for(int i = 0, j = 0; i < expression.length; i++){
if(expression[i].equals("+") && expression[i].equals("-") && expression[i].equals("*") && expression[i].equals("/")){
j = j + 1;
}else if(j == 0){
firstNumber += expression[i];
}else if(j == 1){
secondNumber += expression[i];
}
}