Problème d'affichage JButton

bonjour, j'ai décidé de faire le TP de la calculatrice en java, mon problème est que : les boutons sont initialisé par une boucle for, et son mis dans le bon JPanel :

package fr.skymer.main;

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.GridLayout;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Frame extends JFrame {
	
	private JPanel container = new JPanel();
	private JButton butIni;
	private JPanel numButt = new JPanel();
	private JPanel othButt = new JPanel();
	private JPanel pan = new JPanel();
	private Panel panel = new Panel();
	private int i;
	private String namNum[] = {"1","2","3","4","5","6","7","8","9","0",".","=","C","+","-","/","*"};
	
	public Frame(){
		//Fenêtre
		this.setSize(300,400);
		this.setVisible(true);
		this.setResizable(false);
		this.setLocationRelativeTo(null);
		this.setDefaultCloseOperation(Frame.EXIT_ON_CLOSE);
		this.setTitle("Calculatrice");
		//ContentPane et placement des objets entre eux-mêmes
		this.setContentPane(container);
		this.panel.setPreferredSize(new Dimension(300,50));
		this.othButt.setPreferredSize(new Dimension(100,50));
		this.getContentPane().setLayout(new BorderLayout());
		this.getContentPane().add(panel, BorderLayout.NORTH);
		this.getContentPane().add(pan, BorderLayout.CENTER);
		this.pan.setLayout(new BorderLayout());
		this.pan.add(numButt, BorderLayout.CENTER);
		this.pan.add(othButt, BorderLayout.EAST);
		//Initialisation des boutons
		this.numButt.setLayout(new GridLayout(4,3,5,5));
		this.othButt.setLayout(new GridLayout(5,2,5,5));
		i = 0;
		for (String but : namNum){
			i++;
			butIni = new JButton(but);
			if (i <= 12){
				this.numButt.add(butIni);
			}else {
				this.othButt.add(butIni);
			}
		}
	}
	
}