在 Java 中實現讀取文件的進度條，一般需要結合多線程和 Swing 組件來完成。具體實現步驟如下：

在主界面中創建一個進度條組件，并設置其最小值、最大值和初始值。

創建一個 SwingWorker 類，用于在后臺線程中讀取文件，并更新進度條。

在 SwingWorker 的 doInBackground() 方法中，使用 FileInputStream 讀取文件，每讀取一定數量的數據就更新進度條的值，并在進度條上顯示讀取進度。

在 SwingWorker 的 done() 方法中，更新進度條的值為 100%，并在主界面中添加一個“讀取完成”的提示。

在主界面中創建一個按鈕，點擊按鈕時啟動 SwingWorker。

下面是一個示例：

import java.awt.BorderLayout;

import java.awt.event.ActionEvent;

import java.awt.event.ActionListener;

import java.io.File;

import java.io.FileInputStream;

import java.io.IOException;

import javax.swing.JButton;

import javax.swing.JFrame;

import javax.swing.JPanel;

import javax.swing.JProgressBar;

import javax.swing.SwingWorker;

public class FileProgressBarDemo extends JFrame {

    private JButton btnReadFile;

    private JProgressBar progressBar;

    public FileProgressBarDemo() {

        initComponents();

    }

    private void initComponents() {

        setTitle("File Progress Bar Demo");

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        progressBar = new JProgressBar();

        progressBar.setStringPainted(true);

        progressBar.setMinimum(0);

        progressBar.setMaximum(100);

        progressBar.setValue(0);

        btnReadFile = new JButton("Read File");

        btnReadFile.addActionListener(new ActionListener() {

            public void actionPerformed(ActionEvent e) {

                readFileInBackground();

            }

        });

        JPanel panel = new JPanel(new BorderLayout());

        panel.add(progressBar, BorderLayout.CENTER);

        panel.add(btnReadFile, BorderLayout.SOUTH);

        getContentPane().add(panel);

        setSize(300, 100);

        setLocationRelativeTo(null);

    }

    private void readFileInBackground() {

        SwingWorker<Void, Integer> worker = new SwingWorker<Void, Integer>() {

            @Override

            protected Void doInBackground() throws Exception {

                File file = new File("filename.txt");

                long fileSize = file.length();

                FileInputStream fis = new FileInputStream(file);

                byte[] buffer = new byte[1024];

                int len = 0;

                long bytesRead = 0;

                while ((len = fis.read(buffer)) != -1) {

                    bytesRead += len;

                    publish((int) (bytesRead * 100 / fileSize));

                }

                fis.close();

                return null;

            }

            @Override

            protected void process(java.util.List<Integer> chunks) {

                for (int percent : chunks) {

                    progressBar.setValue(percent);

                }

            }

            @Override

            protected void done() {

                progressBar.setValue(100);

                btnReadFile.setText("Read Complete");

            }

        };

        worker.execute();

    }

    public static void main(String[] args) {

        FileProgressBarDemo frame = new FileProgressBarDemo();

        frame.setVisible(true);

    }

}