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I'm having some trouble with the idea of accessing variables from other classes. I had a post here: Having access to a private variable from other classes in Java where I got some useful information, and thought an example would be better show it, and ask a separate question as well. I have a form that I can input data to, and it has a List variable. I didn't make it static at first, but I thought if I needed to get the total size from another class, then I wouldn't create an instance of that class in order to use the function to getTotalContacts. I basically want to update my status bar with the total number of contacts that are in my list. One of the members said in the above post to use the original Foo member to get the contacts, but I'm not sure how that works in this case. Any thoughts would be appreciated. Thanks.

import java.awt.*; import java.awt.event.*; import javax.swing.*; import java.util.List; import java.util.ArrayList; public class AddressBook { public static void main(String[] args) { EventQueue.invokeLater(new Runnable() { public void run() { AddressBookFrame frame = new AddressBookFrame(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); JMenuBar menuBar = new JMenuBar(); frame.setJMenuBar(menuBar); JMenu fileMenu = new JMenu("File"); JMenuItem openItem = new JMenuItem("Open"); JMenuItem saveItem = new JMenuItem("Save"); JMenuItem saveAsItem = new JMenuItem("Save As"); JMenuItem printItem = new JMenuItem("Print"); JMenuItem exitItem = new JMenuItem("Exit"); fileMenu.add(openItem); fileMenu.add(saveItem); fileMenu.add(saveAsItem); fileMenu.add(printItem); fileMenu.add(exitItem); menuBar.add(fileMenu); JMenu editMenu = new JMenu("Edit"); JMenuItem newItem = new JMenuItem("New"); JMenuItem editItem = new JMenuItem("Edit"); JMenuItem deleteItem = new JMenuItem("Delete"); JMenuItem findItem = new JMenuItem("Find"); JMenuItem firstItem = new JMenuItem("First"); JMenuItem previousItem = new JMenuItem("Previous"); JMenuItem nextItem = new JMenuItem("Next"); JMenuItem lastItem = new JMenuItem("Last"); editMenu.add(newItem); editMenu.add(editItem); editMenu.add(deleteItem); editMenu.add(findItem); editMenu.add(firstItem); editMenu.add(previousItem); editMenu.add(nextItem); editMenu.add(lastItem); menuBar.add(editMenu); JMenu helpMenu = new JMenu("Help"); JMenuItem documentationItem = new JMenuItem("Documentation"); JMenuItem aboutItem = new JMenuItem("About"); helpMenu.add(documentationItem); helpMenu.add(aboutItem); menuBar.add(helpMenu); frame.setVisible(true); } }); } } class AddressBookFrame extends JFrame { public AddressBookFrame() { setLayout(new BorderLayout()); setTitle("Address Book"); setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT); AddressBookToolBar toolBar = new AddressBookToolBar(); add(toolBar, BorderLayout.NORTH); AddressBookStatusBar aStatusBar = new AddressBookStatusBar(); add(aStatusBar, BorderLayout.SOUTH); AddressBookForm form = new AddressBookForm(); add(form, BorderLayout.CENTER); } public static final int DEFAULT_WIDTH = 500; public static final int DEFAULT_HEIGHT = 500; } /* Create toolbar buttons and add buttons to toolbar */ class AddressBookToolBar extends JPanel { public AddressBookToolBar() { setLayout(new FlowLayout(FlowLayout.LEFT)); JToolBar bar = new JToolBar(); JButton newButton = new JButton("New"); JButton editButton = new JButton("Edit"); JButton deleteButton = new JButton("Delete"); JButton findButton = new JButton("Find"); JButton firstButton = new JButton("First"); JButton previousButton = new JButton("Previous"); JButton nextButton = new JButton("Next"); JButton lastButton = new JButton("Last"); bar.add(newButton); bar.add(editButton); bar.add(deleteButton); bar.add(findButton); bar.add(firstButton); bar.add(previousButton); bar.add(nextButton); bar.add(lastButton); add(bar); } } /* Creates the status bar string */ class AddressBookStatusBar extends JPanel { public AddressBookStatusBar() { setLayout(new FlowLayout(FlowLayout.LEFT)); this.statusBarString = new JLabel("Total: " + AddressBookForm.getTotalContacts()); add(this.statusBarString); } public void updateLabel() { contactsLabel.setText(AddressBookForm.getTotalContacts().toString()); } private JLabel statusBarString; private JLabel contactsLabel; } class AddressBookForm extends JPanel { public AddressBookForm() { // create form panel this.setLayout(new GridLayout(2, 1)); JPanel formPanel = new JPanel(); formPanel.setLayout(new GridLayout(4, 2)); firstName = new JTextField(20); lastName = new JTextField(20); telephone = new JTextField(20); email = new JTextField(20); JLabel firstNameLabel = new JLabel("First Name: ", JLabel.LEFT); formPanel.add(firstNameLabel); formPanel.add(firstName); JLabel lastNameLabel = new JLabel("Last Name: ", JLabel.LEFT); formPanel.add(lastNameLabel); formPanel.add(lastName); JLabel telephoneLabel = new JLabel("Telephone: ", JLabel.LEFT); formPanel.add(telephoneLabel); formPanel.add(telephone); JLabel emailLabel = new JLabel("Email: ", JLabel.LEFT); formPanel.add(emailLabel); formPanel.add(email); add(formPanel); // create button panel JPanel buttonPanel = new JPanel(); JButton insertButton = new JButton("Insert"); JButton displayButton = new JButton("Display"); ActionListener insertAction = new AddressBookListener(); ActionListener displayAction = new AddressBookListener(); insertButton.addActionListener(insertAction); displayButton.addActionListener(displayAction); buttonPanel.add(insertButton); buttonPanel.add(displayButton); add(buttonPanel); } public static int getTotalContacts() { return addressList.size(); } //void addContact(Person contact); private JTextField firstName; private JTextField lastName; private JTextField telephone; private JTextField email; private JLabel contacts; private static List<Person> addressList = new ArrayList<Person>(); private class AddressBookListener implements ActionListener { public void actionPerformed(ActionEvent e) { String buttonPressed = e.getActionCommand(); System.out.println(buttonPressed); if (buttonPressed == "Insert") { Person aPerson = new Person(firstName.getText(), lastName.getText(), telephone.getText(), email.getText()); addressList.add(aPerson); } else { for (Person p : addressList) { System.out.println(p); } } } } }

My other question is why do I get the error, "int cannot be dereferenced contactsLabel.setText(AddressbookForm.getTotalContacts().toString()); Thanks!

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2 Answers 2

Reset to default
2

Keep in mind that by making addressList static, it belongs to the AddressbookForm class rather than to an instance of the class. This means that all AddressbookForms will be sharing the same addressList. I'm not sure if that is what you want.

Regarding your error, AddressbookForm.getTotalContacts() returns an int. Since int is a primitive type in Java, it has no toString() method. An int can be converted to a String like:

int a = 5; String aString = a + "";
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4 Comments

Integer.valueOf(a).toString()
You are right, I do not want it to be static. But even still, not sure how to access the variables...
Or String.valueOf(a). / You really don't want to be using static mutables at all.
Why are these approaches better? I am not sure what Java does under the hood here, and even the Sun tutorial lists the string concatenation method (java.sun.com/docs/books/tutorial/java/data/converting.html).
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From my point of view the correct solution is to use the wrapper, Integer, for the primitive type int. Integer, like all the classes in Java, extends Object and in consequence has available toString() to override it or use it.

// http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Integer.html public static Integer getTotalContacts() { return new Integer( addressList.size() ); }
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7 Comments

one should do Integer.valueOf(x) instead of new Integer(x)
@unbeli As the documentation says Integer.valueOf(x) is recommended if you don't require a new Instance. E.g.: if you want the result of the method to apply directly the toString() method. But if you want a new instance to work with it, you should to use the constructor. Using valueOf(x) a cache is used for frequently used values. From my point of view if you're programming under the OOP paradigm you should forget any primitive type, and that is what I want to show in the example.
why would you want a new instance of Integer?
@unbeli For using it after!?!?! in others parts of the code, in other calculations, etc. Depends on how you design the class. Why wouldn't you want a new instance of Integer?
for that you can use what Integer.valueOf() gives you. There is no need to get strictly new instance. More new instances => more garbage. Also, Integer.valueOf() is more readable
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