First we’ll look at Distance. Distance and Current are super similar, they just have different families, so there’s no point in looking at both of them. 
// both of these methods work for constructing a new Distance!
Distance tenMM = new Distance(DistanceUnits.MILLIMETER, 10.0);
// OR
Distances.mm(10);
We can get the actual value of the current as so: 
tenMM.getValue(); // double
And if we want to get the unit that was provided: 
tenMM.getUnit(); // DistanceUnit
We can convert between the units in two different ways: 
// this is a no-op, so its free
Distance asMillimeters = tenMM.into(DistanceUnits.MILLIMETER);
Distance asCentimeters = tenMM.into(DistanceUnits.CENTIMETER);
Distance asMeters = tenMM.into(DistanceUnits.METER);

Distance asInches = tenMM.into(DistanceUnits.INCH);
Distance asFeet = tenMM.into(DistanceUnits.FEET);

// OR

// quick conversions for "batteries included" units
// still a no-op
tenMM.intoMillimeters();
tenMM.intoCentimeters();
tenMM.intoMeters();

tenMM.intoInches();
tenMM.intoFeet();

// if you define your own units, these quick conversion methods won't exist,
// sorry!
All expected mathematical operations are defined: 
tenMM.plus(asInches);
tenMM.minus(asInches);
tenMM.times(asInches);
tenMM.div(asInches);

// just doubles also works for times and divide
tenMM.times(10);
tenMM.div(10);

// the inverse (-tenMM)
tenMM.unaryMinus();
// this is a no-op (+tenMM)
tenMM.unaryPlus();

// the sign of the value (-1.0, 0.0, 1.0)
tenMM.getSign();
// the absolute value
tenMM.getAbsoluteValue();

// if the stored value is Double.NaN
tenMM.isNaN();

// unit specific error calculation (behaves differently for `Angle`s)
tenMM.findError(asInches);

// modulo (%)
// note that there is also rem, which is the same
tenMM.mod(asInches);
tenMM.mod(10);

// gets the value of this when converted to the supplied unit
// returns a double, not a new Distance
tenMM.get(DistanceUnits.INCH);

// coerce's the unit to be in these bounds
tenMM.coerceAtLeast(asInches);
tenMM.coerceAtMost(asInches);
tenMM.coerceIn(asInches, asInches);
Note that all of these operations are non-mutating, the value of tenMM never changes, instead, you need to store the results of the operations, and use the new output instead. These operations are also written to avoid making allocation where possible, which means, where possible, tenMM will be returned, rather than a new Distance. Finally, all operations involving more than one ReifiedUnit value, result in a ReifiedUnit with a unit of the left hand side, so if you chain a whole bunch of operations on a Distance with unit DistanceUnits.METER, then the result should also have unit DistanceUnits.METER. Comparisons are also defined: 
tenMM.lessThan(asInches);
tenMM.lessThanEqualTo(asInches);
tenMM.greaterThan(asInches);
tenMM.greaterThanEqualTo(asInches);