一个关于窗口背景颜色的问题

[code]
import java.awt.*;

import javax.swing.*;

public class AbsolutePosition extends JFrame{
    public AbsolutePosition(){
        setTitle("本窗口使用绝对布局");
        setLayout(null);
        setBounds(200,200,500,350);
        Container c = getContentPane();
        setBackground(Color.BLUE);
        JButton b1 = new JButton("按钮1");
        JButton b2 = new JButton("按钮2");
        b1.setBounds(10, 30, 80, 30);
        b2.setBounds(60, 70, 100, 20);
        c.add(b1);
        c.add(b2);

        setBackground(Color.black);

        setVisible(true);
        //设置窗体关闭方式
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    }
    public static void main(String[] args){
        new AbsolutePosition();
    }

}