1*2-3+4, Java - rextester

1*2-3+4

/**
    Mathematical Expression Evaluator using two Stacks
*/

import java.util.*;
import java.lang.*;

class Rextester
{  
    public static void main(String args[])
    {
        String expr = "4*3+2-1";
        System.out.println(expr + " = " + evaluate(expr));
    }
    
    private static int evaluate(String expr) {
        Stack<Character> opStack = new Stack<Character>();
        Stack<Integer> valueStack = new Stack<Integer>();
        
        char[] tokens = expr.toCharArray();
        
        for(Character c : tokens) {
            if(c >= '0' && c <= '9') {
                // digit
                valueStack.push(Integer.parseInt(c.toString()));
            } else {
                // operator
                if(!opStack.isEmpty() && hasprecedence(c, opStack.peek())) {
                    valueStack.push(getValue(valueStack.pop(), valueStack.pop(), opStack.pop()));
                }
                
                opStack.push(c);
            }
            
            //System.out.println(valueStack);
        }
        
        while(valueStack.size() >= 2 && !valueStack.isEmpty()) {
            valueStack.push(getValue(valueStack.pop(), valueStack.pop(), opStack.pop()));
            //System.out.println("ok: " + valueStack);
        } 
        
        if(!valueStack.isEmpty())
            return valueStack.pop();
        return 0;
    }
    
    private static boolean hasprecedence(char op2, char op1) {
        if(op1 == '*' && op2 == '+') {
            return true;
        } else if(op1 == '*' && op2 == '-') {
            return true;
        } else if(op1 == '+' && op2 == '-') {
            return true;
        } else if(op1 == '/' && op2 == '+') {
            return true;
        } else if(op1 == '/' && op2 == '-') {
            return true;
        } else {
            return false;
        }
    }
                   
    private static int getValue(int val1, int val2, char op) {
        int result = 0;
        
        switch(op) {
            case '+':
                result = val1 + val2;
                break;
            case '-':
                result = val2 - val1;
                break;
            case '*':
                result = val1 * val2;
                break;
        }
        
        return result;
    }
}


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