Question

How can I use something like this (Lambda Expression) in Java with JDK7 ?

public virtual IEnumerable<TEntity> Get(Expression<Func<TEntity, bool>> filter = null)

I am trying to pass this method a where clause to filter the select statement, in C# I can use Lambda expressions and I am searching for a solution to do this in my Java project.

public List<TEntity> Get() {
    List<TEntity> result = emf.createEntityManager().createNamedQuery(ClassName + ".findAll").getResultList();
    return result;
}

This is my Method and i would like to pass a where clause tot this method.

Answer 1

I’m not a C(sharp) person, but isn’t that just the equivalent of:

Iterable<Entity> get();
Iterable<Entity> get(Expression<Func<Entity, Boolean>> filter);

Follow up to your edit: I don’t know the .NET library, but a filtering in Java would look something like:

public interface Predicate<T> {
    boolean matches(T obj);
}
...
public List<Entity> get(Predicate<Entity> filter) {
    List<Entity> filtered = new ArrayList<>();
    for (Entity obj : ...blah...) {
        if (filter.matches(obj)) {
            filtered.add(obj);
         }
    }
    return filtered;
}

In the current verbose syntax that would called like:

List<Entity> entities = thing.get(new Predicate<>() {
    public boolean matches(Entity obj) {
        return obj.isX() && obj.isY();
    }
});

A more concise syntax with random changes to semantics should be added in Java SE 8, along with library additions.