获取JTabbedPane页面的信息出错

Question

我想把JTabbedPane页面输入的信息存放到一个对象里，再加进arraylist里，页面有一个添加的按钮，我给按钮加了个监听器，当点下按钮时，获取文本域的信息，存放到对象里。第一次点按钮时，信息添加成功，修改下信息后第二次点添加，就会报错。代码如下：
 private void jbClassActionPerformed(java.awt.event.ActionEvent evt) {                                        
        Classes newcla = new Classes();
        newcla.grades = Integer.parseInt(classText.getText());  //grades是int型的，初步调试发现该处出错
        newcla.major = majorText.getText();
        newcla.classNum = Integer.parseInt(classNumText.getText(););
        newcla.numOfStu = Integer.parseInt(stuNumText.getText());
        if (newcla.grades == 0 || newcla.major == null || newcla.classNum == 0 || newcla.numOfStu == 0) {
            JOptionPane.showMessageDialog(null, "输入的信息有误，请重新输入");
        } else {
            newcla.stu = new Student[newcla.numOfStu];
            for (int i = 0, j = 0; i < newcla.numOfStu; i++) {
                newcla.stu[i] = new Student();
                newcla.stu[i].name = (String) classTable.getValueAt(i, j);
                newcla.stu[i].id = (String) classTable.getValueAt(i, j + 1);
                j = 0;
            }
            cla.add(newcla);
        }
        classText.setText(" ");
        majorText.setText(" ");
        classNumText.setText(" ");
        stuNumText.setText(" ");
    }错误信息如下：Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: For input string: " 4"
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
at java.lang.Integer.parseInt(Integer.java:447)
at java.lang.Integer.parseInt(Integer.java:497)
at testgrade.TestGrade.jbClassActionPerformed(TestGrade.java:387)
at testgrade.TestGrade.access$100(TestGrade.java:14)
at testgrade.TestGrade$3.actionPerformed(TestGrade.java:134)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.java:6134)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
at java.awt.Component.processEvent(Component.java:5899)
at java.awt.Container.processEvent(Container.java:2023)
at java.awt.Component.dispatchEventImpl(Component.java:4501)
at java.awt.Container.dispatchEventImpl(Container.java:2081)
at java.awt.Component.dispatchEvent(Component.java:4331)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4301)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3965)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3895)
at java.awt.Container.dispatchEventImpl(Container.java:2067)
at java.awt.Window.dispatchEventImpl(Window.java:2458)
at java.awt.Component.dispatchEvent(Component.java:4331)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)经过我的调试，我发现是把String转化为int时出错，我如果把grades等变量改成String型的就不会出错，为什么？

Answer 1

Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: For input string: " 4"
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
4前面多了一个空格
classText.setText(" ");
这句话设置当前内容为空格

Answer 2

NumberFormatException类型转换异常    
先给对象里面的数据附上数据。"";
if (newcla.grades == 0 || newcla.major == null || newcla.classNum == 0 || newcla.numOfStu == 0) {
            JOptionPane.showMessageDialog(null, "输入的信息有误，请重新输入");
        }