BigDecimal e MathContext

Certo?

public void setScale(int num);

setScale(30)

converTo(Unit unit)

public interface AreaMetrics {
 //A base é Metros Quadrados /*Square Metric is the unit base */
	Unit M2 = new Unit("m²", new BigDecimal(String.valueOf(1)));
	Unit ACRE=new Unit( "acre",new BigDecimal(String.valueOf(4046.8564224) ));
	Unit ARES=new Unit("ares",new BigDecimal(String.valueOf(100)));

import java.math.BigDecimal;
//import org.brazilutils.metrics.*;

public class Area implements UnitWithValue,AreaMetrics {

	private static final long serialVersionUID = 210046788217078583L;
	private Unit unit;
	private BigDecimal value;
	private BigDecimal base;
	
	
	public Area(BigDecimal value,Unit unit,int roudingMode,int scale){
		this.base = value.multiply(unit.getValueOnBaseUnit());//,roudingMode,scale);
		this.value = value;
		this.unit = unit;
	}
	
	public Area(BigDecimal value,Unit unit){
		this.base = value.multiply(unit.getValueOnBaseUnit());
		this.value = value;
		this.unit = unit;
	}	
	
	public String toString() {
		return "[value= " + value + ",unit= " + unit + ",base= " + base + "]";
	}

	public UnitWithValue convertTo(Unit unit,int scale,int roundingmode) {
		return new Area(base.divide(unit.getValueOnBaseUnit(),scale,roundingmode),unit);
	}
	
	public UnitWithValue convertTo(Unit unit){
	    return new Area(base.divide(unit.getValueOnBaseUnit(), 34, BigDecimal.ROUND_HALF_EVEN), unit);
	}
	
	public Unit getUnit(){
		return unit;
	}
	
	public void setUnit(Unit u) {
		this.unit = u;
		this.value = base.divide(unit.getValueOnBaseUnit());//, int round);
	}
	
	public static void main(String[] args) {
		Area area1 = new Area(new BigDecimal(10000), Area.M2);
		System.out.println(area1);
		System.out.println(area1.convertTo(Area.HECTARE,34, BigDecimal.ROUND_HALF_EVEN));
		System.out.println(area1.convertTo(Area.ACRE));
	}	

}

public UnitWithValue convertTo(Unit unit)

public UnitWithValue convertTo(Unit unit,int scale,int roundingmode)

import java.io.Serializable;

/**
 * Immutable objects describing settings such as rounding mode and digit
 * precision for numerical operations such as those in the BigDecimal class.
 * @author Anthony Balkissoon abalkiss at redhat dot com
 *
 */
public final class MathContext implements Serializable
{
  /** A MathContext for unlimited precision arithmetic * */
  public static final MathContext UNLIMITED = 
    new MathContext(0, RoundingMode.HALF_UP);
  
  /**
   * A MathContext for the IEEE 754R Decimal32 format - 7 digit preicision and
   * HALF_EVEN rounding.
   */
  public static final MathContext DECIMAL32 = 
    new MathContext(7, RoundingMode.HALF_EVEN);
  
  /**
   * A MathContext for the IEEE 754R Decimal64 format - 16 digit preicision and
   * HALF_EVEN rounding.
   */
  public static final MathContext DECIMAL64 = 
    new MathContext(16, RoundingMode.HALF_EVEN);
  
  /**
   * A MathContext for the IEEE 754R Decimal128 format - 34 digit preicision and
   * HALF_EVEN rounding.
   */
  public static final MathContext DECIMAL128 = 
    new MathContext(34, RoundingMode.HALF_EVEN);
  
  /**
   * This is the serialVersionUID reported here:
   * java.sun.com/j2se/1.5.0/docs/api/serialized-form.html#java.math.MathContext
   */
  private static final long serialVersionUID = 5579720004786848255L;
  
  private int precision;
  
  private RoundingMode roundMode;
  
  /**
   * Constructs a new MathContext with the specified precision and with HALF_UP
   * rounding.
   * @param setPrecision the precision for the new MathContext
   * 
   * @throws IllegalArgumentException if precision is &lt 0.
   */
  public MathContext(int setPrecision)
  {
    this(setPrecision, RoundingMode.HALF_UP);
  }
  
  /**
   * Constructs a new MathContext with the specified precision and rounding
   * mode.
   * @param setPrecision the precision
   * @param setRoundingMode the rounding mode
   * 
   * @throws IllegalArgumentException if precision is &lt 0.
   */
  public MathContext(int setPrecision, RoundingMode setRoundingMode)
  {
    if (setPrecision &lt 0)
      throw new IllegalArgumentException("Precision cannot be less than zero.");
    precision = setPrecision;
    roundMode = setRoundingMode;
  }
  
  /**
   * Constructs a MathContext from a String that has the same form as one
   * produced by the toString() method.
   * @param val
   * 
   * @throws IllegalArgumentException if the String is not in the correct
   * format or if the precision specified is &lt 0.
   */
  public MathContext(String val)
  {
    try
    {
      int roundingModeIndex = val.indexOf("roundingMode", 10);
      precision = Integer.parseInt(val.substring(10, roundingModeIndex - 1));
      roundMode = RoundingMode.valueOf(val.substring(roundingModeIndex + 13));
    }
    catch (NumberFormatException nfe)
    {
      throw new IllegalArgumentException("String not in correct format");
    }
    catch (IllegalArgumentException iae)
    {
      throw new IllegalArgumentException("String not in correct format");
    }
    if (precision &lt 0)
      throw new IllegalArgumentException("Precision cannot be less than 0.");
  }
  
  /**
   * Returns true if x is a MathContext and has the same precision setting
   * and rounding mode as this MathContext.
   * 
   * @return true if the above conditions hold
   */
  public boolean equals(Object x)
  {
    if (!(x instanceof MathContext))
      return false;
    MathContext mc = (MathContext)x;
    return mc.precision == this.precision
           && mc.roundMode.equals(this.roundMode);
  }
  
  /**
   * Returns the precision setting.
   * @return the precision setting.
   */
  public int getPrecision()
  {
    return precision;
  }
  
  /**
   * Returns the rounding mode setting.  This will be one of 
   * RoundingMode.CEILING, RoundingMode.DOWN, RoundingMode.FLOOR, 
   * RoundingMode.HALF_DOWN, RoundingMode.HALF_EVEN, RoundingMode.HALF_UP, 
   * RoundingMode.UNNECESSARY, or RoundingMode.UP.
   * @return the rounding mode setting.
   */
  public RoundingMode getRoundingMode()
  {
    return roundMode;
  }
  
  /**
   * Returns "precision=p roundingMode=MODE" where p is an int giving the 
   * precision and MODE is UP, DOWN, HALF_UP, HALF_DOWN, HALF_EVEN, CEILING,
   * FLOOR, or UNNECESSARY corresponding to rounding modes.
   * 
   * @return a String describing this MathContext
   */
  public String toString()
  {
    return "precision="+precision+" roundingMode="+roundMode;
  }
  
  /**
   * Returns the hashcode for this MathContext.
   * @return the hashcode for this MathContext.
   */
  public int hashCode()
  {
    return precision ^ roundMode.hashCode();
  }
}

public UnitWithValue convertTo(Unit unit,int scale,int roundingmode)

Por mim, deixa assim para a versão 0.1, daí não seria preciso "re-implementar" o MathContext.