Harusnya begini aja sudah bisa loh..
tapi Ane pake IDE..outputnya jadi agak beda..^^

public class bilangan {
    public int i;

void tampil(){
    for(i=0;i<=20;i++)
    {
       System.out.print(i+" ");
        if(i%2==1)
        {
            System.out.print(i+" ");
        }
    }
            }


public static void main(String args[]){
bilangan test = new bilangan();
test.tampil();
}
}