1. java.io.File.listFiles - old java IO
2. java.nio.Files.newDirectoryStream - NIO & NIO2 - new java IO
3. org.apache.commons.io.FileUtils.iterateFiles and org.apache.commons.io.FileUtils.iterateFilesAndDirs (commons-io:2.6) - uses jdk7 but File.listFiles
4. com.google.common.io.Files.fileTreeTraverser (guava:26.0) uses jdk7 but File.listFiles
5. com.google.common.io.Files.fileTraverser - https://google.github.io/guava/releases/snapshot/api/docs/com/google/common/io/Files.html#fileTraverser--
6. java.nio.Files.walk - uses newDirectoryStream and returns a Stream
7. java.nio.Files.walkFileTree - uses newDirectoryStream by using a java.nio.file.FileVisitor

• See how a Finder is implemented at https://docs.oracle.com/javase/tutorial/displayCode.html?code=https://docs.oracle.com/javase/tutorial/essential/io/examples/Find.java
• https://docs.oracle.com/javase/tutorial/essential/io/walk.html
• manually iterate using Files.newDirectoryStream - https://examples.javacodegeeks.com/core-java/nio/java-nio-iterate-files-directory/
• parallel iterate using Files.walkFileTree - https://stackoverflow.com/questions/17732819/parallel-version-of-files-walkfiletree-java-or-scala
• File.listFiles is also an option but is using old java IO library.
• walkFileTree example - https://www.concretepage.com/java/jdk7/traverse-directory-structure-using-files-walkfiletree-java-nio2
• com.google.common.io.Files.fileTreeTraverser is uses File.listFiles but also the deprecated com.google.common.collect.TreeTraverser (better com.google.common.graph.Traverser) that is a generic way to traverse a tree.
• https://stackoverflow.com/questions/20987214/recursively-list-all-files-within-a-directory-using-nio-file-directorystream

Comparison of how to traverse files in jdk7 using Files.walk, Files.walkFileTree and Files.newDirectoryStream. All of them call Files.newDirectoryStream which is implemented natively by JVM. This lists all entries in a directory.

sequenceDiagram
  Client->>+Files:walk => Stream<Path>
  Files->>FileTreeIterator: create
  FileTreeIterator->>FileTreeWalker: create
  Note right of DirectoryStream: lists only a directory
  FileTreeWalker->>DirectoryStream:create
  Files->>FileTreeIterator: iterate
  FileTreeIterator->>FileTreeWalker: walk => Event
  FileTreeWalker->>DirectoryStream:create | walk | next
  Client->>Files:newDirectoryStream
  Files->>DirectoryStream: create
  Note right of DirectoryStream:stream inside a directory
  Client->>Files:walkFileTree(visitor:FileVisitor<Path>)
  Files->>FileTreeWalker:create
  Files->>Files:iterate
  Files->>FileTreeWalker:walk => Event
  FileTreeWalker->>DirectoryStream:create | walk | next