Reference types - Data Structures and Problem Solving Using Java - page 33

The consequences are the same. Here, there are two Button objects that
have been constructed. At the end of the sequence, the first object is being
referenced by both noButton and yesButton , while the second object is
unreferenced.
At first glance, the fact that objects cannot be copied seems like a severe
limitation. Actually, it is not, although this does take a little getting used to.
(Some objects do need to be copied. For those, if a clone method is available,
it should be used. However, clone is not used in this text.)

2.2.5 parameter passing

Because of call-by-value, the actual arguments are sent into the formal param-
eters using normal assignment. If the parameter is a reference type, then we
know that normal assignment means that the formal parameter now references
the same object as does the actual argument. Any method applied to the for-
mal parameter is thus also being applied to the actual argument. In other lan-
guages, this is known as call-by-reference parameter passing . Using this
terminology for Java would be somewhat misleading because it implies that
the parameter passing is different. In reality, the parameter passing has not
changed; rather, it is the parameters that have changed, from nonreference
types to reference types.
As an example, suppose we pass yesButton as a parameter to the
clearButton routine that is defined as follows:

Call-by-value
means that for ref-
erence types, the
formal parameter
references the
same object as
does the actual
argument.

public static void clearButton( Button b )
{
b.setLabel( "No" );
b = null;
}

Then, as Figure 2.3 shows, b references the same object as yesButton , and
changes made to the state of this object by methods invoked through b will be
seen when clearButton returns. Changes to the value of b (i.e., which object it
references) will not have any affect on yesButton .

2.2.6 the meaning of ==

For primitive types, == is true if the stored values are identical. For reference
types, its meaning is different but is perfectly consistent with the previous
discussion.

For reference
types, == is true
only if the two ref-
erences reference
the same object .