local variable is accessed within inner class (jav

Question

I got two errors after I compiled my code.

The errors are:

1.

  local variable input is accessed within inner class; 
  needs to be declared final
     String name = input.getText();

2.

  local variable c_age is accessed within inner class; 
  needs to be declared final
     Object child_age = c_age.getSelectedItem();

This is my code:

import javax.swing.*;
import java.awt.event.*;

public class GUI
{
    public static void main(String[] args)
    {
        JFrame frame = new JFrame("Try GUI");
        JLabel l1 = new JLabel("Please Enter Your Child's Name");
        JTextField input = new JTextField("",10);

        JLabel l2 = new JLabel("Choose Your Child's Age");
        String[] age = {"Age","1","2","3","4","5","6"};
        JComboBox c_age = new JComboBox(age);

        JButton button = new JButton("Search");

        JTextArea result = new JTextArea();
        JScrollPane extend_area = new JScrollPane(result);

        button.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent ae)
            {
                String name = input.getText();
                Object child_age = c_age.getSelectedItem();
            }
        });

        JPanel panel = new JPanel();
        panel.add(l1);
        panel.add(input);
        panel.add(l2);
        panel.add(c_age);
        panel.add(button);
        panel.add(extend_area);
        frame.add(panel);
        frame.setSize(350,350);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }

}

How can I solve this error?

Answer 1

I just created a temporary var in the main class e.g 'tempString' and then it can be set to the var that's causing the problem. So:

tempString = myString

that way I can call tempString without any problem at all. Check my example below:

// Create my temp var here
private String tempUrl;

// my function here
public void updatePage(String url)
{
    // Can use 'url' here because it's not within inner class
    if(!url.equals(""))
    {
        // Set 'tempUrl' to 'url' so I can use it without problem
        tempUrl = url;

        // My inner class that used to cause the problem
        backgroundUpdate = new Thread(new Runnable()
        {
            // From here on I use 'tempUrl' to solve the problem
            public void run()
            {
                // Do something with 'tempUrl' here (where 'url' used to be used)
            }
        });

        backgroundUpdate.start();
    }
}

Answer 2

Any variable that you use inside the actionPerformed method of your inner class will need to be declared final. Try the following:

import javax.swing.*;
import java.awt.event.*;

    public class GUI
    {
        public static void main(String[] args)
        {
            JFrame frame = new JFrame("Try GUI");
            JLabel l1 = new JLabel("Please Enter Your Child's Name");
            final JTextField input = new JTextField("",10);

            JLabel l2 = new JLabel("Choose Your Child's Age");
            String[] age = {"Age","1","2","3","4","5","6"};
            final JComboBox c_age = new JComboBox(age);

            JButton button = new JButton("Search");

            JTextArea result = new JTextArea();
            JScrollPane extend_area = new JScrollPane(result);

            button.addActionListener(new ActionListener()
            {
                    public void actionPerformed(ActionEvent ae)
                    {
                        String name = input.getText();
                        Object child_age = c_age.getSelectedItem();


                    }
            });

            JPanel panel = new JPanel();
            panel.add(l1);
            panel.add(input);
            panel.add(l2);
            panel.add(c_age);
            panel.add(button);
            panel.add(extend_area);
            frame.add(panel);
            frame.setSize(350,350);
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.setVisible(true);
        }

    }