为什么按钮没有显现出来 
//查找
    public void find()
    {
        JDialog findDialog=new JDialog(NotePad.this,"查找",false);
        
        Container findPanel=findDialog.getContentPane();
        
        JPanel fieldPane = new JPanel(),buttonPane=new JPanel();
        JLabel find=new JLabel("查找内容(N):");
        JLabel replace=new JLabel("替换为(P):    ");
        JTextField tofind=new JTextField(18);
        JTextField toreplace=new JTextField(18);

        
        JButton jbtfind=new JButton("查找下一个");
        JButton jbtreplace=new JButton("替换");
        JButton jbtfindnext=new JButton("全部替换");
        JButton jbtcancel=new JButton("取消");
        
        //fieldPane.setLayout(new GridLayout(2,2,3,3));
        fieldPane.add(find);
        fieldPane.add(tofind);
        fieldPane.add(replace);
        fieldPane.add(toreplace);
        
        buttonPane.setLayout(null);
        //jbtfind.setPreferredSize(new Dimension(80,30));
        jbtfind.setSize(78, 30);
        buttonPane.add(jbtfind);
        buttonPane.add(jbtreplace);
        buttonPane.add(jbtfindnext);
        buttonPane.add(jbtcancel);
        
        findPanel.setLayout(new BorderLayout());
        findPanel.add(fieldPane,BorderLayout.CENTER);
        findPanel.add(buttonPane,BorderLayout.EAST);
        
        findDialog.setSize(400,170);
    
        place(findDialog);
        findDialog.setResizable(false);
        findDialog.setVisible(true);
    }
    还有为什么要设成setLayout（null）才能改变按钮的大小