Name: gm110360 Date: 03/22/2004


A DESCRIPTION OF THE REQUEST :

In the Collections class, provide

public class Collections {

    public final static Iterator EMPTY_ITERATOR = new EmptyIterator();

    private final static EmptyIterator implements Iterator {
        public boolean hasNext() { return false; }
        public Object next() { throw new NoSuchElementException(); }
        public void remove() { throw new UnsupportedOperationException(); }
    }

    // ..

}

Make Collections.EMPTY_LIST.iterator() and Collections.EMPTY_SET.iterator()
return Collections.EMPTY_ITERATOR

JUSTIFICATION :
Performance.
EMPTY_LIST and EMPTY_SET are stateless.
So is the iterator returned from them.
The current implementation, however, returns a new object
every time the iterator() method is invoked.

Returning an empty Collection in lieu of null from a method
is a good programming idiom.

For example

    Hr hr = ..
    List employees = hr.getEmployees(Department.ENGR);
    for (Iterator i = employees.iterator(); i.hasNext(); ) {
        // ..
    }

The code works because Hr.getEmployees(..) never returns
null: instead its contract says that it always returns a non-null,
unmodifiable List. When there are no employees to return the
Hr implementation returns Collections.EMPTY_LIST.

But if Hr.getEmployees(..) returns Collections.EMPTY_LIST,
there is no good reason to make the user pay a performance
penalty (the creation of a new object) for invoking iterator().


This fix would take care of Collections.EMPTY_MAP since
it in turn returns EMPTY_SET when either keySet() or
values() is invoked.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
I don't expect any objects to be created when invoking
any method on Collections.EMPTY_SET, EMPTY_LIST
or EMPTY_MAP, or on any object returned by them.
ACTUAL -
A new object is created every time iterator() is invoked
on any of the above.

CUSTOMER SUBMITTED WORKAROUND :
In order to avoid the performance penalty the user has
to check the size of the List. Continuing the above example,

    Hr hr = ..
    List employees = hr.getEmployees(Department.ENGR);
    if ( !empoyees.isEmpty() ) {
        for (Iterator i = employees.iterator(); i.hasNext(); ) {
            // ..
        }
    }

This is as bad as allowing the Hr class return null and making
the user do something like

    Hr hr = ..
    List employees = hr.getEmployees(Department.ENGR);
    if ( employees != null && !empoyees.isEmpty() ) {
        for (Iterator i = employees.iterator(); i.hasNext(); ) {
            // ..
        }
    }
(Incident Review ID: 244382)
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