CalculatorEngine.java

package my_v1;

public class CalculatorEngine {

    private double currentTotal;

    public CalculatorEngine() {
        currentTotal = 0;
    }

    public String getTotalString() {
        return "" + currentTotal;
    }

    public void equal(String number) {
        currentTotal = convertToDouble(number);
    }

    public void add(String number) {
        currentTotal += convertToDouble(number);
    }

    public void subtract(String number) {
        currentTotal -= convertToDouble(number);
    }

    public void multiply(String number) {
        currentTotal *= convertToDouble(number);
    }

    public void divide(String number) {
        currentTotal /= convertToDouble(number);
    }

    public void changeSign(String number) {
        Double d = Double.parseDouble(number);
        currentTotal = d * (-1);

    }

    public void dot(String number) {
        // todo
    }

    private boolean isDouble(String number) {
        double d = Double.parseDouble(number);
        return d % 1.0 != 0;
    }

    private double convertToDouble(String number) {
        return Double.parseDouble(number);
    }

}