MainActivity.java

private int performCalculations() {
    switch (OPERATOR) {
        case '+':
            return FIRST_NUMBER + SECOND_NUMBER;
        case '-':
            return FIRST_NUMBER - SECOND_NUMBER;
        case '*':
            return FIRST_NUMBER * SECOND_NUMBER;
        case '/':
            return FIRST_NUMBER / SECOND_NUMBER;
        case '^':
            return FIRST_NUMBER ^ SECOND_NUMBER;
    }
    return -999;
}