Java Reference

In-Depth Information

ted all the
String
instance methods. We now discuss some of these instance

methods. One of them,
length
, returns the length, or number of characters, of

the string that the instance contains. For example, we have:

"abcd".length()

is 4

("abcd" + "ef").length()

is 6

Non-Java notation for strings

Often, we will be talking about a particular character of a string
s
or a

sequence of adjacent characters of
s
. It helps to have a notation for referring to

these parts of
s
:

s[0]
denotes the first character of
s
.

s[1]
denotes the second character of
s
.

...

s[s.length()-1]
denotes the last character of
s
.

For example, if
s
is the string
"abc"
, then
s[1]
is the character
'b'
. Also, if
int

variable
i
contains
0
, then
s[i+2]
is the character
'c'
. Expression
i
of
s[i]
is

called the
index
of character number
i
. We stress that
s[i]
is not Java notation;

we use it because it helps us discuss strings. Also, when writing
s[i]
, we assume

that
i
lies in the range
0..s.length() - 1
.

A second non-Java notation describes a substring of a string
:

s[h..k]

is the string consisting of the characters
s[h]
,
s[h + 1]
, …,
s[k]
. The length of

s[h..k]
is
k+1-h
. Here are examples, assuming that
s
is
"abcded"
:

s[2..4]
is
"cde";
it has length
4+1-2 = 3

s[2..3]
is
"cd";
it has length
3+1-2 = 2

s[2..2]
is
"c";
it has length
2+1-2 = 1

s[2..1]
is
"";
it has length
1+1-2 = 0

The last line above uses the convention that if the first index (2 in this case)

is one more than the second index (1), then the substring is the empty string. The

abbreviation
s[h..]
refers to the substring
s[h..s.length()-1]
.

Referencing the characters of a String

Function
charAt
retrieves a character from a string. The call
s.charAt(i)

evaluates to the character
s[i]
. Thus,
"At peace".charAt(1)
is
't'
. In refer-

ence
s.charAt(i)
,
i
is the
index
of the character in
s
.

Here is a specification of instance function
charAt
:

/** =
this[i]. Precondition:
0 <= i < this.length */

public char
charAt(
int
i)

Activity

5-3.5

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