Java程序中判断字符串是否为标识符及异常处理

import javax.swing.event.*;
import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.*;
public class WineJFrame extends JFrame implements CaretListener,ActionListener
{
private JTextField text_str,text_char,text_str1;
private JButton button_char,button_uni;
private int temp=0,i,t=0;
public WineJFrame()
{
super("标识符查询器");
this.setBounds(300,280,300,90);
this.setBackground(java.awt.Color.lightGray);
this.setDefaultCloseOperation(EXIT_ON_CLOSE);
this.setLayout(new GridLayout(3,3,3,3));
this.getContentPane().add(new JLabel("请输入字符串",JLabel.RIGHT));
text_str=new JTextField("abcd1234",22);
this.getContentPane().add(text_str);
text_str.addCaretListener(this);
button_char=new JButton("判断");
this.add(button_char);
        button_char.addActionListener(this);
this.getContentPane().add(new JLabel("是否是标识符",JLabel.RIGHT));
text_char=new JTextField(22);
text_char.setHorizontalAlignment(JTextField.RIGHT);
text_char.setEditable(false);
this.getContentPane().add(text_char);
caretUpdate(null);

this.setVisible(true);
    button_uni=new JButton("输出标识符");
    this.add(button_uni);
    button_uni.addActionListener(this);
this.getContentPane().add(new JLabel("输出标识符",JLabel.RIGHT));
text_str1=new JTextField(22);
text_str1.setHorizontalAlignment(JTextField.RIGHT);
text_str1.setEditable(false);
this.getContentPane().add(text_str1);
caretUpdate(null);
this.setVisible(true);
}
public void actionPerformed(ActionEvent ev)
{

if(ev.getSource()==button_char)
{

String str=text_str.getText();
  if(str!=null&&str.length()>0)
       {   
char ch=str.charAt(0);
    if(ch>='A'&&ch<='Z'||ch>='a'&&ch<='z'||ch=='_'||ch=='$')
       {
     for(int i=1;i<str.length();i++)
        {
     ch=str.charAt(i);
     if(ch>='A'&&ch<='Z'||ch>='a'&&ch<='z'||ch=='_'||ch=='$'||ch>='0'&&ch<='9')
     { 
      if(i==(str.length()-1))