import java.io.File;
 
public void listFilesForFolder(final File folder) {
    for (final File fileEntry : folder.listFiles()) {
        if (fileEntry.isDirectory()) {
            listFilesForFolder(fileEntry);
        } else {
            System.out.println(fileEntry.getName());
        }
    }
}

In Java 8, using the Files.walk API:

final File folder = new File("/home/you/Desktop");
listFilesForFolder(folder);
Files.walk API is available from Java 8.
 
try (Stream<Path> paths = Files.walk(Paths.get("/home/you/Desktop"))) {
    paths
        .filter(Files::isRegularFile)
        .forEach(System.out::println);
} 

import java.io.File;
 
File folder = new File("/Users/you/folder/");
File[] listOfFiles = folder.listFiles();
 
for (File file : listOfFiles) {
    if (file.isFile()) {
        System.out.println(file.getName());
    }
}

References:

• https://stackoverflow.com/questions/1844688/read-all-files-in-a-folder

    private void mnuOpenActionPerformed(java.awt.event.ActionEvent evt) {                                        
        try {
            // Read text file
            Path pth = Paths.get(System.getProperty("user.dir") + "/src/com/acme/myproj/data/", "customer.txt");
 
            Stream<String> data = Files.lines(pth);
            List<String> asList = data.collect(Collectors.toList());
            System.out.println("Working Directory = " + System.getProperty("user.dir"));
            System.out.println(asList);
 
            DefaultListModel<String> lst = new DefaultListModel();
            //for(int i=0; i< asList.size(); i++) {
            //    String str = asList.get(i);
            //    lst.addElement(str);
            //}
            for(String str: asList) {
                lst.addElement(str);
            }
            lstAdminRegions.setModel(lst);
        } catch (IOException ex) {
            Logger.getLogger(frmMain.class.getName()).log(Level.SEVERE, null, ex);
        }
    }