How can I override a base class method using a lambda expression in java 8?

Question

Lambda expressions must be cast to a functional interface. They cannot extend a class as far as I know but I want to know if there is a way to get something similar.

I have java.nio.file.SimpleFileVisitor<Path> as base class and I want to override a method of it but I wish to do so inside another method. I can do it with an anonymous class this way:

public static void printContent(Path path) throws IOException {
    FileVisitor<Path> visitor = new SimpleFileVisitor<Path>() {
        @Override
        public FileVisitResult visitFile(Path file, BasicFileAttributes attrs)
                throws IOException {
            System.out.println(file);
            return super.visitFile(file, attrs);
        }
    };
    Files.walkFileTree(path, visitor);
}

Is there a way to remove that load of code with the help of a lambda?

I think the lambda would be (f) -> System.out.println(f);

I thought about forgetting SimpleFileVisitor and creating a equivalent interface with default methods but, how could I select what method to override? Would I need to leave the method I want to override without default implementation? In that case I would need several Interfaces for different cases with different not implemented methods.

Thank you.

Answer 1

Use Delegation. For this task you need a helper class that has to be implemented only once:

interface IoBiFunction<T, U, R> {
  R apply(T t, U u) throws IOException;
}
class LambdaFileVisitor<T> extends SimpleFileVisitor<T> {
    IoBiFunction<T, BasicFileAttributes, FileVisitResult> preVisitDir=super::preVisitDirectory;
    IoBiFunction<T, BasicFileAttributes, FileVisitResult> visitFile=super::visitFile;
    IoBiFunction<T, IOException, FileVisitResult> visitFailed=super::visitFileFailed;
    IoBiFunction<T, IOException, FileVisitResult> postVisitDir=super::postVisitDirectory;

    public LambdaFileVisitor<T> onVisitFile(IoBiFunction<T, BasicFileAttributes, FileVisitResult> f) {
        this.visitFile = Objects.requireNonNull(f);
        return this;
    }
    public LambdaFileVisitor<T> onVisitFailed(IoBiFunction<T, IOException, FileVisitResult> f) {
        this.visitFailed = Objects.requireNonNull(f);
        return this;
    }
    public LambdaFileVisitor<T> onPreVisitDir(IoBiFunction<T, BasicFileAttributes, FileVisitResult> f) {
        this.preVisitDir = Objects.requireNonNull(f);
        return this;
    }
    public LambdaFileVisitor<T> onPostVisitDir(IoBiFunction<T, IOException, FileVisitResult> f) {
        this.postVisitDir = Objects.requireNonNull(f);
        return this;
    }
    @Override
    public FileVisitResult visitFile(T file, BasicFileAttributes attrs) throws IOException {
        return visitFile.apply(file, attrs);
    }
    @Override
    public FileVisitResult visitFileFailed(T file, IOException exc) throws IOException {
        return visitFailed.apply(file, exc);
    }
    @Override
    public FileVisitResult preVisitDirectory(T dir, BasicFileAttributes attrs) throws IOException {
        return preVisitDir.apply(dir, attrs);
    }
    @Override
    public FileVisitResult postVisitDirectory(T dir, IOException exc) throws IOException {
        return postVisitDir.apply(dir, exc);
    }
}

Once you have your helper class you can use it together with lambda expressions, e.g.

FileVisitor<Path> fv=new LambdaFileVisitor<Path>()
  .onVisitFile((f,a)->{System.out.println(f); return CONTINUE; })
  .onVisitFailed((f,e)->{ throw e; });

or

FileVisitor<Path> fv2=new LambdaFileVisitor<Path>()
  .onPreVisitDir((f,a)->{System.out.println("ENTER "+f); return CONTINUE; })
  .onPostVisitDir((f,e)->{
      System.out.println("LEAVE "+f);
      if(e!=null) throw e; else return CONTINUE;
  });