数字格式输入字符串的异常

Question

目前我正在制作一款专注软件，但这里遇到运行时错误的是我的代码： -

public class gui015 extends JFrame {

private JTextField[] tf;
private JRadioButton a;
private JRadioButton b;
private JTextField t2;
private JOptionPane op;
private JButton c;
public gui015()
{
    super("Attenedence");
    setLayout(new FlowLayout());
    t2 = new JTextField("Enter no. of students");
    add(t2);
    String s = t2.getText();
    int a = Integer.parseInt(s);
    tf = new JTextField[a];
    while(a>0)
    {
        tf[a] = new JTextField();
        add(tf[a]);
    }
}

我得到了一堆这样的错误： -

Exception in thread "main" java.lang.NumberFormatException: For input string: "Enter no. of students"
    at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
    at java.lang.Integer.parseInt(Integer.java:580)
    at java.lang.Integer.parseInt(Integer.java:615)
    at tutorial.gui015.<init>(gui015.java:26)
    at tutorial.gui016.main(gui016.java:7)