为什么我在contentpane里面加JButton的时候要最大化后才能看见?

package test;
import java.awt.event.*;
import javax.swing.*;
import java.awt.*;
public class LayeredPan2 extends JFrame{
    public LayeredPan2() {
        Container contentpane = getContentPane();
        contentpane.setLayout(new GridLayout(2, 2));
        setSize(400, 400);
        setVisible(true);        Color[] colors = {Color.red, Color.blue, Color.magenta, Color.cyan,
                         Color.yellow, Color.green, Color.pink};
        Integer[] layerconstants = {new Integer( -100), JLayeredPane.PALETTE_LAYER,new Integer(101), JLayeredPane.MODAL_LAYER,new Integer(201),
                                   JLayeredPane.POPUP_LAYER, JLayeredPane.DRAG_LAYER};
        Point positision = new Point(10, 10);
        JLayeredPane layeredpane = getLayeredPane();
        JButton[] bts = new JButton[7];
        for (int i = 0; i < 7; i++) {
            bts[i] = creatbutton("button"+(i+1), colors[i], positision);
            positision.x = positision.x + 20;
            positision.y = positision.y + 20;
            layeredpane.add(bts[i], layerconstants[i]);
        }
        JButton bt1 = new JButton("按钮一");
        JButton bt2 = new JButton("按钮二");
        JButton bt3 = new JButton("按钮三");
        JButton bt4 = new JButton("按钮四");
        contentpane.add(bt1);
        contentpane.add(bt2);
        contentpane.add(bt3);
        contentpane.add(bt4);        System.out.println("contentpane 的层次值为:" +layeredpane.getLayer(contentpane));
        System.out.println("按钮一的层次值为:" + layeredpane.getLayer(bt1));
        System.out.println("按钮二的层次值为:" + layeredpane.getLayer(bt2));
        System.out.println("bts[0]的层次值为:" + layeredpane.getLayer(bts[0]));
        System.out.println("bts[1]的层次值为:" + layeredpane.getLayer(bts[1]));
        System.out.println("bts[2]的层次值为:" + layeredpane.getLayer(bts[2]));
        addWindowListener(new WindowAdapter(){
           public void windowClosing(WindowEvent e){
               System.exit(0);
           }
       });    }    public JButton creatbutton(String content, Color color, Point positision) {
        JButton newbt = new JButton(content);
        newbt.setVerticalAlignment(JButton.TOP);
        newbt.setBackground(color);
        newbt.setForeground(Color.black);
        newbt.setBounds(positision.x,positision.y,100,100);
        return newbt;    }
    public static void main(String args[]){
        new LayeredPan2();
    }
}
我执行后,那几个在contentpane上的按钮总是要最大化后才显示,这是为什么?
还有数组Integer我编程int 后就不行了,请问两者有什么区别啊?