Error with setSelectedValue method for a JLIST

I am having troubles with JList class. I am getting data from a vector to fill the JList and the list looks fine when I show it on screen. The list contains table names,and the selected one is the table开发者_Go百科 to be displayed. So the proyect structure is based on the getSelectedValue() of the List. When I first run the program getSelectedValue returned null, so I tried modifiying it by myself with setSelectedValue("Alumnos", true) and I get this: Exception in thread "main" java.lang.NullPointerException at interfaz.DataBaseManagerJList.setSelectedValue(DataBaseManagerJList.java:53)

This makes no sense since Alumnos is in the list when I display it on screen. Here is the JList class code:

public class DataBaseManagerJList extends JPanel
{

private static final long serialVersionUID = 1L;

private static JList tablas;

DefaultListModel model;
DatabaseTableManagers dtm = DatabaseTableManagers.getInstance();
private static DataBaseManagerJList instance = null;

public static DataBaseManagerJList getInstance()
{
    if (instance == null)
    {
        instance = new DataBaseManagerJList();
    }
    return instance;
}

public DataBaseManagerJList()
{
    model = new DefaultListModel();
    ArrayList<String> aux = new ArrayList<String>(dtm.getTableNames());
    for(int i =0;i<aux.size();i++)
    {
        model.addElement(aux.get(i));
        System.out.println(aux.get(i));
    }
    JList tablas = new JList(model);    
    //tablas.setSelectedValue("Alumnos",true);
    add(tablas);
}

public String devolver()
{
    return (String) tablas.getSelectedValue(); 
}


public void setSelectedValue(String name)
{

    tablas.setSelectedValue(name, true);
}


}

Any ideas? Thanks everyone :)

I would guess that the String in your model has a space or some other difference than the one you are trying to select. Instead of hardcoding a value. Try just selecting the first item from the model. For example:

 JList tablas = new JList(model);    
 tablas.setSelectedValue(aux.get(0), true);
 add(tablas);

That way you are guaranteed that the value is in the model. (Assuming the list is not empty).

Another advantage is Alumnos is no longer a hardcoded value so that if it is not longer an option in your list you don't have to change the code that selects it.

Instead of

JList tablas = new JList(model);

write

tablas = new JList(model);

You create local instance but then use class field in setter.