如何在java中使用来自不同方法的变量？

Question

First off let me say that I am very new to java so this is really simple but I have this date object and I made an actionlistener method but it won't let me use the object in there. How do I make it so that I can access in the method?

            jp = new JPanel();
        jta = new JTextArea(100, 100);
        jta.setPreferredSize(new Dimension(460,420));
        jta.setLineWrap(true);
        jta.setWrapStyleWord(true);
        DateFormat dateFormat = new SimpleDateFormat("yyyy/MM/dd");
        Date date = new Date();
        jb = new JButton("Tommorow");
        jl = new JLabel();
        jf = new JFrame();

        jf.setBackground(Color.BLUE);

        jl.setText(dateFormat.format(date));
        add(jl, BorderLayout.NORTH);

        add(jp);
        jp.add(jb, BorderLayout.EAST);
        add(jta);

        jb.addActionListener(this);
}

public void actionPerformed(ActionEvent e) {
       jta.setText("");
        Calendar cal = Calendar.getInstance();
        cal.setTime(date);
        cal.add(Calendar.DATE, 1);
        date = cal.getTime();
}

Answer 1

You are currently having a problem with the scope of your variables. You cannot use variables from your another method in the method public void actionPerformed(ActionEvent e) . Above you used:

jta = new JTextArea(100, 100);
jta.setPreferredSize(new Dimension(460,420));
jta.setLineWrap(true);
jta.setWrapStyleWord(true);

The variable jta is local to that method and cannot be accessed by other methods. The same goes for Date date = new Date(); . If you want to continue to use these variables down below create an object contains these variables. It could look something like this:

public class MyClass {

    private JTextArea textArea;
    private Date date;

    public MyClass(JTextArea textArea, Date date) {
        this.textArea = textArea;
        this.date = date;
    }

    public JTextArea getTextArea() {
        return this.textArea;
    }

    public Date getDate() {
        return this.date;
    }

}

Answer 2

You're having a scope problem. You can't access variables from other functions unless they are first declared as globals, or passed around in function parameters