Java Reference
In-Depth Information
Assembling Strings with String Concatenation
Java allows you to assemble
String
objects into larger strings by using operators
+
or
+=
.
This is known as
string concatenation
. When both operands of operator
+
are
String
ob-
jects, operator
+
creates a new
String
object in which the characters of the right operand
are placed at the end of those in the left operand—e.g., the expression
"hello " + "there"
creates the
String
"hello
there"
.
In line 24 of Fig. 6.3, the expression
"Maximum
is: "
+
result
uses operator
+
with
operands of types
String
and
double
.
Every primitive value and object in Java can be rep-
resented as a
String
.
When one of the
+
operator's operands is a
String
, the other is con-
verted to a
String
, then the two are
concatenated
. In line 24, the
double
value is converted
to its
String
representation and placed at the end of the
String
"Maximum
is: "
. If there
are any
trailing zeros
in a
double
value, these will be
discarded
when the number is con-
verted to a
String
—for example 9.3500 would be represented as 9.35.
Primitive values used in
String
concatenation are converted to
String
s. A
boolean
concatenated with a
String
is converted to the
String
"true"
or
"false"
.
All objects have
a
toString
method that returns a
String
representation of the object.
(We discuss
toString
in more detail in subsequent chapters.) When an object is concatenated with a
String
, the
object's
toString
method is implicitly called to obtain the
String
representation of the
object. Method
toString
also can be called explicitly.
You can break large
String
literals into several smaller
String
s and place them on
multiple lines of code for readability. In this case, the
String
s can be reassembled using
concatenation. We discuss the details of
String
s in Chapter 14.
Common Programming Error 6.2
It's a syntax error to break a
String
literal across lines. If necessary, you can split a
String
into several smaller
String
s and use concatenation to form the desired
String
.
Common Programming Error 6.3
Confusing the
+
operator used for string concatenation with the
+
operator used for addi-
tion can lead to strange results. Java evaluates the operands of an operator from left to
right. For example, if integer variable
y
has the value
5
, the expression
"y+2="+y+2
results in the string
"y
+
2
=
52"
, not
"y
+
2
=
7"
, because first the value of
y
(5) is con-
catenated to the string
"y
+
2
=
"
, then the value
2
is concatenated to the new larger string
"y
+
2
=
5"
. The expression
"y
+
2="
+
(y
+
2)
produces the desired result
"y
+
2
=
7"
.
There are three ways to call a method:
1.
Using a method name by itself to call another method of the
same
class—such as
maximum(number1, number2, number3)
in line 21 of Fig. 6.3.
2.
Using a variable that contains a reference to an object, followed by a dot (
.
) and
the method name to call a non-
static
method of the referenced object—such as
the method call in line 16 of Fig. 3.2,
myAccount.getName()
, which calls a meth-
od of class
Account
from the
main
method of
AccountTest
. Non-
static
meth-
ods are typically called
instance methods
.
