We find this code on page 61, for ListSelectFiles.java

final String[] files =
        new File("fpij").list(new java.io.FilenameFilter() {
            public boolean accept(final File dir, final String name) {
                return name.endsWith(".java");
            }
        });
System.out.println(files);

But this just prints the identifier of the array object, not the array contents.

The last line needs to be for example

System.out.println(Arrays.stream(files).collect(Collectors.joining("\n")));

The original code also NullPointerExcepts if “fpij” is nonexistent or not a directory.

Thus

    public void listSelectFiles() {
        String dir = "/home/aloy";
        final String[] files =
                new File(dir).list(new java.io.FilenameFilter() {
                    public boolean accept(final File _dir, final String name) {
                        return name.endsWith(".java");
                    }
                });
        String res =
                (files == null) ?
                        ("Looks like '" + dir + "' is not a directory") :
                        (Arrays.stream(files).collect(Collectors.joining("\n")));
        System.out.println(res);
    }