java - java中的边框布局

Question

我正在关注一本java编程书，但我刚刚发现了一些我没有得到的东西。在下面的代码中，更具体地说是在“MainFrame”类的构造函数中如果我理解正确的话，我们会添加标签并同时设置框架的布局。我认为我们需要首先设置布局的类型，然后向框架添加元素。

package fontframe;
import java.awt.BorderLayout;
import java.awt.Font;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.ButtonGroup;
import javax.swing.JButton;
import javax.swing.JCheckBox;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JRadioButton;
import javax.swing.border.EtchedBorder;
import javax.swing.border.TitledBorder;

public class MainFrame extends JFrame{
private static final int FRAME_WIDTH = 300;
private static final int FRAME_HEIGHT = 300;

private JLabel label;
private JCheckBox italicCheckBox;
private JCheckBox boldCheckBox;
private JRadioButton smallButton;
private JRadioButton mediumButton;
private JRadioButton largeButton;
private JComboBox facenameCombo;
private ActionListener listener;

public MainFrame(){
    label = new JLabel("Big Java");
    add(label,BorderLayout.CENTER);
.......
}

Answer 1

I thought we need to set out the type of the layout first and than to add elements to the frame.

是的，你是对的，但是JFrame的默认布局是BorderLayout，标签的默认布局是FlowLayout。因此不需要为 JFrame 指定布局，它已经是一个 BorderLayout。

来自documentation :

And the child will be added to the contentPane. The content pane will always be non-null. Attempting to set it to null will cause the JFrame to throw an exception. The default content pane will have a BorderLayout manager set on it.