Background em interface gráfica

import java.util.Scanner;


public class Main {

   
    public static void main(String[] args) {
       int op =1;
       Scanner ler = new Scanner(System.in);
       Metodos m = new Metodos();
       do{
           System.out.println("1 - Experiencia pra lvl");
           System.out.println("2 - Experiencia tnl");
           System.out.println("0 - Sair");
           op = ler.nextInt();
           switch (op){
               case 1:
                   System.out.println("Qual level que deseja saber experiencia necessária?");
                   m.setLvl(ler.nextInt());
                   m.calcularExpPlvl();
                   System.out.println(m.getExp());
                   break;
               case 2:
                   System.out.println("Qual level que você está?");
                   m.setLvl(ler.nextInt());
                   m.calcularExpTnl();
                   System.out.println(m.getExp());
                   break;
               case 0:
                   System.out.println("Bye bye");
                   break;
                   
           }
       }while(op !=0);
    }
    
}

public class Metodos {
    private int lvl;
    
    private long exp;
    private long exp1;

    public int getLvl() {
        return lvl;
    }

    public void setLvl(int lvl) {
        this.lvl = lvl;
    }

        public long getExp() {
        return exp;
    }

    public void setExp(long exp) {
        this.exp = exp;
    }

    
    
    public void calcularExpPlvl(){
        exp = (50 * (lvl - 1) * (lvl- 1) * (lvl- 1) - 150 * (lvl- 1) * (lvl- 1) + 400 * (lvl- 1)) / 3;
    }
    public void calcularExpTnl(){
        long ex1;
        long ex2;
        ex1 = (50 * (lvl - 1) * (lvl- 1) * (lvl- 1) - 150 * (lvl- 1) * (lvl- 1) + 400 * (lvl- 1)) / 3;
        ex2 =    (50 * (lvl) * (lvl) * (lvl) - 150 * (lvl) * (lvl) + 400 * (lvl)) / 3;
        exp = ex2 - ex1;
    }
    
}