java为什么显示不出按钮

package 线程;

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.FlowLayout;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.WindowConstants;

public class Ceshi extends JFrame{

public Ceshi(){
Container c=getContentPane();
JButton jb1=new JButton();
JButton jb2=new JButton();
JLabel jl1=new JLabel();
JLabel jl2=new JLabel();
setTitle("测试");
setBounds(10,10,300,300);
setVisible(true);
setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
jl2.setLayout(new FlowLayout());
jb1.setText("输入");
jb2.setText("清除");
jl2.add(jb1);
jl2.add(jb2);

c.add(jl1);
jl1.add(jl2,BorderLayout.NORTH);
}

public static void main(String[] args) {
// TODO 自动生成的方法存根
new Ceshi();
}

}
JLable改为JPanel即可