java图形用户界面添加背景颜色不成功的解决方案

package clientFrame;

import java.awt.Color;
import java.awt.FlowLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;
import javax.swing.JPasswordField;

public class see extends JFrame {
// 做一个登陆的密码，账户框。2个h文本框，2个label
JLabel jl1, jl2;
JButton jb;
JPasswordField pf;
JTextField tf1, tf2;

public see() {
jl1 = new JLabel("用户名：");
jb = new JButton("登  陆");
jl2 = new JLabel("密   码：");
tf1 = new JTextField(10);
pf = new JPasswordField(10);
tf2 = new JTextField(10);
this.setVisible(true);

this.setLayout(new FlowLayout());// 添加背景色
this.getContentPane().setBackground(Color.red);// 核心，设置背景颜色，用这种方法。getContentPane()
this.add(jl1);

this.add(tf1);
this.add(jl2);
this.add(pf);
// this.add(tf2);
this.add(jb);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setSize(200, 200);
}

public static void main(String[] args) {

see s = new see();
}
}