即使在上面的类中也找不到变量

Question

So I have a GUI class which contains the JButtons, and then I have an actionPerformed class below which shows what will happen if I press the button. Now, when I do the if(e.getSource()==search) the search comes up red and says the variable cannot be found in the class GUI. Even though it's in that very class.

I need some help with this.

package crimedata;

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


class GUI extends JFrame implements ActionListener {

    Connection con = null;

    GUI() {
        JPanel p = new JPanel();
        JFrame f = new JFrame("Crime Data");

        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setSize(600, 600);

        f.setResizable(true);
        f.setLocationRelativeTo(null);
        GridBagConstraints gbc = new GridBagConstraints();
        getContentPane().setLayout(new GridBagLayout());

        gbc.insets = new Insets(5, 5, 5, 5);

        JLabel LongLabel = new JLabel("Enter Longitude here");
        LongLabel.setForeground(Color.gray);
        gbc.gridx = 0;
        gbc.gridy = 1;
        getContentPane().add(LongLabel, gbc);
        JTextField LongText = new JTextField(20);
        getContentPane().add(LongText, gbc);

        JLabel LatLabel = new JLabel("Enter Latitude here");
        LatLabel.setForeground(Color.gray);
        gbc.gridx = 0;
        gbc.gridy = 2;
        getContentPane().add(LatLabel, gbc);
        JTextField LatText = new JTextField(20);
        getContentPane().add(LatText, gbc);

        JLabel LSOANameLabel = new JLabel("Enter LSOA Name here");
        LSOANameLabel.setForeground(Color.gray);
        gbc.gridx = 0;
        gbc.gridy = 3;
        getContentPane().add(LSOANameLabel, gbc);
        JTextField LSOANameText = new JTextField(20);
        getContentPane().add(LSOANameText, gbc);

        JButton search = new JButton("Search");
        gbc.gridx = 0;
        gbc.gridy = -1;
        getContentPane().add(search, gbc);
        search.addActionListener(this);

        JButton exportnoid = new JButton("Export No Crime ID");
        gbc.gridx = 0;
        gbc.gridy = -1;
        getContentPane().add(exportnoid, gbc);

        JButton exportdup = new JButton("Export Duplicate ID");
        gbc.gridx = 0;
        gbc.gridy = -1;
        getContentPane().add(exportdup, gbc);

        f.add(getContentPane());
        pack();
        f.setVisible(true);
    }

   @Override
   public void actionPerformed(ActionEvent e) {
      if(e.getSource()==search){
          System.out.println("");
      } else {

      }
   }

}

class GUIHandler {
    public static void main(String[] args) {
        GUI a = new GUI();
    }
}

Answer 1

No, there is no variable search in the scope of your class GUI .

There is a local variable search in the constructor of your class GUI .

The only instance variable I see so far, is the variable con of type Connection .

You should declare search as an instance variable of GUI , just like con , if you want to use it in another method. My suggestion is, you make it private .

Alternatively you could use setActionCommand(String cmd) to set an action command name for your JButton. Then you can use getActionCommand() of your ActionEvent to retrieve the name of the action and react accordingly. But I do not like this approach, since in this case the compiler cannot help you much to detect name clashes.

Answer 2

search variable is not an instance variable, its scope is limited to the constructor, so you cannot use it outside of it.

Declare JButton search variable after Connection con = null; .

Also note that have empty 'else' statement in actionPerformed method.

Answer 3

class GUI extends JFrame implements ActionListener {

Connection con = null;
JButton search;

GUI() {
    JPanel p = new JPanel();
    JFrame f = new JFrame("Crime Data");

    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setSize(600, 600);

    f.setResizable(true);
    f.setLocationRelativeTo(null);
    GridBagConstraints gbc = new GridBagConstraints();
    getContentPane().setLayout(new GridBagLayout());

    gbc.insets = new Insets(5, 5, 5, 5);

    JLabel LongLabel = new JLabel("Enter Longitude here");
    LongLabel.setForeground(Color.gray);
    gbc.gridx = 0;
    gbc.gridy = 1;
    getContentPane().add(LongLabel, gbc);
    JTextField LongText = new JTextField(20);
    getContentPane().add(LongText, gbc);

    JLabel LatLabel = new JLabel("Enter Latitude here");
    LatLabel.setForeground(Color.gray);
    gbc.gridx = 0;
    gbc.gridy = 2;
    getContentPane().add(LatLabel, gbc);
    JTextField LatText = new JTextField(20);
    getContentPane().add(LatText, gbc);

    JLabel LSOANameLabel = new JLabel("Enter LSOA Name here");
    LSOANameLabel.setForeground(Color.gray);
    gbc.gridx = 0;
    gbc.gridy = 3;
    getContentPane().add(LSOANameLabel, gbc);
    JTextField LSOANameText = new JTextField(20);
    getContentPane().add(LSOANameText, gbc);
    search = new JButton("Search");
    gbc.gridx = 0;
    gbc.gridy = -1;
    getContentPane().add(search, gbc);
    search.addActionListener(this);

    JButton exportnoid = new JButton("Export No Crime ID");
    gbc.gridx = 0;
    gbc.gridy = -1;
    getContentPane().add(exportnoid, gbc);

    JButton exportdup = new JButton("Export Duplicate ID");
    gbc.gridx = 0;
    gbc.gridy = -1;
    getContentPane().add(exportdup, gbc);

    f.add(getContentPane());
    pack();
    f.setVisible(true);
}

}