Answer:
Here is the Circle class:
public class Circle {  
// class definition
 private int radius;    // private data member of type int of class Circle named radius that stores the value of radius 
 public Circle(int r) {  
// parameterized constructor of class Circle that takes radius as argument
  radius = r;  }    // sets value of radius as r
 public int getRadius() {  
// accessor method to get the value of radius
  return radius;  }  
// returns the current value of radius
 public int Diameter() {  
// method to compute diameter of a circle
  return radius * 2;  }  
// multiply radius value by 2 to compute diameter of Circle
  
 public double Area() {  
//
method to compute area of a circle
  return Math.PI  * Math.pow(radius, 2);   }  
//the formula of area is 
π x radius² where value of π is get using Math.PI
  
  public double Circumference() {  
//
//
method to compute circumference of a circle
  return 2* Math.PI * radius;  	}   }  //the formula of circumference is 
2 x π x radius where value of π is get using Math.PI
Explanation:
Here is the Main class:
import java.util.Scanner;  //to accept input from user
public class Main {  //definition of Main class
 public static void main(String[] args) {  //start of main method
    
     Scanner scanner = new Scanner (System.in);  //creates Scanner object to take input from user
     System.out.println("Enter radius: ");  //prompts user to enter radius
     int r = scanner.nextInt();  //reads the value of radius from user
  Circle c = new Circle(r);  // calls Constructor of Circle passing r as argument to it using the object c of class Circle
  
     if(c.getRadius()<=0){  //calls getRadius method to get current value of radius using objec and checks if this value (input value of r ) is less than or equal to 0
         System.out.println("Error!");   }  //when above if condition evaluates to true then print this Error! message 
     else
{  //if the value of radius is greater than 0
 System.out.println("the radius of this Circle is: " +c.getRadius());  //calls getRadius method to return current value of r (input value by user)
 System.out.println("the diameter of this Circle  is: " + c.Diameter());  //calls Diameter method to compute the diameter of Circle and display the result on output screen
 System.out.printf("the circumference of this Circle  is: %.2f", c.Circumference());  //calls Circumference method to compute the Circumference of Circle and display the result on output screen
 System.out.println();  //prints a line
 System.out.printf("the Area of this Circle  is: %.2f", c.Area()); } }  } 
//calls Area method to compute the Area of Circle and display the result on output screen
The program and its output is attached.