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import java.*:Strings

June 2000/import java.*

Like many of the generation of "seasoned" programmers, my career in coding began with number crunching. Most of the time my FORTRAN programs just emitted numbers accompanied by terse labels. As I stood in line at the university computing center to submit my deck of punched cards to the IBM OS/360 expediters (as they were called), I would feel sorry for the COBOL programmers who hefted decks 10-20 times the size of mine. "The poor things," I would mutter to myself, "their hundreds of cards only print a silly report while my less-than-one-inch deck solves a system of differential equations. I hope I never have to learn that stupid language!" I didn't. But when it came time to report my findings in a more formal way, I had to get out a typewriter.

Don't get me wrong. I'm pleased and proud to never have written a single line of COBOL, but FORTRAN IV's string handling capabilities were nothing to write home about, manually or electronically. No string data type, no dynamic memory allocation, no string nothing. When I left school to embark on a career as a "scientific programmer," I found that business required at least as much text and data processing as mathematics, and found myself wanting a more suitable programming language. And if you want the whole truth, let me admit that most of the projects at my first job used COBOL for I/O.

Back to BASICs

Once when I needed to do some sophisticated string manipulation, someone suggested I use SNOBOL. "Sure, why not? It's just another programming language," I thought. Ha! I just wasn't ready for such a weird syntax. Then I remembered having run across BASIC in one of my college classes. Like most "real programmers," I had dismissed it as a toy, but boy, I certainly could have used a + operator for concatenation or a MID$() for searching right about then! But BASIC wasn't available in our environment, and besides, C was becoming the language of choice almost everywhere, so I quickly learned the joys of null-terminated arrays of char. And malloc. And realloc. And memory leaks.

So here we are programming in Internet Time and have now, with C++ and Java, come full circle to what BASIC provided originally: strings that manage their own memory. What took so long? I wish I knew.

Java provides two string classes: String and StringBuffer. Both are defined in the package java.lang, and both represent sequences of char (a 16-bit Unicode character). The key difference between the two classes is that String objects are immutable, and hence can be easily pooled and shared. Whenever you use a string literal (characters between pairs of double quotes), the JVM (Java Virtual Machine) first checks if it has appeared previously. If not, the JVM stores the literal in the string pool, and returns a reference to it. The next time you use that literal, the JVM just gives you back a reference to it, so all occurrences of a particular string literal refer to a single String instance. You can even request a reference for a string built at run time, as the program in Figure 1 illustrates.

The variable s2 refers to the same object as s1, since the compiler can determine at compile time that the character sequences are the same. A new object is explicitly created for s3, so the references are distinct. (Let this be a lesson on how not to create strings from literals — if there is no need for a distinct object, don't use new). s4 is also a unique object since its value is determined at run time. To force a run-time check as to whether the string being assigned already exists in the string pool, use function intern, as I did for s5.

The String class comes with the usual methods for searching and extracting substrings, changing case (for locales where case applies), and converting strings to numbers. Here's a grep-like program that extracts lines from a text file that contain a given string:


class Search
   public static void
   main(String[] args)
      throws IOException
      // Read Standard Input:
      BufferedReader in =
         new BufferedReader(
            new InputStreamReader(

     // Process each line:
     String line;
     while ((line = in.readLine())
               != null)
        if (line.indexOf(args[0])>=0)

The indexOf method returns the zero-based index where its argument appears in its string, or returns -1 if it's not there. Like many of String's methods, there is also version of indexOf that specifies the search start position as well (i.e., indexOf(String str, int fromIndex)), so you can find multiple instances within a single line. String also includes a lastIndexOf method for searching from the right, and startsWith and endsWith methods, which are special cases of the indexOf and lastIndexOf, respectively.

Executing on its own source searching for the string "read" gives the following output:

while ((line = in.readLine()) != null)

That's because indexOf is case-sensitive, as you would expect, so the lines that contain the substring "Read" don't qualify. To extract lines without regard to case, I'll have to convert both the search string and each line to the same case (see Figure 2). Although the String class includes an equalsIgnoreCase method, that won't help here since it compares entire strings and I'm looking for a substring. This time the output is as follows:

C:> java Search2 read <
        BufferedReader in =
            new BufferedReader(
                new InputStreamReader(;
        while ((line = in.readLine()) != null)

The program in Figure 3 reads standard input and uses indexOf and the substring method to replace all occurrences of the string in args[0] with the string in args[1]. The output in Figure 3 comes from the following command line:

C:> java Replace in $$$ <

Both String and StringBuffer are thread-safe: String because its instances are immutable, and StringBuffer because its mutator methods are synchronized (hence String is more efficient with respect to thread overhead). While most of the time String does the job, StringBuffer is more efficient if you need to append to a string repeatedly. In fact, StringBuffer is used internally whenever you use the + concatenation operator with instances of String. The expression a + b + c, for example, is equivalent to the following expression:

new StringBuffer(a).append(b).

Using StringBuffer instead of String can sometimes bring noticeable gains in efficiency. To see this, replace the body of the inner while loop in Figure 3 with the following:

    StringBuffer temp =  
      new StringBuffer(newLine);
    newLine =  
      temp.replace(pos2,  pos2 + fromLen,
    pos1 = pos2 + toLen;

Running javap -c Replace (the byte-code disassembler) both before and after the change reveals that the StringBuffer version generates 11 fewer byte code instructions. StringBuffer.replace works within a single buffer, allocating memory only when necessary. Since the creation of the substrings is not needed in the second version, the code is more efficient. For further optimization, if you know the largest size a StringBuffer will ever need to be, you can allocate it once and for all with the method StringBuffer.ensureCapacity(int minimumCapacity). The loop above would be even simpler if StringBuffer had an indexOf operator, since it could just declare newLine as a StringBuffer and do without temp altogether. Too bad the Java library folks didn't include it.

The program in Figure 4 illustrates the method String.charAt in determining whether a string is a palindrome (i.e., whether it reads the same backward as it does forward). If String had a reverse method, as StringBuffer does, you might be tempted to use it to just compare a string to its reversal, but that would be much less efficient than using Palindrome.isSymmetric.

Palindromes are more interesting and readable if they can have whitespace and punctuation, which you then ignore when testing for symmetry. (Such strings are called imperfect palindromes.) The program in Figure 5 does just that by using StringBuffer.deleteCharAt in Palindrome2.normalize to strip out non-letters (via Character.isLetter) before calling isSymmetric. The normalize method also changes each character to lower case so the comparison will be case insensitive. The logic is a little tricky: notice that the variable i is incremented only if the character is not deleted. Since the length of the string can change, I also have to call s.length explicitly every time the loop iterates, instead of caching it like I did in Figure 4. And once again, I have to traverse the string one character at a time, searching for non-letters, since StringBuffer does not have an indexOf method.


Much of the time a program that processes text needs to extract tokens. Java provides a class in the java.util package, StringTokenizer, for this purpose. In its simplest application, a StringTokenizer object extracts tokens delimited by whitespace from a string. The program in Figure 6 places up to 256 space-delimited tokens from the file tokens.dat in Figure 7 into an array, converts them to lower case, and sorts them with the Arrays.sort algorithm. (I'll say more about java.util.Arrays in the September 2000 issue.) After initializing a StringTokenizer with the string to parse, the hasMoreTokens method will return true if there are any tokens left to extract, and nextToken will return the next one as a String. You can call StringTokenizer.countTokens at any time if you want to know how many tokens are left without advancing the tokenization position.

Whitespace-only delimiters are not useful very often, however. The output in Figure 6, for example, contains unwanted punctuation. To extract only alphanumeric tokens, you can give a StringTokenizer an optional string representing the delimiters to ignore during tokenization, as I did with the string delims in Figure 8. This behaves pretty much the same as strtok from the standard C library, except that the string is not altered in the process. And like strtok, nextToken can take an optional string argument, so you can change the set of delimiters with each call.

Yet another usage of StringTokenizer extracts the delimiters themselves, one character at a time. In Figure 9, I give the StringTokenizer constructor a third argument of true, which tells the object that I want only the delimiters back. I use this behavior to count the number of vowels in the input file.


Java's string classes provide most of what you need for everyday text processing: substring searching and construction, case conversion, and basic token parsing. I hope you managed to pick up on a bit of irony in this article, however. I began by talking about number crunching, yet I've carefully avoided talking about formatting numeric I/O. There's a good reason, which you'll see in my next installment when I explain locales and Java's formatting classes. (Yes formatting is in separate classes, not in format strings!)

Chuck Allison is Consulting Editor and a columnist with CUJ. He is the owner of Fresh Sources, a company specializing in object-oriented software development, training, and mentoring. He has been a contributing member of J16, the C++ Standards Committee, since 1991, and is the author of C and C++ Code Capsules: A Guide for Practitioners, Prentice-Hall, 1998. You can email Chuck at [email protected].

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