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