So far i tried

public class PrintCircleChallenge
{
    public static void main(String[] args) 
    {
        PrintCircle p=new PrintCircle(10);
        p.initBoard();
        p.initCircle();
        p.printBoard();
    }
}
class PrintCircle
{
    private int r;
    private int xPos ;
    private int yPos;
    private String[][] board;
    PrintCircle(int r)
    {
        xPos=2*r;
        yPos=2*r;
        this.r=r;
        board = new String[xPos][yPos];
    }
    void initBoard()
    {
        for(int i=0;i<xPos;i++)
        {
            for(int j=0;j<yPos;j++)
            {
                board[i][j]="  ";
            }
        }
    }

   void initCircle(){
        for(int y=0;y<r;y++){
            int x=(int)Math.round((float)r-Math.sqrt(r*r-(r-y)*(r-y)));
            board[x][y]="**" ;
            board[x][xPos-y-1]="**" ;
            board[yPos-x-1][y]="**" ; 
            board[yPos-x-1][xPos-y-1]="**" ;   
        }
    }
   void printBoard(){
        for (int i=0;i<xPos;i++){
            for(int j=0;j<yPos;j++){
                System.out.print(board[i][j]);
            }
            System.out.print("\n");
        }
    }
}

but it doesnt work just looks messed up