public class CalculatorEngine {

    private int value;
    private int keep;
    private char toDo;

    void binaryOperation(char op){
        keep = value;
        value = 0;
        toDo = op;
    }

    void add(){
        binaryOperation('+');
    }

    void subtract(){
     binaryOperation('-');
    }

    void multiply(){
     binaryOperation('*');
    }

    void divide(){
     binaryOperation('/');
    }

    void compute(){
        if(toDo == '+')
            value = keep + value;
        else if (toDo == '-' )
            value = keep - value;
        else if(toDo == '*')
            value = keep * value;
        else if(toDo == '/')
            value = keep / value;
        keep = 0;
    }

    void clear(){
        value = 0;
        keep = 0;
    }

    void digit(int x){
        value = value*10 + x;
    }

    int display(){
        return value;
    }

        CalculatorEngine(){
            clear();
    }

}