calculatrice.java

package calculatrice;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Calculatrice {
    public  static final char[] list_operators = new char[] {'+', '-', '*', '/'};
    private static List<Integer>           numbers;
    private static Map<Integer, Character> operators;
    
    public static String execute(String input) {   
        // CAS SPECIAUX
        if (input.isEmpty())
            return "";
        
        if (input.matches("[0-9]+"))
            return input;

        input = input.replaceAll("\\s","");
        
        // INITIALISATION
        numbers   = new ArrayList<>();
        operators = new HashMap<Integer, Character>();
        
        // DECOMPOSITION
        for (int i = 0; i < input.length(); i++) {
            if (isInto(input.toCharArray()[i], list_operators))
                operators.put(i, input.toCharArray()[i]);
        }
        
        // TESTS
//        System.out.println("Les operateurs :");
//        for (Object c : operators.values().toArray()) {
//            System.out.println(c);
//        }
        // END TESTS
        
        Integer[] idxOperators = Arrays.copyOf(operators.keySet().toArray(), operators.values().size(), Integer[].class);
        
        // TESTS
//        System.out.println("IDX operateurs :");
//        for (Integer i : idxOperators) {
//            System.out.println(i);
//        }
        // END TESTS
        
        for (int i = 0; i < idxOperators.length + 1; i++) {
            if (i == 0) {
                numbers.add(Integer.parseInt(input.substring(0, idxOperators[0])));
            } else if (i == idxOperators.length) {
                numbers.add(Integer.parseInt(input.substring(idxOperators[idxOperators.length - 1] + 1, input.length())));
            } else {
                numbers.add(Integer.parseInt(input.substring(idxOperators[i - 1] + 1, idxOperators[i])));
            }
        }
        
        // TESTS
//        System.out.println("Les nombres :");
//        for (Integer i : numbers)
//            System.out.println(i);
        // END TESTS        
        
        // CALCUL 
        Double res = new Double(numbers.get(0));
        for (int i = 1; i < numbers.size(); i++) {
            switch (operators.get(idxOperators[i - 1])) {
                case '+':
                    res += numbers.get(i);
                    break;
                case '-':
                    res -= numbers.get(i);
                    break;
                case '*':
                    res *= numbers.get(i);
                    break;
                case '/':
                    res /= numbers.get(i);
                    break;
                default:
                    break;
            }
        }
        
        if (res.intValue() == res)
            return String.valueOf(res.intValue());
        
        // END
        return String.valueOf(res);
    }
    
    private static boolean isInto(char c, char[] list) {
        for (char ch : list) {
            if (ch == c)
                return true;
        }
        return false;
    }
    
    public static void main(String[] args) {
        System.out.println("LE RESULTAT : " + Calculatrice.execute("1/2"));
    }
}