Java Replace Nested If statements with 2-D Map

Playing around again with adding Units to a Java program. Trying another approach, I ended up with a monolithic method, which the code is so ugly I'll post it lower down, and just describe here the simple idea of what it does.

It's basically a huge nested if-else statement, which determines which units we currently have and what units we want to convert to, and then calling the appropriate conversion.

For my test example I have two Measurement types: Temperature and Length.

Temperature measurements have 3 units to choose from: Fahrenheit, Celsius, and Kelvin.

Length measurements have 3 units to choose from: Inches, Feet, Miles.

So basically I first determine if this is a Temperature measurement or a Length measurement.

For a Temperature measurement the possibilities are:

Fahrenheit → Fahrenheit
Fahrenheit → Celsius
Fahrenheit → Kelvin
Celsius → Fahrenheit
Celsius → Celsius
Celsius → Kelvin
Kelvin → Fahrenheit
Kelvin → Celsius
Kelvin → Kelvin

(Note how this looks like an Array.)

           Fahrenheit Celsius Kelvin
          +----------+-------+-------+
Fahrenheit|          |       |       |
          +----------+-------+-------+
Celsius   |          |       |       |
          +----------+-------+-------+
Kelvin    |          |       |       |
          +----------+-------+-------+

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For a Length measurement the possibilities are:

Inches → Inches
Inches → Feet
Inches → Miles
Feet → Inches
Feet → Feet
Feet → Miles
Miles → Inches
Miles → Feet
Miles → Miles

As you can imagine this leads to a very ugly nested if-else statement which while in theory it works, I'm looking for a nicer alternative.


 that looks like this:

	public double getValueAs(T targetUnits) {

		// TEMPERATURE MEASUREMENT *******************************************
		if (this.measurementUnits.getClass() == TemperatureUnits.class) {
			
			if (targetUnits == TemperatureUnits.FAHRENHEIT) {
				if (this.measurementUnits == TemperatureUnits.FAHRENHEIT) {
					return this.value;
				}
				else if (this.measurementUnits == TemperatureUnits.CELSIUS) {
					return convertCelsiusToFahrenheit(this.value);
				}
				else if (this.measurementUnits == TemperatureUnits.KELVIN) {
					return convertKelvinToFahrenheit(this.value);
				}
				else {
					//can not convert
				}
			}
			else if (targetUnits == TemperatureUnits.CELSIUS) {
				if (this.measurementUnits == TemperatureUnits.FAHRENHEIT) {
					return convertFahrenheitToCelsius(this.value);
				}
				else if (this.measurementUnits == TemperatureUnits.CELSIUS) {
					return this.value;
				}
				else if (this.measurementUnits == TemperatureUnits.KELVIN) {
					return convertKelvinToCelsius(this.value);
				}
				else {
					//can not convert
				}
				
			}
			else if (targetUnits == TemperatureUnits.KELVIN) {
				if (this.measurementUnits == TemperatureUnits.FAHRENHEIT) {
					return convertFahrenheitToKelvin(this.value);
				}
				else if (this.measurementUnits == TemperatureUnits.CELSIUS) {
					return convertCelsiusToKelvin(this.value);
				}
				else if (this.measurementUnits == TemperatureUnits.KELVIN) {
					return value;
				}
				else {
					//can not convert
				}
				
			}
			else {
				//can not convert
			}
		}

		// LENGTH MEASUREMENT *******************************************
		else if (this.unitType == LengthUnits.class) {
			if (targetUnits == LengthUnits.INCHES) {
			
				if (this.measurementUnits == LengthUnits.INCHES) {
					return this.value;
				}
				else if (this.measurementUnits == LengthUnits.FEET) {
					return convertFeetToInches(this.value);
				}
				else if (this.measurementUnits == LengthUnits.MILES) {
					return convertMilesToInches(this.value);
				}
				else {
					//can not convert
				}
			}
			else if (targetUnits == LengthUnits.FEET) {
				if (this.measurementUnits == LengthUnits.INCHES) {
					return convertInchesToFeet(this.value);
				}
				else if (this.measurementUnits == LengthUnits.FEET) {
					return value;
				}
				else if (this.measurementUnits == LengthUnits.MILES) {
					return convertMilesToFeet(this.value);
				}
				else {
					//can not convert
				}				
			}
			else if (targetUnits == LengthUnits.MILES) {
				if (this.measurementUnits == LengthUnits.INCHES) {
					return convertInchesToMiles(this.value);
				}
				else if (this.measurementUnits == LengthUnits.FEET) {
					return convertFeetToMiles(this.value);
				}
				else if (this.measurementUnits == LengthUnits.MILES) {
					return value;
				}
				else {
					//can not convert
				}
			
			}
			else {
				//can not convert
			}

		}
	
		return 0;
	}

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My first attempt was to try Polymorphism and Strategy Pattern, but I realized that was just spreading out the if-else statements so they were all over the place, rather than reducing them or eliminating them.

Next (and current) idea is to create a HashMap, with an array of keys for the units (from this unit to that unit), and the value will be the appropriate conversion method (I'm expecting something implementing the callable interface).

My units are enums: Temperature={FAHRENHEIT, CELSIUS, KELVIN}, Length={INCHES, FEET, MILES}

Now I'm wondering how to implement this Map of Array → Values. I can think of two possibilities:

1. Somehow flatten my 2-D Array of enum Keys to a 1-D array of Keys, and then use HashMap.

OR

2. Create a nested tree of HashMaps.

Any ideas or suggestions? Preference for idea #1 or #2? Perhaps a #3 idea I haven't thought of yet?