This should be close to what you want, albeit at a very basic level. It should handle any equation that contains only integers and the 4 basic operators, provided they're seperated by spaces, as in "4 + 2 / 3 - 6 * 7"
Note that it doesn't check for errors in the formatting of the string, and it may not even compile (haven't tested it), but it should give you a more-than-ample nudge in the right direction. The basic algorithm is: look for an operator (following the order-of-operations, which I only halfway implemented). If you find one, perform that operation on the numbers on either side of the operator, store the result in the position of the left operand, and delete the operator and right operand. This eventually leaves just one entry in the list, which is your final answer.
Here's the code:
public class EquationParser{
String equation;
public EquationParser(String equation) { this.equation = equation; }
public int ParseMe()
{
String[] split = equation.split(" ");
ArrayList pieces = new ArrayList();
for (int i=0; i<split.length; i++) pieces.add(split[i]);
for(int i=0; i<pieces.size(); i++)
{
int temp;
if (pieces.get(i).equals("/"))
{
temp = Integer.parseInt((String)pieces.get(i-1)) / Integer.parseInt((String)pieces.get(i+1));
pieces.set(i-1, Integer.toString(temp));
pieces.removeRange(i, i+2);
i--;
}
}
for(int i=0; i<pieces.size(); i++)
{
int temp;
if (pieces.get(i).equals("*"))
{
temp = Integer.parseInt((String)pieces.get(i-1)) * Integer.parseInt((String)pieces.get(i+1));
pieces.set(i-1, Integer.toString(temp));
pieces.removeRange(i, i+2);
i--;
}
}
for(int i=0; i<pieces.size(); i++)
{
int temp;
if (pieces.get(i).equals("-"))
{
temp = Integer.parseInt((String)pieces.get(i-1)) - Integer.parseInt((String)pieces.get(i+1));
pieces.set(i-1, Integer.toString(temp));
pieces.removeRange(i, i+2);
i--;
}
}
for(int i=0; i<pieces.size(); i++)
{
int temp;
if (pieces.get(i).equals("+"))
{
temp = Integer.parseInt((String)pieces.get(i-1)) + Integer.parseInt((String)pieces.get(i+1));
pieces.set(i-1, Integer.toString(temp));
pieces.removeRange(i, i+2);
i--;
}
}
int result = Integer.parseInt((String)pieces.get(0));
return(result);
}
}