Java Reference
In-Depth Information
memory from the system to store the
Account
object, then implicitly calls the class's con-
structor to
initialize
the object. The call is indicated by the parentheses after the class
name, which contain the
argument
"Jane
Green"
that's used to initialize the new object's
name. The class instance creation expression in line 10 returns a
reference
to the new ob-
ject, which is assigned to the variable
account1
. Line 11 repeats this process, passing the
argument
"John
Blue"
to initialize the name for
account2
. Lines 14-15 use each object's
getName
method to obtain the names and show that they were indeed initialized when the
objects were
created
. The output shows
different
names, confirming that each
Account
maintains its
own copy
of instance variable
name
.
1
// Fig. 3.6: AccountTest.java
2
// Using the Account constructor to initialize the name instance
3
// variable at the time each Account object is created.
4
5
public class
AccountTest
6
{
7
public static void
main(String[] args)
8
{
9
// create two Account objects
Account account1 =
new
Account(
"Jane Green"
);
Account account2 =
new
Account(
"John Blue"
);
10
11
12
13
// display initial value of name for each Account
14
System.out.printf(
"account1 name is: %s%n"
, account1.getName());
15
System.out.printf(
"account2 name is: %s%n"
, account2.getName());
16
}
17
}
// end class AccountTest
account1 name is: Jane Green
account2 name is: John Blue
Fig. 3.6
|
Using the
Account
constructor to initialize the
name
instance variable at the time each
Account
object is created.
Constructors Cannot Return Values
An important difference between constructors and methods is that
constructors cannot re-
turn values
, so they
cannot
specify a return type (not even
void
). Normally, constructors
are declared
public
—later in the topic we'll explain when to use
private
constructors.
Default Constructor
Recall that line 13 of Fig. 3.2
Account myAccount =
new
Account();
used
new
to create an
Account
object. The
empty
parentheses after “
new
Account
” indicate a
call to the class's
default constructor
—in any class that does
not
explicitly declare a construc-
tor, the compiler provides a default constructor (which always has no parameters). When a
class has only the default constructor, the class's instance variables are initialized to their
de-
fault values
. In Section 8.5, you'll learn that classes can have multiple constructors.
