Java Reference
In-Depth Information
You can override the
equals
method in the
Circle
class to compare whether two circles
are equal based on their radius as follows:
public boolean
equals(Object o) {
if
(o
instanceof
Circle) {
return
radius == ((Circle)o).radius;
}
else
return false
;
}
Note
The
==
comparison operator is used for comparing two primitive data type values or for
determining whether two objects have the same references. The
equals
method is
intended to test whether two objects have the same contents, provided that the method
is overridden in the defining class of the objects. The
==
operator is stronger than the
equals
method, in that the
==
operator checks whether the two reference variables
refer to the same object.
==
vs. equals
Caution
Using the signature
equals(SomeClassName obj)
(e.g.,
equals(Circle c))
to override the
equals
method in a subclass is a common mistake. You should use
equals(Object obj
). See CheckPoint Question 11.28.
equals(Object)
11.27
✓
✓
Does every object have a
toString
method and an
equals
method? Where do they
come from? How are they used? Is it appropriate to override these methods?
Check
Point
11.28
When overriding the
equals
method, a common mistake is mistyping its signature in
the subclass. For example, the
equals
method is incorrectly written as
equals(Circle circle)
, as shown in (a) in following the code; instead, it should
be
equals(Object circle)
, as shown in (b). Show the output of running class
Test
with the
Circle
class in (a) and in (b), respectively.
public class
Test {
public static void
main(String[] args) {
Object circle1 =
new
Circle();
Object circle2 =
new
Circle();
System.out.println(circle1.equals(circle2));
}
}
class
Circle {
double
radius;
class
Circle {
double
radius;
public boolean
equals( ) {
return this
.radius == circle.radius;
Circle circle
public boolean
equals(
Object circle
) {
return this
.radius ==
((Circle)circle).radius;
}
}
}
}
(a)
(b)
Key
Point
VideoNote
Now we are ready to introduce a very useful class for storing objects. You can create an array
to store objects. But, once the array is created, its size is fixed. Java provides the
ArrayList
The
ArrayList
class
