Passing method as a parameter - Is this possible?

Question

I am trying to migrate to java 8 and have a number of methods in my dao classes which do the following

@Override
@SuppressWarnings("unchecked")
public List<Group> getGroups()
{
    Session session = sessionFactory.openSession();
    List<Group> allGroups = (List<Group>)session.createQuery("from Group").list();
    session.close();
    return allGroups;
}

Here the same boiler plate sessionFactory.open and session.close is repeated for all methods.

Is it possible in Java 8 to have a method which does the open and close and takes a function which is the rest of my code and execute it inbetween?

If so - what is the name of this process , or can anyone provide some help on how this might be achieved

Answer 1

Since you want to express code which works on a Session instance (so you can abstract the creation and cleanup of it) and might return an arbitrary result, a Function<Session,T> would be the right type for encapsulating such code:

public <T> T doWithSession(Function<Session,T> f) {
    Session session = sessionFactory.openSession();
    try {
        return f.apply(session);
    }
    finally {
        session.close();
    }
}

then you can use it like:

@Override
@SuppressWarnings("unchecked")
public List<Group> getGroups()
{
    return doWithSession(session -> (List<Group>)session.createQuery("from Group").list());
}

This isn’t a special Java 8 technique. You can do the same in earlier Java version, see also What is the “execute around” idiom (Thanks to gustafc for the link). What makes it easier is that Java 8 provides you with an existing Function interface and that you can implement that interface using a lambda expression instead of having to resort to anonymous inner classes or such alike.

If your desired operation consists of a single method invocation and doesn’t require a type cast, you can implement it as a method reference like doWithSession(Session::methodName), see Method References in the tutorial