Chapter 7 ■ String proCeSSing

Table 7-2. The regionMatches() Methods and Descriptions

Listing 7-4. MatchRegionInString.java

// This example demonstrates how to search a "region" of text within a string public class MatchRegionInString {

public static void main(String[] s) {

String chat = "Tarzan: Hi Jane, wanna ride an Elephant? \n Jane: No thanks! I'm preparing for OCPJP now!";

String matchString = " Jane: No thanks!";

//first get the index of the position from which the search region starts int startIndex = chat.indexOf('\n');

System.out.println("Jane's response starts at the index: " + startIndex);

//if '\n' found, then try matching for the string " Jane: No thanks!" from there if(startIndex > −1) {

//remember that the index starts from 0 and not 1, so add 1 to startIndex boolean doesMatch = chat.regionMatches(startIndex + 1, matchString, 0,

matchString.length());

if(doesMatch)

System.out.println("Jane's response matches with the string" +

matchString);

}

}

}

This program prints the following output, as you expected:

Jane's response starts at the index: 41

Jane's response matches the string Jane: No thanks!

String Parsing

Parsing is an interesting and useful operation on strings. (The word parse means “to analyze to break down into constituent parts based on an assumed structure.”) We will introduce you to the basic parsing operation in this section; we will discuss advanced parsing topics later in this chapter when discussing regular expressions.

String Conversions

In your programs, you’ll find it is often necessary to convert strings to and from primitive types such as floats, ints, and booleans. To convert from a primitive type value to String type, you can use the overloaded valueOf() method from the String class.

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Chapter 7 ■ String Processing

Let’s start with converting an integer value 10 to String. Here’s how to do it:

String str1 = String.valueOf(10); // right way to convert from an integer to String

Note that direct assignments or casts will result in compiler error, such as these two statements:

String str1 = 10; // compiler error—cannot convert from int to String String str1 = (String) 10; // compiler error—cannot convert from int to String

How about the conversion the other way around: if a string has value of some primitive type (say an integral value), how can you perform the conversion? Obviously, the following two statements, which attempt to directly assign or change type through an explicit cast, will result in compiler errors:

To make this type conversion, you need to use the parseInt() static method available in the Integer class, like so:

int i = Integer.parseInt("10"); // right way to convert from a String to an int  

This parseInt() method is an overloaded method. There is another parseInt() method that takes an additional argument: the base (or radix) of the integral value such as octal and hexadecimal. The wrapper classes Byte, Short, Long, Float, and Double have the equivalent parse methods to convert a string to the corresponding primitive type value. What if you pass an invalid argument to one of these parse methods? For example,

float f = Float.parseFloat("no such value");

For this code, you’ll get a runtime exception of java.lang.NumberFormatException since the string “no such value” cannot be converted to float type value.

The Split() Method

Listing 7-5 shows an example of how to split a sentence and print all words in the string using the split() method of the String class.

Listing 7-5.  ParseString1.java

// this example demonstrates the usage of split() method public class ParseString1 {

public static void main(String[] s) {

String quote = "Never lend books-nobody ever returns them!";

String [] words = quote.split(" "); // split strings based on the delimiter " "

(space)

for (String word : words) { System.out.println(word);

}

}

}

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Chapter 7 ■ String Processing

It prints the following:

Never lend

books-nobody ever returns them!

The split() method takes a delimiter as a regular expression (you will explore regular expression later in this chapter). In this example, you provide a whitespace as a delimiter, so you are able to extract all the words in the sentence. Note how the characters “-” and “!” are part of the strings “books-nobody” and “them!” since you did not specify any punctuation characters as delimiters. You’ll revisit this problem shortly when you learn about regular expressions.

The argument of the split() method is a delimiter string, which is a regular expression. If the regular expression you pass has invalid syntax, you’ll get a PatternSyntaxException exception.

Now, let’s assume that you have a string containing the path of a folder, and you want to parse this string and print individual folder names. Listing 7-6 shows the implementation.

Listing 7-6.  ParseString2.java

public class ParseString2 {

public static void main(String[] args) {

String str = "c:\\work\\programs\\parser"; String [] dirList = str.split("\\\\"); for (int i=0; i<dirList.length; i++) {

System.out.println(dirList[i]);

}

}

}

It prints the following:

c: work

programs parser

In this example, two things may surprise you.

•First, the use of “\\” instead of “\” is interesting. In order to show special characters in a string, you need to use the escape sequence for such special characters. For instance, if you want to put a new line character, you have to use “\n”; similarly you have to use “\t” for a tab symbol. Here, you want to use a backslash, which can be shown in a string as “\\”.

•The second thing is the delimiter used with the split() method. Well, a regular expression was used here to parse the path using “\” as a delimiter. We will soon discuss the usage of four consecutive backslashes instead of one when we discuss regular expressions.

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