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POUse TreeSet to sort JList
 

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    <p>I'm doing an assignment where I'm making a program that generates 20 random numbers and puts them in an arraylist, a hashset and a treeset. It is supposed to show those numbers in JList1. Then, I am going to use HashSet to display unique numbers only, and TreeSet to display the numbers sorted. I made three JLists, and made one function for each of them to update the GUI (there's probably an easier way to do this, but thats all I could come up with due to me being quite new to java). So, for example my updateGUI function used to display the arraylist in my JList looks like this:</p> <pre><code>public void updateGUI(JList someList, ArrayList&lt;Integer&gt; anyarraylist){ DefaultListModel m = new DefaultListModel(); for (Object s : anyarraylist) { m.addElement(s); } someList.setModel(m); } </code></pre> <p>I then call that function when I press a button by using ActionEvent:</p> <pre><code>@Override public void actionPerformed(ActionEvent arg0) { generateNumbers(); this.updateGUI(this.numbers1, numberlist); </code></pre> <p>Where generateNumbers is my function to generate 20 random numbers, numbers1 is my first JList and numberlist is the name of my arraylist.</p> <p>I did pretty much the exact same thing for HashSet to display a JList with unique numbers, and it worked. However, I then tried to do the same for TreeSet, which gave me a "Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException"-error. The function looks like this:</p> <pre><code> public void oppdaterGUI3(JList someList, TreeSet&lt;Integer&gt; anysortedlist){ DefaultListModel m = new DefaultListModel(); for (Object s : anysortedlist) { m.addElement(s); } someList.setModel(m); } </code></pre> <p>Why doesn't this work? I thought TreeSet would work the same way as HashSet. Any help is appreciated. I can post the full error and code if necessary.</p> <p>Thank you.</p>
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