a question on conversion logic

I am designing a unit converter using android.i m using logic which is not nice logic i think and i m giving some code here too.can anyone suggest me any better logic????

public void temperature(String value1,String value2)
        {
            if(value1.equals("Celsius") && value2.equals("fahrenheit") )
            {
                double i=Double.parseDouble(at1.getText().toSt开发者_JS百科ring());
                double value=i*33.8;
                Double d=new Double(value);
                at2.setText(d.toString());
            }
            else if(value1.equals("fahrenheit") && value2.equals("Celsius"))
            {
                double i=Double.parseDouble(at1.getText().toString());
                double value=i*(-17.2222);
                Double d=new Double(value);
                at2.setText(d.toString());
            }

There is like this many unit in every category like angle,computer.this is only a small example of temperature category.

Why not just

at2.setText(String.valueOf(Double.parseDouble(at1.getText().toString()) * 33.8);

Anyway, it's just to shorten your code, the logic stays the same.

P.S. Define some constants for values like 33.8 and -17.2222.

There's a lot of duplicated code in your example. Try to refactor it so you common actions are expressed only once.

Example:

public void temperature(String value1, String value2) {
    double i = Double.parseDouble(at1.getText().toString());
    double value;
    if (value1.equals("fahrenheit") && value2.equals("celsius")) {
        value = /* convert i */;
    } else if (....) {
        value = ...;
    }
    at2.setText(String.valueOf(value));
}

If your list of if statements becomes large, consider using a switch statement instead. You might want to change representation of the units from String to something else like integer constants or enums.

My suggestion would be to use the command pattern.

For temperature, I believe you can use Kelvin to convert to a common scale:

public class TemperatureConverter {

    private final static String CELSIUS = "Celsius";
    private final static String FARENHEIT = "Farenheit";

    public double temperature(double temperature, String temp1Type, String temp2Type) {
        Converter fromConverter, toConverter;

        if (temp1Type.equals(CELSIUS)) {
            fromConverter = new CelsiusConverter();
        } else if (temp1Type.equals(FARENHEIT)) {
            fromConverter = new FarenheitConverter();
        } else {
            fromConverter = new noopConverter();
        }

        if (temp2Type.equals(CELSIUS)) {
            toConverter = new CelsiusConverter();
        } else if (temp2Type.equals(FARENHEIT)) {
            toConverter = new FarenheitConverter();
        } else {
            toConverter = new noopConverter();
        }

        return toConverter.fromKelvin(fromConverter.toKelvin(temperature));
    }
}

Converter is an interface, and FarenheitConverter and CelsiusConverter are concrete classes that implement this interface. They contain the details to convert from Farenheit and Celsius (respectively) to Kelvin. They also contain the details of converting from Kelvin to their respective scales. This keeps the logic cleanly separated, and easily extended in the future.

Say in the future you need to convert from degrees blignox to degrees fizzbuzz. It's as simple as writing the BlignoxConverter and the FizzbuzzConverter, implementing the two methods, and plugging them in. Once they're written and integrated, you can convert from either of those to or from F and C as well.