JList getSelectionValue returns null after clear

Question

I have 2 JLists among which I'd like only one item to be selected. I created a separate program using only the code necessary to replicate the error: Main Class:

public class Main {

    public static void main(String[] args) {

        new Staff("John", "D", 123456);
        new Staff("Bob", "X", 123455);
        Staff.lookup("John","D").setActive(true);
        Staff.lookup("Bob","X").setActive(true);
        new Staff("Jerry","Smith",384938);
        new Staff("Bob","Hope",834802);

        new InstructorGUI(1);



    }

}

Staff Class:

import java.util.ArrayList;
import java.util.List;


public class Staff{

    private String fName;
    private String lName;
    private int emtID;
    public static List staffIndex = new ArrayList<>();
    public static ArrayList activeStaffIndex = new ArrayList<>();

    Staff(String fName, String lName, int NJEMS) {
        this.fName = fName;
        this.lName = lName;
        staffIndex.add(this);
        emtID = NJEMS;
    }

    //Getters
    public int getEmtID() {
        return emtID;
    }
    public String getFirstName() {return fName; }
    public String getLastName() {return lName; }

    @Override
    public String toString() {
        return String.format("%s, %s", lName, fName);
    }

    //Lookups

    public static Staff lookup(String fName, String lName) {
        for (Staff s : staffIndex) {
            if (s.getFirstName().equalsIgnoreCase(fName) && s.getLastName().equalsIgnoreCase(lName)) {
                return s;
            }
        }
        return null;
    }

    //Setters
    public void setEmtID(int NJEMS) {
        emtID = NJEMS;
    }
    public void setFirstName(String s) {
        fName = s;
    }
    public void setLastName(String s) {
        lName = s;
    }

    //Removers
    public static void removeStaff(Staff s) {
        staffIndex.remove(s);
        activeStaffIndex.remove(s);
    }

    //Utilities
    public boolean isActive() {
        if(activeStaffIndex.contains(this)) {
            return true;
        } else {
            return false;
        }
    }
    public void setActive(Boolean b) {
        if (b) {
            if (!activeStaffIndex.contains(this)) {
                activeStaffIndex.add(this);
            }
        } else {
            if (activeStaffIndex.contains(this)) {
                activeStaffIndex.remove(this);
            }
        }
    }

}

InstructorGUI Class:

import javax.swing.*;
import java.awt.*;
import java.util.ArrayList;
import java.util.HashSet;

public class InstructorGUI extends JFrame {

    private static HashSet instructorGUIIndex = new HashSet<>();
    private int identifier;

    private JList

Hovercraft Full Of Eels · Accepted Answer

Your problem lies within your ListSelectionListener. First, let's create a much better and simpler MCVE, one that simplifie the code to only the essentials needed to reproduce the problem:

import java.awt.GridLayout;
import javax.swing.*;
import javax.swing.event.ListSelectionEvent;
import javax.swing.event.ListSelectionListener;

@SuppressWarnings("serial")
public class Main2 extends JPanel {
    private JList list1 = new JList<>(new String[] { "one", "two", "three" });
    private JList list2 = new JList<>(new String[] { "hello", "goodbye", "yes" });

    public Main2() {
        list1.setName("list 1");
        list2.setName("list 2");
        list1.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        list2.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);

        list1.addListSelectionListener(new MySelectionListener(list2));
        list2.addListSelectionListener(new MySelectionListener(list1));

        setLayout(new GridLayout(1, 0));
        add(new JScrollPane(list1));
        add(new JScrollPane(list2));
    }

    private class MySelectionListener implements ListSelectionListener {
        private JList otherList;

        public MySelectionListener(JList otherList) {
            this.otherList = otherList;
        }

        @Override
        public void valueChanged(ListSelectionEvent e) {
            otherList.clearSelection();
        }
    }

    private static void createAndShowGui() {
        Main2 mainPanel = new Main2();

        JFrame frame = new JFrame("Main2");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }
}

Run this, and you'll see that the problem is reproduced -- the newly selected does not appropriately select the selected item. In addition, if you comment out the line, otherList.clearSelection();, you'll see that the newly selected list will show the new selection just, fine, and so this line is at fault for messing up your desired behavior.

This can be fixed by limiting when the other list is cleared to only when the selected value is adjusting:

@Override
public void valueChanged(ListSelectionEvent e) {
    // otherList.clearSelection();
    if (e.getValueIsAdjusting()) {
        otherList.clearSelection();
    }
}

Why does this work? I honestly can't say for sure, but I do know that this will clear the other list's selection before the new list's item is being selected, and so it works. Also, you'll want to extract the selected value only when the value is not adjusting and so will need an if / else block as shown below:

import java.awt.BorderLayout;
import java.awt.GridLayout;
import javax.swing.*;
import javax.swing.event.ListSelectionEvent;
import javax.swing.event.ListSelectionListener;

@SuppressWarnings("serial")
public class Main2 extends JPanel {
    private JList list1 = new JList<>(new String[] { "one", "two", "three" });
    private JList list2 = new JList<>(new String[] { "hello", "goodbye", "yes" });
    private JTextField selectedItemTxtFld = new JTextField(10);

    public Main2() {
        list1.setName("list 1");
        list2.setName("list 2");
        list1.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        list2.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);

        list1.addListSelectionListener(new MySelectionListener(list2));
        list2.addListSelectionListener(new MySelectionListener(list1));

        JPanel listPanel = new JPanel(new GridLayout(1, 0));
        listPanel.add(new JScrollPane(list1));
        listPanel.add(new JScrollPane(list2));

        JPanel topPanel = new JPanel();
        topPanel.add(new JLabel("Selection:"));
        topPanel.add(selectedItemTxtFld);

        setLayout(new BorderLayout());
        add(topPanel, BorderLayout.PAGE_START);
        add(listPanel, BorderLayout.CENTER);
    }

    private class MySelectionListener implements ListSelectionListener {
        private JList otherList;

        public MySelectionListener(JList otherList) {
            this.otherList = otherList;
        }

        @Override
        public void valueChanged(ListSelectionEvent e) {
            // otherList.clearSelection();
            if (e.getValueIsAdjusting()) {
                otherList.clearSelection();
            } else {
                JList thisList = (JList) e.getSource();
                if (!thisList.isSelectionEmpty()) {
                    String selectedText = thisList.getSelectedValue().toString();
                    selectedItemTxtFld.setText(selectedText);
                }
            }
        }
    }

    private static void createAndShowGui() {
        Main2 mainPanel = new Main2();

        JFrame frame = new JFrame("Main2");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }
}

JList getSelectionValue returns null after clear

Answers (1)

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