when or why do i use Property.forName()?

Question

What is the difference between :

List cats = session.createCriteria(Cat.class)  
.add( Restrictions.like("name", "F%")
.list();   

and

List cats = session.createCriteria(Cat.class)  
.add( Property.forName("name").like("F%") )  
.list(); 

Or for that matter, difference between :

Criteria cr = session.createCriteria(User.class)
    .setProjection(Projections.projectionList() 
    .add(Property.forName("id").as("id")) 
    .add(Property.forName("name").as("name"))

and

Criteria cr = session.createCriteria(User.class)
    .setProjection(Projections.projectionList()
    .add(Projections.property("id"), "id")
    .add(Projections.property("Name"), "Name"))

Answer 1

Property.forName("propName") always returns you the matching Property instance.

Having said this, means there is no difference between the first two code snippets posted in your question. You should use Property.forName("propName") when you need to use that property multiple times in Criteria or Query. It is equivalent to using direct no. (e.g. 11) or using variable assigned to the no. (e.g. int x = 11) and use the variable everytime you need to use the no.

For more details, see this.

Now if I talk about the 2nd question (3rd & 4th code snippets), working of both is the same. The only difference is in the API being used.

In 3rd code snippet, you're getting the instance of Property, calling its as() method which is used to generate alias for that particular property and returns an instance of SimpleProjection (subclass of Projection).

While in 4th code snippet, you're getting instance of PropertyProjection (subclass of Projection) by doing Projections.property("Name").

So in both cases you're getting instance of Projection, which you're adding to ProjectionsList. Now ProjectionList is having 2 overloaded methods called add(). In 3rd code snippet you're calling add() that takes only instance of Projection as argument. In 4th code snippet you're calling another version of add(), that takes instance of Projection as first argument & alias for the property of Projection as the 2nd argument. So ultimately working of both is the same.