Create JTree using data from text file

How can I create a JTree using data from a .txt file? The text file is formatted as follows:

Root Node
        Category A
            Sub-Category A1
                Item A1.0
                Item A1.1
                Item A1.2
            Sub-Category A2
                Item A2.0
                Item A2.1
                Item A2.2       
        Category B
            Sub-Category B1
                Item B1.0
                Item B1.1
            Sub-Category B2
                Item B2.0
                Item B2.1
            Sub-Category B3
                Item B3.0
                Item B3.1
                Sub-Sub-Category B3_1
                    Item B3_1.0
                    Item B3_1.1
        Category C
            ... etc

The .txt file contains over 700 lines.

Just as PhoenixS has mentioned in his answer above, the procedure involves the following steps:

1. Read the .txt file line by line. In this example we use BufferedReader in conjunction with InputStreamReader OR FileReader, depending on whether we are reading the .txt file as a resource or from the File System respectively.
2. Depending on the formatting of the .txt file, use the delimiter to determine the hierarchy level of each line as it is being read. Use this to create the relevant nodes of your Jtree. In this case the .txt file is tab-delimited, with the number of tabs indicating the hierarchy level.

Here is the TreeFromTextFile class I wrote:

/*
 * File: TreeFromTextFile.java
 * Created 2013-02-06
 * This Class creates a JTree using data from a Specially formatted text File.
 * The supplied text file is tab-delimited, as illustrated at:
 * https://stackoverflow.com/questions/14724014/create-jtree-using-data-from-text-file
 * 
 * You can use either InputStreamReader to read thetext file as a resource
 * or use FileReader to read the text file from the file System
 */


import java.io.BufferedReader;
//import java.io.FileReader;
import java.io.InputStreamReader;
import java.io.LineNumberReader;
import javax.swing.JTree;
import javax.swing.tree.DefaultMutableTreeNode;
import javax.swing.tree.TreeSelectionModel;

/**
 * @author engineervix
 *
 */
public class TreeFromTextFile {

private BufferedReader in;
private LineNumberReader ln;
private String line;    //value of a line in the text file
private String root;    //value to be used for the root Node of our JTree                         
private String filename = "TheTextFile.txt";
private String encoding = "UTF-8";
private DefaultMutableTreeNode top;
private JTree tree;

public TreeFromTextFile() {
    getRootNode();
    top = new DefaultMutableTreeNode(root);
    createNodes(top);

    //Create a tree that allows one selection at a time.
    tree = new JTree(top);
    tree.getSelectionModel().setSelectionMode(TreeSelectionModel.SINGLE_TREE_SELECTION);
}

//this method reads the file and prints all the lines to standard output
//for testing purposes
public void readFile() {
    try {
        //in = new BufferedReader(new FileReader("Path\\To\\File.txt"));
        in = new BufferedReader(new InputStreamReader(getClass().getResourceAsStream(filename), encoding));

        while ((line = in.readLine()) != null) {
            System.out.println(line);
        }
        in.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

//this method reads the first line in the text file and assigns it 
//to the root variable which will be used for the root node of our JTree
private void getRootNode() {
    try {
        //in = new BufferedReader(new FileReader("Path\\To\\File.txt"));
        in = new BufferedReader(new InputStreamReader(getClass().getResourceAsStream(filename), encoding));
        ln = new LineNumberReader(in);

        if (ln.getLineNumber() == 0) {
            root = ln.readLine();
            //System.out.println(root);
        }

        in.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

/**
 * this method counts the number of occurrences of a given
 * <code>char</code> in the Specified
 * <code>String</code> source:
 * https://stackoverflow.com/questions/275944/how-do-i-count-the-number-of-occurrences-of-a-char-in-a-string
 */
private int countOccurrences(String haystack, char needle) {
    int count = 0;
    for (int i = 0; i < haystack.length(); i++) {
        if (haystack.charAt(i) == needle) {
            count++;
        }
    }
    return count;
}

//create the Nodes
private void createNodes(DefaultMutableTreeNode top) {
    DefaultMutableTreeNode category = null;     // Level 1 in Hierarchy
    DefaultMutableTreeNode subCategory = null;  // Level 2 in Hierarchy
    DefaultMutableTreeNode leaf = null;         // Level 3 in Hierarchy    

    try {
        //in = new BufferedReader(new FileReader("Path\\To\\File.txt"));
        in = new BufferedReader(new InputStreamReader(getClass().getResourceAsStream(filename), encoding));

        while ((line = in.readLine()) != null) {
            if (countOccurrences(line, '\t') == 1) {
                category = new DefaultMutableTreeNode(line);
                top.add(category);
            } else if (countOccurrences(line, '\t') == 2) {
                subCategory = new DefaultMutableTreeNode(line);
                category.add(subCategory);
            } else if (countOccurrences(line, '\t') == 3) {
                leaf = new DefaultMutableTreeNode(line);
                subCategory.add(leaf);
            } //continue the else...if - if you have more levels
        }
        in.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

    public JTree getTree() {
        return tree;
    }
}

To test the above, I wrote the TreeFromTextFileDemo class, which is as shown below:

/*
 * Requires TreeFromTextFile.java
 */

import java.awt.BorderLayout;
import java.awt.Container;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTree;

/**
 * @author engineervix
 */
public class TreeFromTextFileDemo {

private static TreeFromTextFile tr = new TreeFromTextFile();

public static void main(String[] args) {
    JFrame frame = new JFrame("Demo | Creating JTree From File.txt");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    Container content = frame.getContentPane();

    JTree t = tr.getTree();

    content.add(new JScrollPane(t), BorderLayout.CENTER);
    frame.setSize(275, 300);
    frame.setLocationByPlatform(true);
    frame.setVisible(true);

    }
}

---

The following screenshot illustrates the output. You can use this file as a sample text file.