comfirmlbl=new JLabel();
newpasslbl=new JLabel();
passlbl=new JLabel();
numberlbl=new JLabel();
this.setLayout(null);
numberlbl.setText("帐号:");
numberlbl.setBounds(30,40,75,30);
passlbl.setText("密码:");
passlbl.setBounds(30,90,75,30);
newpasslbl.setText("新密码:");
newpasslbl.setBounds(30,140,75,30);
comfirmlbl.setText("重新输入:");
comfirmlbl.setBounds(30,185,75,30);
this.setBounds(0, 0,400,300);
this.add(numberlbl);
this.add(passlbl,null);
this.add(newpasslbl,null);
this.add(comfirmlbl,null);
以上是程序的某一段,其中this.add(passlbl,null);中的null是什么意思?我去掉null程序照样能正确执行,这是为什么?