《Java编程思想第四版》学习笔记44--关于按钮组

//: ButtonGroups.java
// Uses reflection to create groups of different
// types of AbstractButton.
package c13.swing;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.*;
import java.lang.reflect.*;
public class ButtonGroups extends JPanel {
    static String[] ids = {
            "June", "Ward", "Beaver",
            "Wally", "Eddie", "Lumpy",
    };
    static JPanel
    makeBPanel(Class bClass, String[] ids) {
        ButtonGroup bg = new ButtonGroup();
        JPanel jp = new JPanel();
        String title = bClass.getName();
        title = title.substring(
                title.lastIndexOf('.') + 1);
        jp.setBorder(new TitledBorder(title));
        for(int i = 0; i < ids.length; i++) {
            AbstractButton ab = new JButton("failed");
            try {
// Get the dynamic constructor method
// that takes a String argument:
                Constructor ctor = bClass.getConstructor(
                        new Class[] { String.class });
// Create a new object:
                ab = (AbstractButton)ctor.newInstance(
                        new Object[]{ids[i]});
            } catch(Exception ex) {
                System.out.println("can't create " +
                        bClass);
            }
            bg.add(ab);
            jp.add(ab);
        }
        return jp;
    }
    public ButtonGroups() {
        add(makeBPanel(JButton.class, ids));
        add(makeBPanel(JToggleButton.class, ids));
        add(makeBPanel(JCheckBox.class, ids));
        add(makeBPanel(JRadioButton.class, ids));
    }
    public static void main(String args[]) {
        Show.inFrame(new ButtonGroups(), 500, 300);
    }
} ///:~

                                                                                                                                                    P.472

这个例程中24秆的AbstractButton ab = new JButton("failed");为什么不写在循环的外面？37、38行的bg.add(ab);jp.add(ab);原以为应该是bg.add(ab);jp.add(bg);上机后才知道只有这样写才行。