The String and StringBuffer Classes |
For the
String
ClassIn addition to thelength
andcharAt
accessors you saw on the previous page, theString
class provides two accessors that return the position within the string of a specific character or string:indexOf
andlastIndexOf
. TheindexOf
method searches forward from the beginning of the string, andlastIndexOf
searches backward from the end of the string.The
indexOf
andlastIndexOf
methods are frequently used withsubstring
, which returns a substring of the string. The following class illustrates the use oflastIndexOf
andsubstring
to isolate different parts of a filename.
Note: The methods in the followingFilename
class don't do any error checking and assume that their argument contains a full directory path and a filename with an extension. If these methods were production code they would verify that their arguments were properly constructed.
Here's a small program that constructs a// This class assumes that the string used to initialize // fullPath has a directory path, filename, and extension. // The methods won't work if it doesn't. public class Filename { private String fullPath; private char pathSeparator, extensionSeparator; public Filename(String str, char sep, char ext) { fullPath = str; pathSeparator = sep; extensionSeparator = ext; } public String extension() { int dot = fullPath.lastIndexOf(extensionSeparator); return fullPath.substring(dot + 1); } public String filename() { int dot = fullPath.lastIndexOf(extensionSeparator); int sep = fullPath.lastIndexOf(pathSeparator); return fullPath.substring(sep + 1, dot); } public String path() { int sep = fullPath.lastIndexOf(pathSeparator); return fullPath.substring(0, sep); } }Filename
object and calls all of its methods:And here's the output from the program:public class FilenameTest { public static void main(String[] args) { Filename myHomePage = new Filename("/home/mem/index.html", '/', '.'); System.out.println("Extension = " + myHomePage.extension()); System.out.println("Filename = " + myHomePage.filename()); System.out.println("Path = " + myHomePage.path()); } }TheExtension = html Filename = index Path = /home/mem/public_htmlextension
method useslastIndexOf
to locate the last occurrence of the period (.) in the filename. Thensubstring
uses the return value oflastIndexOf
to extract the filename extension--that is, the substring from the period to the end of the string. This code assumes that the filename actually has a period in it; if the filename does not have a period , thenlastIndexOf
returns -1, and thesubstring
method throws aStringIndexOutOfBoundsException
.Also, notice that
extension
usesdot + 1
as the argument tosubstring
. If the period character is the last character of the string, thendot + 1
is equal to the length of the string which is one larger than the largest index into the string (because indices start at 0). However,substring
accepts an index equal to but not greater than the length of the string and interprets it to mean "the end of the string."Try this: Inspect the other methods in the
Filename
class and notice how thelastIndexOf
andsubstring
methods work together to isolate different parts of a filename.While the methods in the example above use only one version of the
lastIndexOf
method, theString
class actually supports four different versions of both theindexOf
andlastIndexOf
methods. The four versions work as follows:
indexOf(int character)
lastIndexOf(int character)
- Return the index of the first (last) occurrence of the specified character.
indexOf(int character, int from)
lastIndexOf(int character, int from)
- Return the index of the first (last) occurrence of the specified character, searching forward (backward) from the specified index.
indexOf(String string)
lastIndexOf(String string)
- Return the index of the first (last) occurrence of the specified String.
indexOf(String string, int from)
lastIndexOf(String string, int from)
- Return the index of the first (last) occurrence of the specified String, searching forward (backward) from the specified index.
For the
StringBuffer
ClassLikeString
,StringBuffer
provideslength
andcharAt
accessor methods. In addition to these two accessors,StringBuffer
also has a method calledcapacity
. Thecapacity
method differs fromlength
in that it returns the amount of space currently allocated for theStringBuffer
, rather than the amount of space used. For example, the capacity of theStringBuffer
in thereverseIt
method shown here never changes, while the length of theStringBuffer
increases by one for each iteration of the loop:public class ReverseString { public static String reverseIt(String source) { int i, len = source.length(); StringBuffer dest = new StringBuffer(len); for (i = (len - 1); i >= 0; i--) dest.append(source.charAt(i)); return dest.toString(); } }
The String and StringBuffer Classes |