I am working on a unit converter i have got my code to start working but now i a

Question

I am working on a unit converter i have got my code to start working but now i am having problems figuring out the best way to implement the rejection of imcompatible conversions. this is the code i have so far:

   import java.util.Scanner;
    /**
       This class converts between two units.
    */
    public class ConversionCalculator
    {
       public static void main(String[] args)
       {
         Scanner in = new Scanner(System.in);
         System.out.println("Convert from:");
       String fromUnit = in.nextLine();
       System.out.println("Convert to: ");
       String toUnit = in.nextLine();
        
          UnitConverter from = new UnitConverter(fromUnit);
          UnitConverter to = new UnitConverter(toUnit);
          System.out.println("Value:");
          double val = in.nextDouble();
          double meters = from.toMeters(val);
          double converted = to.fromMeters(meters);
          System.out.println(val + " " + fromUnit + " = " + converted + " " + toUnit);
       }
    }

this is the UnitConverter:

    public class UnitConverter
    {
       static double INCHES = 0.0254001;
       static double FEET = 0.3048;
       static double MILES = 1609.35;
       static double MILLIMETERS = 0.001;
       static double CENTIMETERS = 0.01;
       static double METERS = 1;
       static double KILOMETERS = 1000;
       private double val ,meters ,converted;
       String afromUnit;
   
     public UnitConverter(String fromUnit)
     {
       afromUnit = fromUnit;
     }
      
     public double toMeters(double val)
     {
      if(afromUnit.equals("in"))
       {
           meters = (val*INCHES);
       }
      else if(afromUnit.equals("ft"))
       {  
           meters = (val*FEET);
       }
      else if(afromUnit.equals("mi"))
       {
           meters = (val*MILES);
       }
      else if(afromUnit.equals("mm"))
       {
           meters = (val*MILLIMETERS);
       }
      else if(afromUnit.equals("cm"))
       {
           meters = (val*CENTIMETERS);
       }
       else if(afromUnit.equals("m"))
       {
           meters = (val*METERS);
       }
       else
       {
           meters = (val*KILOMETERS);
       }
      return meters;
     }
   
     public double fromMeters(double meters)
     {
       if(afromUnit.equals("in"))
       {
           converted = Math.round(meters*39.369923740457715);
       }
       else if(afromUnit.equals("ft"))
       {
           converted = Math.round(meters*3.280839895013123);
       }
       else if(afromUnit.equals("mi"))
       {
           converted = Math.round(meters*0.0006213688756330196);
       }
       else if(afromUnit.equals("mm"))
       {
           converted = Math.round(meters*1000);
       }
       else if(afromUnit.equals("cm"))
       {
           converted = Math.round(meters*100);
       }
       else if(afromUnit.equals("m"))
       {
           converted = Math.round(meters*1);;
       }
       else
       {
           converted = Math.round(meters*0.001);
       }
       return converted;
     }  
    }

Explanation / Answer

Note:I have integrated two classes

This code is succesfully working for all conversions

ConversionCalculator.java

import java.util.Scanner;
/**
This class converts between two units.
*/
public class ConversionCalculator
{//main class
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);//key board inputting
System.out.println("Convert from:");//enter from unit name
String fromUnit = in.nextLine();
System.out.println("Convert to: ");//enter to unit name
String toUnit = in.nextLine();

UnitConverter from = new UnitConverter(fromUnit);//creating object and passing argument
UnitConverter to = new UnitConverter(toUnit);//creating object and passing argument
System.out.println("Value:");
double val = in.nextDouble();
double meters = from.toMeters(val);
double converted = to.fromMeters(meters);
System.out.println(val + " " + fromUnit + " = " + converted + " " + toUnit);
}
}

class UnitConverter
{
static double INCHES = 0.0254001;
static double FEET = 0.3048;
static double MILES = 1609.35;
static double MILLIMETERS = 0.001;
static double CENTIMETERS = 0.01;
static double METERS = 1;
static double KILOMETERS = 1000;
private double val ,meters ,converted;
String afromUnit;

public UnitConverter(String fromUnit)
{//constructor with arguments
afromUnit = fromUnit;
}

public double toMeters(double val)
{//method with returns double and one argument
//which converts meters into other converisons
if(afromUnit.equals("in"))
{//which converts inches into meters
meters = (val*INCHES);
}
else if(afromUnit.equals("ft"))
{//which converts feet into meters
meters = (val*FEET);
}
else if(afromUnit.equals("mi"))
{//which converts miles into meters
meters = (val*MILES);
}
else if(afromUnit.equals("mm"))
{//which converts millimeters into other meters
meters = (val*MILLIMETERS);
}
else if(afromUnit.equals("cm"))
{//which converts centimeterss into meters
meters = (val*CENTIMETERS);
}
else if(afromUnit.equals("m"))
{
meters = (val*METERS);
}
else if(afromUnit.equals("km"))
{//km into meters
meters = (val*KILOMETERS);
}
else
{
System.out.println("INcompetable conversion");
System.exit(1);
}
return meters;
}

public double fromMeters(double meters)
{ //which converts meters into other converisons
if(afromUnit.equals("in"))
{//inches into any other unit
converted = Math.round(meters*39.369923740457715);
}
else if(afromUnit.equals("ft"))
{//feet into any other unit
converted = Math.round(meters*3.280839895013123);
}
else if(afromUnit.equals("mi"))
{//miles into any ther unit
converted = Math.round(meters*0.0006213688756330196);
}
else if(afromUnit.equals("mm"))
{//millimeters into any other unit
converted = Math.round(meters*1000);
}
else if(afromUnit.equals("cm"))
{//cm into any other unit
converted = Math.round(meters*100);
}
else if(afromUnit.equals("m"))
{//meters into meters
converted = Math.round(meters*1);;
}
else if(afromUnit.equals("km"))
{
converted = Math.round(meters*0.001);
}
else
{
System.out.println("Incompetable conversion");
System.exit(1);
}

return converted;
}
}

output

run:
Convert from:
gal
Convert to:
m
Value:
1
INcompetable conversion
Java Result: 1
BUILD SUCCESSFUL (total time: 7 seconds)

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