Method References

Since a lambda expression is like an object-less method, wouldn't be nice if we could refer to existing methods instead of using a lamda expression? This is exactly what we can do with method references.

For example, imagine you frequently need to filter a list of Files based on file types. Assume you have the following set of methods for determining a file's type:

public class FileFilters {
    public static boolean fileIsPdf(File file) {/*code*/}
    public static boolean fileIsTxt(File file) {/*code*/}
    public static boolean fileIsRtf(File file) {/*code*/}
}

Whenever you want to filter a list of files, you can use a method reference as in the following example (assuming you already defined a method getFiles() that returns a Stream):

Stream<File> pdfs = getFiles().filter(FileFilters::fileIsPdf);
Stream<File> txts = getFiles().filter(FileFilters::fileIsTxt);
Stream<File> rtfs = getFiles().filter(FileFilters::fileIsRtf);

Method references can point to:

• Static methods.
• Instance methods.
• Methods on particular instances.
• Constructors (ie. TreeSet::new)

For example, using the new java.nio.file.Files.lines method:

Files.lines(Paths.get("Nio.java"))
            .map(String::trim)
            .forEach(System.out::println);

The above reads the file "Nio.java", calls trim() on every line, and then prints out the lines.

Notice that System.out::println refers to the println method on an instance of PrintStream.