Calling a method from another class upon button click

Question

I've been stuck with this problem for quite some time already. I've been searching online but I never found a way for it to work. Whenever I click a button, I am expecting my program to call a method from another class; however, it creates a multitude of errors in my terminal window as listed below:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at TrackerGUI.actionPerformed(TrackerGUI.java:84) at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018) at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341) at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402) at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259) at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252) at java.awt.Component.processMouseEvent(Component.java:6505) at javax.swing.JComponent.processMouseEvent(JComponent.java:3321) at java.awt.Component.processEvent(Component.java:6270) at java.awt.Container.processEvent(Container.java:2229) at java.awt.Component.dispatchEventImpl(Component.java:4861) at java.awt.Container.dispatchEventImpl(Container.java:2287) at java.awt.Component.dispatchEvent(Component.java:4687) at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832) at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492) at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422) at java.awt.Container.dispatchEventImpl(Container.java:2273) at java.awt.Window.dispatchEventImpl(Window.java:2719) at java.awt.Component.dispatchEvent(Component.java:4687) at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:729) at java.awt.EventQueue.access$200(EventQueue.java:103) at java.awt.EventQueue$3.run(EventQueue.java:688) at java.awt.EventQueue$3.run(EventQueue.java:686) at java.security.AccessController.doPrivileged(Native Method) at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76) at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87) at java.awt.EventQueue$4.run(EventQueue.java:702) at java.awt.EventQueue$4.run(EventQueue.java:700) at java.security.AccessController.doPrivileged(Native Method) at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76) at java.awt.EventQueue.dispatchEvent(EventQueue.java:699) at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242) at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161) at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138) at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

Here is my code in my GUI:

public void actionPerformed(ActionEvent ae)
{
    if (ae.getSource() == button)
    {
        String a = firstField.getText();
        double b = Double.parseDouble(secondField.getText());

        c.call(a, b);
    }
}

firstField and secondField are my text fields from the GUI. c is my object to another class. call(a,b) is my method from the other class.

Here is my method from the other class:

public boolean call(String a, double b)
{
    if (names.isEmpty() == true)
    {
        names.add(new Name(a, b));
        return true;
    }
    else
    {
        boolean updated = false;

        for (int i = 0; i < names.size(); i++)
        {
            if (names.get(i).getName().contains(f.name))
            {
                names.set(i, new Name(a,b));
                updated = true;
            } 
        }

        if(!updated)
        {
            names.add(new Name(a,b));
        }         
        return true;
    }
}

I hope someone can help me out here! Thank you.

Answer 1

Try changing this:

if (names.isEmpty() == true)
{
    names.add(new Name(a, b));
    return true;
}

to this:

if (names == null)
    names = new ArrayList<>(); //or whatever type names is.

if (names.isEmpty())
{
    names.add(new Name(a, b));
    return true;
}