptions.

If you so wish, you can remain on a lower level and do all this by hand. Simply make a string and then read through your character array character by character until you reach a delimiter. You could do that in a loop which would end when you reach a delimiter. Then you could use the class to parse number from the string. Did you notice the capital D in Double? It means that we're calling function of a class called Double which will parse the number from the string. There are also classes Integer for int, Float for single precision floating point numbers and so on.

If the expression is longer, you can still use StringTokenizer, since there's a method called hasMoreElements which you can use to check if you're at the end of input.

Answer 2

This might be a total overkill if you are not going to extend your program and want to keep it as simple as possible. (We had this method in our first semester of Java programming so it should be a good exercise) You could use the Reverse Polish Notation ( Idea: Wikipedia ) using a stack for your numbers. This would enable you to use parentheses and perform multiple operations in the right order (*,/ before +,-). On the other hand you would need the parentheses around each basic operation (eg " ((A+B)+C) ").

When I implemented a calculator this way I went character by character through the string adding up numbers with two digits or digits after the comma.