Java Reference
In-Depth Information
int aIndex = -1; // Position of 1st 'a'
int bIndex = -1; // Position of 1st 'b' after 'a'
aIndex = text.indexOf('a'); // Find first 'a'
if(aIndex >= 0) { // Make sure you found 'a'
bIndex = text.indexOf('b', aIndex+1); // Find 1st 'b' after 1st 'a'
}
After you have the index value from the initial search for
'a'
, you need to check that
'a'
was really
found by verifying that
aIndex
is not negative. You can then search for
'b'
from the position following
'a'
.
As you can see, the second argument of this version of the method
indexOf()
is separated from the first
argument by a comma. Because the second argument is the index position from which the search is to start,
and
aIndex
is the position at which
'a'
was found, you should increment
aIndex
to the position following
'a'
before using it in the search for
'b'
to avoid checking for
'b'
in the position you already know contains
'a'
.
If
'a'
happened to be the last character in the string, it wouldn't matter because the
indexOf()
method
just returns −1 if the index value is beyond the last character in the string. If you somehow supplied a neg-
ative index value to the method, it simply searches the whole string from the beginning.
Of course, you could use the
indexOf()
method to count how many times a particular character occurred
in a string:
int aIndex = -1; // Search start position
int count = 0; // Count of 'a' occurrences
while((aIndex = text.indexOf('a', ++aIndex)) > -1) {
++count;
}
The
while
loop condition expression calls the
indexOf()
method for the
String
object referenced by
text
and stores the result in the variable
aIndex
. If the value stored is greater than
-1
, it means that
'a'
was
found, so the loop body executes and
count
is incremented. Because
aIndex
has
-1
as its initial value, the
search starts from index position
0
in the string, which is precisely what you want. When a search reaches
the end of the string without finding
'a'
,
-1
is returned by the
indexOf()
method and the loop ends.
Searching for Substrings
The
indexOf()
and
lastIndexOf()
methods also come in versions that accept a string as the first argument,
which searches for this string rather than a single character. In all other respects they work in the same way
as the character searching methods you have just seen. I summarize the complete set of
indexOf()
methods
DESCRIPTION
Returns the index position of the first occurrence of the character
ch
in the
String
for which the
method is called. If the character
ch
does not occur, −1 is returned.
indexOf(int
ch)
Same as the preceding method, but with the search starting at position
index
in the string. If the
value of
index
is less than or equal to 0, the entire string is searched. If
index
is greater than or
equal to the length of the string, −1 is returned.
indexOf(int
ch,
int index)
Returns the index position of the first occurrence of the substring
str
in the
String
object for which
the method is called. If the substring
str
does not occur, −1 is returned.
indexOf(String
str)
