Java Reference
In-Depth Information
As stated earlier, Java programs work by having things called
objects
perform actions.
The actions performed by an object are called
methods
.
System.out
is an object used
for sending output to the screen;
println
is the method (that is, the action) that this
object performs. The action is to send what is in parentheses to the screen. When an
object performs an action using a method, it is called
invoking
(or
calling
) the method.
In a Java program, you write such a method invocation by writing the object followed
by a
dot
(period), followed by the method name, and some parentheses that may or
may not have something inside them. The thing (or things) inside the parentheses is
called an
argument(s)
, which provides information needed by the method to carry out
its action. In each of these two lines and the similar line that follows them, the method
is
println
. The method
println
writes something to the screen, and the argument
(a string in quotes) tells it what it should write.
Invoking a method
is also sometimes called
sending a message
to the object. With
this view, a message is sent to the object (by invoking a method) and in response, the
object performs some action (namely the action taken by the method invoked). We
seldom use the terminology
sending a message
, but it is standard terminology used by
some programmers and authors.
Variable declarations in Java are similar to what they are in other programming
languages. The following line from Display 1.1 declares the variable
answer
:
invoking
dot
argument
sending a
message
variable
int
int
answer;
The type
int
is one of the Java types for integers (whole numbers). So, this line says
that
answer
is a variable that can hold a single integer (whole number).
The following line is the only real computing done by this first program:
equal sign
assignment
operator
answer = 2 + 2;
In Java, the equal sign is used as the
assignment operator
, which is an instruction
to set the value of the variable on the left-hand side of the equal sign. In the preceding
program line, the equal sign does not mean that
answer
is equal to
2 + 2
. Instead, the
equal sign is an instruction to the computer to
make
answer
equal to
2 + 2
.
The last program action is
System.out.println("2 plus 2 is " + answer);
This is an output statement of the same kind as we discussed earlier, but there is
something new in it. Note that the string "
2 plus 2 is
" is followed by a plus sign and
the variable
answer
. In this case, the plus sign is an operator to concatenate (connect)
two strings. However, the variable
answer
is not a string. If one of the two operands
to
+
is a string, Java will convert the other operand, such as the value of
answer
, to a
string. In this program,
answer
has the value
4
, so
answer
is converted to the string
"
4
" and then concatenated to the string "
2 plus 2 is
", so the output statement
under discussion is equivalent to
System.out.println("2 plus 2 is 4");