Java Reference
In-Depth Information
A string is a sequence of characters.
Key
Point
The
char
type represents only one character. To represent a string of characters, use the data
type called
String
. For example, the following code declares
message
to be a string with
the value
"Welcome to Java"
.
String message =
"Welcome to Java"
;
VideoNote
Introduce strings and objects
String
is a predefined class in the Java library, just like the classes
System
and
Scanner
.
The
String
type is not a primitive type. It is known as a
reference type
. Any Java class can
be used as a reference type for a variable. The variable declared by a reference type is known
as a reference variable that references an object. Here,
message
is a reference variable that
references a string object with contents
Welcome to Java
.
Reference data types will be discussed in detail in Chapter 9, Objects and Classes. For the
time being, you need to know only how to declare a
String
variable, how to assign a string
to the variable, and how to use the methods in the
String
class. More details on using strings
will be covered in Chapter 10.
Table 4.7 lists the
String
methods for obtaining string length, for accessing characters
in the string, for concatenating strings, for converting a string to upper or lowercases, and for
trimming a string.
T
ABLE
4.7
Simple Methods for
String
Objects
Method
Description
length()
Returns the number of characters in this string.
Returns the character at the specified index from this string.
charAt(index)
Returns a new string that concatenates this string with string s1.
concat(s1)
Returns a new string with all letters in uppercase.
toUpperCase()
Returns a new string with all letters in lowercase
toLowerCase()
Returns a new string with whitespace characters trimmed on both sides.
trim()
Strings are objects in Java. The methods in Table 4.7 can only be invoked from a spe-
cific string instance. For this reason, these methods are called
instance methods
. A non-
instance method is called a
static method
. A static method can be invoked without using
an object. All the methods defined in the
Math
class are static methods. They are not tied
to a specific object instance. The syntax to invoke an instance method is
reference-
Variable.methodName(arguments)
. A method may have many arguments or no argu-
ments. For example, the
charAt(index)
method has one argument, but the
length()
method has no arguments. Recall that the syntax to invoke a static method is
ClassName
.methodName(arguments)
. For example, the
pow
method in the
Math
class can be invoked
using
Math.pow(2, 2.5)
.
instance method
static method
4.4.1 Getting String Length
You can use the
length()
method to return the number of characters in a string. For exam-
ple, the following code
String message =
"Welcome to Java"
;
System.out.println(
"The length of "
+ message +
" is "
+ message.length());
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