Answer:
// program in java.
// package
import java.util.*;
// class definition
class Main
{
    // main method of the class
 public static void main (String[] args) throws java.lang.Exception
 {
    try{
   // Scanner object to read input
 Scanner scr=new Scanner(System.in);
  // variables
 double rad;
 String unit1="in";
 String unit2="ft";
 String unit3="cm";
 String unit4="m";
 // ask user to enter radius
 System.out.print("Enter the radius: ");
  // read radius
 rad=scr.nextDouble();
  // check radius is negative or not
 while(rad<0)
 {
     System.out.println("Radius can't be negative:");
      // ask again to enter radius
     System.out.print("Enter the Radius again:");
      // read radius again
     rad=scr.nextInt();
 }
// ask user to enter the unit of measurement
 System.out.print("Enter the unit (\"in\" for inch, \"ft\" for feet, \"cm\" for centimeter, and \"m\" for meter):");
  // read unit
  String unt=scr.next();
  // calculate area
 double area=3.14*rad*rad;
  // print area and unit
 if(unt.equals(unit1))
     System.out.println(area+" inch.");
 else if(unt.equals(unit2))
     System.out.println(area+" feet.");
 else if(unt.equals(unit3))
     System.out.println(area+" centimeter.");
 else if(unt.equals(unit4))
     System.out.println(area+" meter.");
    }catch(Exception ex){
        return;}
 }
 }
Explanation:
Read radius of circle from user with Scanner object and assign it to variable "rad".Check the radius is negative or not.If the radius is negative ask user to enter radius again until user enters a positive radius.Then read the unit of measurement.calculate the area of circle and print area with unit.
Output:
Enter the radius: -4
Radius can't be negative:
Enter the Radius again:6
Enter the unit ("in" for inch, "ft" for feet, "cm" for centimeter, and "m" for meter):cm
Area of circle is 113.03999999999999 centimeter.