Thanks everybody for his help!
I've solved the problem!
Generally to call all my JInternalFrames I used a general global declaration:
private JInternalFrame jif;
But to solve my problem, I declared a new constructor using directly the class of my JInternalFrame:
private LogInJInternalFrame logIn;
Before the call, I declared a public global constructor in the Log In JIF:
public MainJFrame mainJF;
With that, using the new constructor in my JF, I could able to access to all the public declarations in my JIF. Then, I made the call using something like this:
private void logInCall(){
logIn = new LogInJInternalFrame(); //--New instance of my JIF
logIn.mainJF = this; /*Set the class to the constructor instead to declare a new one*/
logIn.setVisible(true);
}
After that, in my JIF, when the credentials were right, I've used this:
mainJF.setNewUserMenu(); /*The class's instance let me "invoke" the method I've wanted*/
And that's all, after the results all I can said was "I really need to relax more and think better the things".
Thanks again!!!