Error displayed during run in swing tree pgm

The error is :
Exceptionin thread "main" java.lang.Error: Do not use jtrexp.setLayout() use )jtrexp.geContentPane().setLayout() instead
atjavax.swing.JFrame.createRootPaneException(JFram e.java:406)

atjavax.swing.JFrame.setLayout(JFrame.java:472)
at jtrexp.<init>(jtrexp.java.13)
at jtrexp.main(jtrexp.java:54)

MY code is:

Code:

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import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.tree.*;
public class jtrexp extends JFrame
{
JTree jt;
DefaultMutableTreeNode top,ff,df,ff1,ff2,ff3,df1,df2,df3;
JTextField f1;
        public jtrexp()
        {
               
                setLayout(new BorderLayout());               
                top=new DefaultMutableTreeNode("Fruits");
                ff=new DefaultMutableTreeNode("Fresh Fruits");
                df=new DefaultMutableTreeNode("Dry Fruits");
                ff1=new DefaultMutableTreeNode("Apple");
                ff2=new DefaultMutableTreeNode("Orange");
                ff3=new DefaultMutableTreeNode("Grapes");
                df1=new DefaultMutableTreeNode("Strawberry");
                df2=new DefaultMutableTreeNode("Dates");
                df3=new DefaultMutableTreeNode("Beaches");
                ff.add(ff1);
                ff.add(ff2);
                ff.add(ff3);
                df.add(df1);
                df.add(df2);
                df.add(df3);
                top.add(ff);
                top.add(df);
                jt=new JTree(top);
                jt.setFont(new Font("Times New Roman",Font.BOLD,20));
                jt.setRowHeight(40);
                add(jt);
                f1=new JTextField(20);
                add(f1,"south");
                jt.addMouseListener(new MouseAdapter()
                {
                        public void mouseClicked(MouseEvent me)
                        {
                        TreePath tp=jt.getPathForLocation(me.getX(),me.getY());
                        if(tp!=null)
                                f1.setText(tp.toString());
                        else
                                f1.setText("");
               
                        }
                });
               

        }
        public static void main(String arg[])
        {
                jtrexp jj=new jtrexp();
                jj.setSize(400,400);
                jj.setVisible(true);
       
        }
}

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When posting code use the code tags, Put [code] at the start of the code and [/code] at the end so that the formatting is preserved.

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The error message is quite straight forward as regards how you should change the code. What don't you understand about what to change? To see why it occurs you should read a textbook or tutorial. Oracle have an online tutorial which explains the point at Using Top-Level Containers (The Java™ Tutorials > Creating a GUI With JFC/Swing > Using Swing Components)

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Also, Java clases should begin with a capital letter.

thank you...

You're welcome.