Arredondamento de datas para dias 10, 20 e 30, Java

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//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_72

import java.util.*;
import java.lang.*;
import java.util.Date;

class Rextester
{  
    public static void main(String args[])
    {
        Integer dias = 45;
        
        Calendar c = Calendar.getInstance();
        c.setTime(new Date());
        
        System.out.println("dia recebido = "+c.getTime());
        
        c.add(Calendar.DAY_OF_MONTH, +dias);
        
        System.out.println("dia recebido + 20 dias = "+c.getTime());
        
        Integer dia = c.get(Calendar.DAY_OF_MONTH);
        
        if(dia <= 4 || dia >= 25){
            if(dia<=4){
                    c.set(Calendar.MONTH,c.get(Calendar.MONTH)-1);
            }
            dia = 30;
        }
        else if(dia <= 14 && dia >= 5){
            dia = 10;
        }
        else if(dia <= 24 && dia >= 15){
            dia = 20;
        }

c.set(Calendar.DAY_OF_MONTH, dia);
        System.out.println("data correta:"+c.getTime());
        
    }
}

[ + ] Show input
Compilation time: 0.52 sec, absolute running time: 0.14 sec, cpu time: 0.09 sec, memory peak: 21 Mb, absolute service time: 0,67 sec