java中缀表达式转后缀表达式为何会提示下标越界？

public void actionPerformed(ActionEvent e)
{
String s = new String(); //接收用户输入的中缀表达式
char [] str = new char[255]; //存放用户输入的中缀表达式（字符数组形式）
char [] str2 = new char[255]; //保存将用户输入的中缀表达式转化的后缀表达式
Stack st = new Stack();
char temp;
s = tf.getText();
str = s.toCharArray();
int i;
int n = 0;
int len = s.length();

for (i=0; i {
while (str[i] >= '0' && str[i] <= '9')
{
str2[n++] = str[i++];

if (str[i] < '0' || str[i] > '9')
{
str2[n++] = ' ';
break;
}
}
if (str[i] == '(')
{
st.push(str[i]);
}
else if (str[i] == ')')
{
temp = (char)st.pop();

while ('(' != temp)
{
str2[n++] = temp;
str2[n++] = ' ';
temp = (char)st.pop();
}
}
else if (str[i] == '+' || str[i] == '-')
{
if (st.empty())
{
st.push(str[i]);
}
else
{
do
{
temp = (char)st.pop();

if ('(' == temp)
{
st.push(temp);
}
else
{
str2[n++] = temp;
str2[n++] = ' ';
}
}while (!st.empty() && temp != '(');

st.push(str[i]);
}
}
else if (str[i] == '*' || str[i] == '/')
{
if (st.empty())
{
st.push(str[i]);
}
else
{
do
{
temp = (char)st.pop();

if ('(' == temp || '+' == temp || '-' == temp)
{
st.push(temp);
break;
}
else
{
str2[n++] = temp;
str2[n++] = ' ';
}
}while (!st.empty());

st.push(str[i]);
}
}
else if (i == len)
{
break;
}
else
{
System.out.println("出错");
System.exit(-1);
}
}

while (!st.empty())
{
temp = (char)st.pop();
str2[n++] = temp;
str2[n++] = ' ';
}