How can I avoid making this method and all other variables static?

Question

I am kind of new to Java and I am trying some stuff with swing.

    class CustomListSelectionListener implements ListSelectionListener {
    public void valueChanged (ListSelectionEvent e)
    {
        //All this listener does is to return the NAME of the selected connection to the static method gotocategories.
        JList lsm = (JList)e.getSource();
        Home.goToCategories((String)lsm.getSelectedValue());
    }

problem is, the listener is a class, so basically if I call a static method from the "home class" I need to make all the variables I have to use in the "goToCategories" function static.

    public static void goToCategories(String collectionname)
{
    Statement stmt;
    try
    {
        stmt = privconn.createStatement();
        //Getting the collection ID;
        String firststmt = "SELECT * FROM collection WHERE Name='"+collectionname+"' AND User_ID = "+userID+"";
        ResultSet rs = stmt.executeQuery(firststmt);
        rs.next();
        int id = rs.getInt("ID");
        JOptionPane.showMessageDialog(null, id);

    } catch(Exception E)
    {
        E.printStackTrace();
    }
}

This code works , but I think I am creating a lot of static variables and I am not sure that's the best way to do it. Of course as soon as I try and remove the static it'll say "Cannot make static reference to non-static field ..."

You should read up and understand exactly what `static` means. http://stackoverflow.com/questions/3903537/i-want-to-know-the-difference-between-static-method-and-non-static-method and http://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html are good references. — Mike B, Jan 22 '14 at 21:06
Note: You don't want to [create SQL queries like that](http://en.wikipedia.org/wiki/SQL_injection). Use [prepared statements](http://docs.oracle.com/javase/tutorial/jdbc/basics/prepared.html) instead. — Alexis C., Jan 22 '14 at 21:08
Or a library that handles all that stuff for you like Hibernate. — corsiKa, Jan 22 '14 at 21:09

score 1 · Accepted Answer · answered Jan 22 '14 at 21:09

1

If your CustomSelectionListener class has a reference to an instance of your Home class, you will be able to call non-static methods on that instance.

It would look something like this:

MyClass someObject = new MyClass();
someObject.nonStaticMethod();

However static methods are called without an instance of the class, by using the name of the class:

MyClass.someStaticMethod();

It is a design decision whether a method or member variable should be static, and as the comments say you should fully understand what static means in order to correctly make this decision.

answered Jan 22 '14 at 21:09

user1445967

1,520
4
14
30

I do understand what you wrote, I chose the second option here. What I would like to understand is: In terms of memory usage, what is better? – arocketman Jan 22 '14 at 21:14
Instantiating objects does consume memory, in proportion to what is required to store the data of that objects members. – user1445967 Jan 22 '14 at 22:03

score 0 · Answer 2 · answered Jan 22 '14 at 21:10

0

I think the best way to do that, is making a class without a main method,
do all your stuff in the constructer, and then make a new instance of that class from a main method in an other class(new my class),

answered Jan 22 '14 at 21:10

java-love

516
1
8
23

How can I avoid making this method and all other variables static?

2 Answers2