JTextField textStart=new JTextField(3);
我限制输入的是3列。。为什么我能一直输入数据，而不是我定义的3列？  


也就是说当我输入3列后这个文本就不允许我们再输入数据。。那要怎么定义才行

textField = new JTextField(20);
textField.addActionListener(this);

The integer argument passed to the JTextField constructor, 20 in the example, indicates the number of columns in the field. This number is used along with metrics provided by the field's current font to calculate the field's preferred width. It does not limit the number of characters the user can enter. To do that, you can either use a formatted text field or a document listener, as described in Text Component Features.