10: The Java
IO system

Creating a good input/output (IO) system is one of the more difficult tasks for the language designer.

This is evidenced by the number of different approaches. The challenge seems to be in covering all eventualities. Not only are there different kinds of IO that you want to communicate with (files, the console, network connections), but you need to talk to them in a wide variety of ways (sequential, random-access, binary, character, by lines, by words, etc.).

The Java library designers attacked the problem by creating lots of classes. In fact, there are so many classes for Java’s IO system that it can be intimidating at first (ironically, the Java IO design actually prevents an explosion of classes). There has also been a significant change in the IO library between Java 1.0 and Java 1.1. Instead of simply replacing the old library with a new one, the designers at Sun extended the old library and added the new one alongside it. As a result you can sometimes end up mixing the old and new libraries and creating even more intimidating code.

This chapter will help you understand the variety of IO classes in the standard Java library and how to use them. The first portion of the chapter will introduce the “old” Java 1.0 IO stream library, since there is a significant amount of existing code that uses that library. The remainder of the chapter will introduce the new features in the Java 1.1 IO library. Note that when you compile some of the code in the first part of the chapter with a Java 1.1 compiler you can get a “deprecated feature” warning message at compile time. The code still works; the compiler is just suggesting that you use certain new features that are described in the latter part of this chapter. It is valuable, however, to see the difference between the old and new way of doing things and that’s why it was left in – to increase your understanding (and to allow you to read code written for Java 1.0).

The Java library classes for IO are divided by input and output, as you can see by looking at the online Java class hierarchy with your Web browser. By inheritance, all classes derived from InputStream have basic methods called read( ) for reading a single byte or array of bytes. Likewise, all classes derived from OutputStream have basic methods called write( ) for writing a single byte or array of bytes. However, you won’t generally use these methods; they exist so more sophisticated classes can use them as they provide a more useful interface. Thus, you’ll rarely create your stream object by using a single class, but instead will layer multiple objects together to provide your desired functionality. The fact that you create more than one object to create a single resulting stream is the primary reason that Java’s stream library is confusing.

It’s helpful to categorize the classes by their functionality. The library designers started by deciding that all classes that had anything to do with input would be inherited from InputStream and all classes that were associated with output would be inherited from OutputStream.

InputStream’s job is to represent classes that produce input from different sources. These sources can be (and each has an associated subclass of InputStream):

In addition, the FilterInputStream is also a type of InputStream, to provide a base class for "decorator" classes that attach attributes or useful interfaces to input streams. This is discussed later.

Table 10-1. Types of InputStream

Class	Function	Constructor Arguments
How to use it
ByteArray-InputStream	Allows a buffer in memory to be used as an InputStream.	The buffer from which to extract the bytes.
As a source of data. Connect it to a FilterInputStream object to provide a useful interface.
StringBuffer-InputStream	Converts a String into an InputStream.	A String. The underlying implementation actually uses a StringBuffer.
As a source of data. Connect it to a FilterInputStream object to provide a useful interface.
File-InputStream	For reading information from a file.	A String representing the file name, or a File or FileDescriptor object.
As a source of data. Connect it to a FilterInputStream object to provide a useful interface.

Piped-InputStream	Produces the data that’s being written to the associated PipedOutput-Stream. Implements the “piping” concept.	PipedOutputStream
As a source of data in multithreading. Connect it to a FilterInputStream object to provide a useful interface.
Sequence-InputStream	Coverts two or more InputStream objects into a single InputStream.	Two InputStream objects or an Enumeration for a container of InputStream objects.
As a source of data. Connect it to a FilterInputStream object to provide a useful interface.
Filter-InputStream	Abstract class which is an interface for decorators that provide useful functionality to the other InputStream classes. See Table 10-3.	See Table 10-3.
See Table 10-3.

This category includes the classes that decide where your output will go: an array of bytes (no String, however; presumably you can create one using the array of bytes), a file, or a “pipe.”

In addition, the FilterOutputStream provides a base class for "decorator" classes that attach attributes or useful interfaces to output streams. This is discussed later.

Table 10-2. Types of OutputStream

Class	Function	Constructor Arguments
How to use it
ByteArray-OutputStream	Creates a buffer in memory. All the data that you send to the stream is placed in this buffer.	Optional initial size of the buffer.
To designate the destination of your data. Connect it to a FilterOutputStream object to provide a useful interface.
File-OutputStream	For sending information to a file.	A String representing the file name, or a File or FileDescriptor object.
To designate the destination of your data. Connect it to a FilterOutputStream object to provide a useful interface.
Piped-OutputStream	Any information you write to this automatically ends up as input for the associated PipedInput-Stream. Implements the “piping” concept.	PipedInputStream
To designate the destination of your data for multithreading. Connect it to a FilterOutputStream object to provide a useful interface.
Filter-OutputStream	Abstract class which is an interface for decorators that provide useful functionality to the other OutputStream classes. See Table 10-4.	See Table 10-4.
See Table 10-4.

The use of layered objects to dynamically and transparently add responsibilities to individual objects is referred to as the decorator pattern. (Patterns[44] are the subject of Chapter 16.) The decorator pattern specifies that all objects that wrap around your initial object have the same interface, to make the use of the decorators transparent – you send the same message to an object whether it’s been decorated or not. This is the reason for the existence of the “filter” classes in the Java IO library: the abstract “filter” class is the base class for all the decorators. (A decorator must have the same interface as the object it decorates, but the decorator can also extend the interface, which occurs in several of the “filter” classes).

Decorators are often used when subclassing requires a large number of subclasses to support every possible combination needed – so many that subclassing becomes impractical. The Java IO library requires many different combinations of features which is why the decorator pattern is a good approach. There is a drawback to the decorator pattern, however. Decorators give you much more flexibility while you’re writing a program (since you can easily mix and match attributes), but they add complexity to your code. The reason that the Java IO library is awkward to use is that you must create many classes – the “core” IO type plus all the decorators – in order to get the single IO object that you want.

The classes that provide the decorator interface to control a particular InputStream or OutputStream are the FilterInputStream and FilterOutputStream – which don’t have very intuitive names. They are derived, respectively, from InputStream and OutputStream, and they are abstract classes, in theory to provide a common interface for all the different ways you want to talk to a stream. In fact, FilterInputStream and FilterOutputStream simply mimic their base classes, which is the key requirement of the decorator.

The FilterInputStream classes accomplish two significantly different things. DataInputStream allows you to read different types of primitive data as well as String objects. (All the methods start with “read,” such as readByte( ), readFloat( ), etc.) This, along with its companion DataOutputStream, allows you to move primitive data from one place to another via a stream. These “places” are determined by the classes in Table 10-1. If you’re reading data in blocks and parsing it yourself, you won’t need DataInputStream, but in most other cases you will want to use it to automatically format the data you read.

The remaining classes modify the way an InputStream behaves internally: whether it’s buffered or unbuffered, if it keeps track of the lines it’s reading (allowing you to ask for line numbers or set the line number), and whether you can push back a single character. The last two classes look a lot like support for building a compiler (that is, they were added to support the construction of the Java compiler), so you probably won’t use them in general programming.

You’ll probably need to buffer your input almost every time, regardless of the IO device you’re connecting to, so it would have made more sense for the IO library to make a special case for unbuffered input rather than buffered input.

Table 10-3. Types of FilterInputStream

Class	Function	Constructor Arguments
How to use it
Data-InputStream	Used in concert with DataOutputStream, so you can read primitives (int, char, long, etc.) from a stream in a portable fashion.	InputStream
Contains a full interface to allow you to read primitive types.

Buffered-InputStream	Use this to prevent a physical read every time you want more data. You’re saying “Use a buffer.”	InputStream, with optional buffer size.
This doesn’t provide an interface per se, just a requirement that a buffer be used. Attach an interface object.
LineNumber-InputStream	Keeps track of line numbers in the input stream; you can call getLineNumber( ) and setLineNumber(int).	InputStream
This just adds line numbering, so you’ll probably attach an interface object.
Pushback-InputStream	Has a one byte push-back buffer so that you can push back the last character read.	InputStream
Generally used in the scanner for a compiler and probably included because the Java compiler needed it. You probably won’t use this.

The complement to DataInputStream is DataOutputStream, which formats each of the primitive types and String objects onto a stream in such a way that any DataInputStream, on any machine, can read them. All the methods start with “write,” such as writeByte( ), writeFloat( ), etc.

If you want to do true formatted output, for example, to the console, use a PrintStream. This is the endpoint that allows you to print all of the primitive data types and String objects in a viewable format as opposed to DataOutputStream, whose goal is to put them on a stream in a way that DataInputStream can portably reconstruct them. The System.out static object is a PrintStream.

The two important methods in PrintStream are print( ) and println( ), which are overloaded to print out all the various types. The difference between print( ) and println( ) is that the latter adds a newline when it’s done.

BufferedOutputStream is a modifier and tells the stream to use buffering so you don’t get a physical write every time you write to the stream. You’ll probably always want to use this with files, and possibly console IO.

Table 10-4. Types of FilterOutputStream

Class	Function	Constructor Arguments
How to use it
Data-OutputStream	Used in concert with DataInputStream so you can write primitives (int, char, long, etc.) to a stream in a portable fashion.	OutputStream
Contains full interface to allow you to write primitive types.
PrintStream	For producing formatted output. While DataOutputStream handles the storage of data, PrintStream handles display.	OutputStream, with optional boolean indicating that the buffer is flushed with every newline.
Should be the “final” wrapping for your OutputStream object. You’ll probably use this a lot.
Buffered-OutputStream	Use this to prevent a physical write every time you send a piece of data. You’re saying “Use a buffer.” You can call flush( ) to flush the buffer.	OutputStream, with optional buffer size.
This doesn’t provide an interface per se, just a requirement that a buffer is used. Attach an interface object.

RandomAccessFile is used for files containing records of known size so that you can move from one record to another using seek( ), then read or change the records. The records don’t have to be the same size; you just have to be able to determine how big they are and where they are placed in the file.

At first it’s a little bit hard to believe that RandomAccessFile is not part of the InputStream or OutputStream hierarchy. It has no association with those hierarchies other than that it happens to implement the DataInput and DataOutput interfaces (which are also implemented by DataInputStream and DataOutputStream). It doesn’t even use any of the functionality of the existing InputStream or OutputStream classes – it’s a completely separate class, written from scratch, with all of its own (mostly native) methods. The reason for this may be that RandomAccessFile has essentially different behavior than the other IO types, since you can move forward and backward within a file. In any event, it stands alone, as a direct descendant of Object.

Essentially, a RandomAccessFile works like a DataInputStream pasted together with a DataOutputStream and the methods getFilePointer( ) to find out where you are in the file, seek( ) to move to a new point in the file, and length( ) to determine the maximum size of the file. In addition, the constructors require a second argument (identical to fopen( ) in C) indicating whether you are just randomly reading (“r”) or reading and writing (“rw”). There’s no support for write-only files, which could suggest that RandomAccessFile might have worked well if it were inherited from DataInputStream.

What’s even more frustrating is that you could easily imagine wanting to seek within other types of streams, such as a ByteArrayInputStream, but the seeking methods are available only in RandomAccessFile, which works for files only. BufferedInputStream does allow you to mark( ) a position (whose value is held in a single internal variable) and reset( ) to that position, but this is limited and not too useful.

The File class has a deceiving name – you might think it refers to a file, but it doesn’t. It can represent either the name of a particular file or the names of a set of files in a directory. If it’s a set of files, you can ask for the set with the list( ) method, and this returns an array of String. It makes sense to return an array rather than one of the flexible collection classes because the number of elements is fixed, and if you want a different directory listing you just create a different File object. In fact, “FilePath” would have been a better name. This section shows a complete example of the use of this class, including the associated FilenameFilter interface.

Suppose you’d like to see a directory listing. The File object can be listed in two ways. If you call list( ) with no arguments, you’ll get the full list that the File object contains. However, if you want a restricted list, for example, all of the files with an extension of .java, then you use a “directory filter,” which is a class that tells how to select the File objects for display.

Here’s the code for the example:

The DirFilter class “implements” the interface FilenameFilter. (Interfaces were covered in Chapter 7.) It’s useful to see how simple the FilenameFilter interface is:

It says that all that this type of object does is provide a method called accept( ). The whole reason behind the creation of this class is to provide the accept( ) method to the list( ) method so that list( ) can call back accept( ) to determine which file names should be included in the list. Thus, this technique is often referred to as a callback or sometimes a functor (that is, DirFilter is a functor because its only job is to hold a method). Because list( ) takes a FilenameFilter object as its argument, it means that you can pass an object of any class that implements FilenameFilter to choose (even at run-time) how the list( ) method will behave. The purpose of a callback is to provide flexibility in the behavior of code.

DirFilter shows that just because an interface contains only a set of methods, you’re not restricted to writing only those methods. (You must at least provide definitions for all the methods in an interface, however.) In this case, the DirFilter constructor is also created.

The accept( ) method must accept a File object representing the directory that a particular file is found in, and a String containing the name of that file. You might choose to use or ignore either of these arguments, but you will probably at least use the file name. Remember that the list( ) method is calling accept( ) for each of the file names in the directory object to see which one should be included – this is indicated by the boolean result returned by accept( ).

To make sure that what you’re working with is only the name and contains no path information, all you have to do is take the String object and create a File object out of it, then call getName( ) which strips away all the path information (in a platform-independent way). Then accept( ) uses the String class indexOf( ) method to see if the search string afn appears anywhere in the name of the file. If afn is found within the string, the return value is the starting index of afn, but if it’s not found the return value is -1. Keep in mind that this is a simple string search and does not have regular expression “wildcard” matching such as “fo?.b?r*” which is much more difficult to implement.

The list( ) method returns an array. You can query this array for its length and then move through it selecting the array elements. This ability to easily pass an array in and out of a method is a tremendous improvement over the behavior of C and C++.

This example is ideal for rewriting using an anonymous inner class (described in Chapter 7). As a first cut, a method filter( ) is created that returns a handle to a FilenameFilter:

Note that the argument to filter( ) must be final. This is required by the anonymous inner class so that it can use an object from outside its scope.

This design is an improvement because the FilenameFilter class is now tightly bound to DirList2. However, you can take this approach one step further and define the anonymous inner class as an argument to list( ), in which case it’s even smaller:

The argument to main( ) is now final, since the anonymous inner class uses args[0] directly.

This shows you how anonymous inner classes allow the creation of quick-and-dirty classes to solve problems. Since everything in Java revolves around classes, this can be a useful coding technique. One benefit is that it keeps the code that solves a particular problem isolated together in one spot. On the other hand, it is not always as easy to read, so you must use it judiciously.

Ah, you say that you want the file names sorted? Since there’s no support for sorting in Java 1.0 or Java 1.1 (although sorting is included in Java 1.2), it will have to be added into the program directly using the SortVector created in Chapter 8:

A few other improvements have been made. Instead of creating path and list as local variables to main( ), they are members of the class so their values can be accessible for the lifetime of the object. In fact, main( ) is now just a way to test the class. You can see that the constructor of the class automatically sorts the list once that list has been created.

The sort is case-insensitive so you don’t end up with a list of all the words starting with capital letters, followed by the rest of the words starting with all the lowercase letters. However, you’ll notice that within a group of file names that begin with the same letter the capitalized words are listed first, which is still not quite the desired behavior for the sort. This problem will be fixed in Java 1.2.

The File class is more than just a representation for an existing directory path, file, or group of files. You can also use a File object to create a new directory or an entire directory path if it doesn’t exist. You can also look at the characteristics of files (size, last modification date, read/write), see whether a File object represents a file or a directory, and delete a file. This program shows the remaining methods available with the File class:

In fileData( ) you can see the various file investigation methods put to use to display information about the file or directory path.

The first method that’s exercised by main( ) is renameTo( ), which allows you to rename (or move) a file to an entirely new path represented by the argument, which is another File object. This also works with directories of any length.

If you experiment with the above program, you’ll find that you can make a directory path of any complexity because mkdirs( ) will do all the work for you. In Java 1.0, the -d flag reports that the directory is deleted but it’s still there; in Java 1.1 the directory is actually deleted.

Although there are a lot of IO stream classes in the library that can be combined in many different ways, there are just a few ways that you’ll probably end up using them. However, they require attention to get the correct combinations. The following rather long example shows the creation and use of typical IO configurations so you can use it as a reference when writing your own code. Note that each configuration begins with a commented number and title that corresponds to the heading for the appropriate explanation that follows in the text.

Of course, one common thing you’ll want to do is print formatted output to the console, but that’s already been simplified in the package com.bruceeckel.tools created in Chapter 5.

Parts 1 through 4 demonstrate the creation and use of input streams (although part 4 also shows the simple use of an output stream as a testing tool).

To open a file for input, you use a FileInputStream with a String or a File object as the file name. For speed, you’ll want that file to be buffered so you give the resulting handle to the constructor for a BufferedInputStream. To read input in a formatted fashion, you give that resulting handle to the constructor for a DataInputStream, which is your final object and the interface you read from.

In this example, only the readLine( ) method is used, but of course any of the DataInputStream methods are available. When you reach the end of the file, readLine( ) returns null so that is used to break out of the while loop.

The String s2 is used to accumulate the entire contents of the file (including newlines that must be added since readLine( ) strips them off). s2 is then used in the later portions of this program. Finally, close( ) is called to close the file. Technically, close( ) will be called when finalize( ) is run, and this is supposed to happen (whether or not garbage collection occurs) as the program exits. However, Java 1.0 has a rather important bug, so this doesn’t happen. In Java 1.1 you must explicitly call System.runFinalizersOnExit(true) to guarantee that finalize( ) will be called for every object in the system. The safest approach is to explicitly call close( ) for files.

This piece takes the String s2 that now contains the entire contents of the file and uses it to create a StringBufferInputStream. (A String, not a StringBuffer, is required as the constructor argument.) Then read( ) is used to read each character one at a time and send it out to the console. Note that read( ) returns the next byte as an int and thus it must be cast to a char to print properly.

The interface for StringBufferInputStream is limited, so you usually enhance it by wrapping it inside a DataInputStream. However, if you choose to read the characters out a byte at a time using readByte( ), any value is valid so the return value cannot be used to detect the end of input. Instead, you can use the available( ) method to find out how many more characters are available. Here’s an example that shows how to read a file one byte at a time:

Note that available( ) works differently depending on what sort of medium you’re reading from – it’s literally “the number of bytes that can be read without blocking.” With a file this means the whole file, but with a different kind of stream this might not be true, so use it thoughtfully.

You could also detect the end of input in cases like these by catching an exception. However, the use of exceptions for control flow is considered a misuse of that feature.

This example shows the use of the LineNumberInputStream to keep track of the input line numbers. Here, you cannot simply gang all the constructors together, since you have to keep a handle to the LineNumberInputStream. (Note that this is not an inheritance situation, so you cannot simply cast in4 to a LineNumberInputStream.) Thus, li holds the handle to the LineNumberInputStream, which is then used to create a DataInputStream for easy reading.

This example also shows how to write formatted data to a file. First, a FileOutputStream is created to connect to the file. For efficiency, this is made a BufferedOutputStream, which is what you’ll virtually always want to do, but you’re forced to do it explicitly. Then for the formatting it’s turned into a PrintStream. The data file created this way is readable as an ordinary text file.

One of the methods that indicates when a DataInputStream is exhausted is readLine( ), which returns null when there are no more strings to read. Each line is printed to the file along with its line number, which is acquired through li.

You’ll see an explicit close( ) for out1, which would make sense if the program were to turn around and read the same file again. However, this program ends without ever looking at the file IODemo.out. As mentioned before, if you don’t call close( ) for all your output files, you might discover that the buffers don’t get flushed so they’re incomplete.

The two primary kinds of output streams are separated by the way they write data: one writes it for human consumption, and the other writes it to be re-acquired by a DataInputStream. The RandomAccessFile stands alone, although its data format is compatible with the DataInputStream and DataOutputStream.

A PrintStream formats data so it’s readable by a human. To output data so that it can be recovered by another stream, you use a DataOutputStream to write the data and a DataInputStream to recover the data. Of course, these streams could be anything, but here a file is used, buffered for both reading and writing.

Note that the character string is written using writeBytes( ) and not writeChars( ). If you use the latter, you’ll be writing the 16-bit Unicode characters. Since there is no complementary “readChars” method in DataInputStream, you’re stuck pulling these characters off one at a time with readChar( ). So for ASCII, it’s easier to write the characters as bytes followed by a newline; then use readLine( ) to read back the bytes as a regular ASCII line.

The writeDouble( ) stores the double number to the stream and the complementary readDouble( ) recovers it. But for any of the reading methods to work correctly, you must know the exact placement of the data item in the stream, since it would be equally possible to read the stored double as a simple sequence of bytes, or as a char, etc. So you must either have a fixed format for the data in the file or extra information must be stored in the file that you parse to determine where the data is located.

As previously noted, the RandomAccessFile is almost totally isolated from the rest of the IO hierarchy, save for the fact that it implements the DataInput and DataOutput interfaces. So you cannot combine it with any of the aspects of the InputStream and OutputStream subclasses. Even though it might make sense to treat a ByteArrayInputStream as a random access element, you can use RandomAccessFile to only open a file. You must assume a RandomAccessFile is properly buffered since you cannot add that.

The one option you have is in the second constructor argument: you can open a RandomAccessFile to read (“r”) or read and write (“rw”).

Using a RandomAccessFile is like using a combined DataInputStream and DataOutputStream (because it implements the equivalent interfaces). In addition, you can see that seek( ) is used to move about in the file and change one of the values.

Since there are certain canonical forms that you’ll be using regularly with files, you may wonder why you have to do all of that typing – this is one of the drawbacks of the decorator pattern. This portion shows the creation and use of shorthand versions of typical file reading and writing configurations. These shorthands are placed in the package com.bruceeckel.tools that was begun in Chapter 5 (See page 196). To add each class to the library, simply place it in the appropriate directory and add the package statement.

The creation of an object that reads a file from a buffered DataInputStream can be encapsulated into a class called InFile:

Both the String versions of the constructor and the File versions are included, to parallel the creation of a FileInputStream.

Now you can reduce your chances of repetitive stress syndrome while creating files, as seen in the example.

The same kind of approach can be taken to create a PrintStream that writes to a buffered file. Here’s the extension to com.bruceeckel.tools:

Note that it is not possible for a constructor to catch an exception that’s thrown by a base-class constructor.

Finally, the same kind of shorthand can create a buffered output file for data storage (as opposed to human-readable storage):

It is curious (and unfortunate) that the Java library designers didn’t think to provide these conveniences as part of their standard.

Following the approach pioneered in Unix of “standard input,” “standard output,” and “standard error output,” Java has System.in, System.out, and System.err. Throughout the book you’ve seen how to write to standard output using System.out, which is already pre-wrapped as a PrintStream object. System.err is likewise a PrintStream, but System.in is a raw InputStream, with no wrapping. This means that while you can use System.out and System.err right away, System.in must be wrapped before you can read from it.

Typically, you’ll want to read input a line at a time using readLine( ), so you’ll want to wrap System.in in a DataInputStream. This is the “old” Java 1.0 way to do line input. A bit later in the chapter you’ll see the Java 1.1 solution. Here’s an example that simply echoes each line that you type in:

The reason for the try block is that readLine( ) can throw an IOException. Note that System.in should also be buffered, as with most streams

It’s a bit inconvenient that you’re forced to wrap System.in in a DataInputStream in each program, but perhaps it was designed this way to allow maximum flexibility.

The PipedInputStream and PipedOutputStream have been mentioned only briefly in this chapter. This is not to suggest that they aren’t useful, but their value is not apparent until you begin to understand multithreading, since the piped streams are used to communicate between threads. This is covered along with an example in Chapter 14.

Although StreamTokenizer is not derived from InputStream or OutputStream, it works only with InputStream objects, so it rightfully belongs in the IO portion of the library.

The StreamTokenizer class is used to break any InputStream into a sequence of “tokens,” which are bits of text delimited by whatever you choose. For example, your tokens could be words, and then they would be delimited by white space and punctuation.

Consider a program to count the occurrence of words in a text file:

It makes sense to present these in a sorted form, but since Java 1.0 and Java 1.1 don’t have any sorting methods, that will have to be mixed in. This is easy enough to do with a StrSortVector. (This was created in Chapter 8, and is part of the package created in that chapter. Remember that the starting directory for all the subdirectories in this book must be in your class path for the program to compile successfully.)

To open the file, a FileInputStream is used, and to turn the file into words a StreamTokenizer is created from the FileInputStream. In StreamTokenizer, there is a default list of separators, and you can add more with a set of methods. Here, ordinaryChar( ) is used to say “This character has no significance that I’m interested in,” so the parser doesn’t include it as part of any of the words that it creates. For example, saying st.ordinaryChar('.') means that periods will not be included as parts of the words that are parsed. You can find more information in the online documentation that comes with Java.

In countWords( ), the tokens are pulled one at a time from the stream, and the ttype information is used to determine what to do with each token, since a token can be an end-of-line, a number, a string, or a single character.

Once a token is found, the Hashtable counts is queried to see if it already contains the token as a key. If it does, the corresponding Counter object is incremented to indicate that another instance of this word has been found. If not, a new Counter is created – since the Counter constructor initializes its value to one, this also acts to count the word.

SortedWordCount is not a type of Hashtable, so it wasn’t inherited. It performs a specific type of functionality, so even though the keys( ) and values( ) methods must be re-exposed, that still doesn’t mean that inheritance should be used since a number of Hashtable methods are inappropriate here. In addition, other methods like getCounter( ), which get the Counter for a particular String, and sortedKeys( ), which produces an Enumeration, finish the change in the shape of SortedWordCount’s interface.

In main( ) you can see the use of a SortedWordCount to open and count the words in a file – it just takes two lines of code. Then an enumeration to a sorted list of keys (words) is extracted, and this is used to pull out each key and associated Count. Note that the call to cleanup( ) is necessary to ensure that the file is closed.

A second example using StreamTokenizer can be found in Chapter 17.

Although it isn’t part of the IO library, the StringTokenizer has sufficiently similar functionality to StreamTokenizer that it will be described here.

The StringTokenizer returns the tokens within a string one at a time. These tokens are consecutive characters delimited by tabs, spaces, and newlines. Thus, the tokens of the string “Where is my cat?” are “Where”, “is”, “my”, and “cat?” Like the StreamTokenizer, you can tell the StringTokenizer to break up the input in any way that you want, but with StringTokenizer you do this by passing a second argument to the constructor, which is a String of the delimiters you wish to use. In general, if you need more sophistication, use a StreamTokenizer.

You ask a StringTokenizer object for the next token in the string using the nextToken( ) method, which either returns the token or an empty string to indicate that no tokens remain.

As an example, the following program performs a limited analysis of a sentence, looking for key phrase sequences to indicate whether happiness or sadness is implied.

For each string being analyzed, a while loop is entered and tokens are pulled off the string. Notice the first if statement, which says to continue (go back to the beginning of the loop and start again) if the token is neither an “I” nor an “Are.” This means that it will get tokens until an “I” or an “Are” is found. You might think to use the == instead of the equals( ) method, but that won’t work correctly, since == compares handle values while equals( ) compares contents.

The logic of the rest of the analyze( ) method is that the pattern that’s being searched for is “I am sad,” “I am not happy,” or “Are you sad?” Without the break statement, the code for this would be even messier than it is. You should be aware that a typical parser (this is a primitive example of one) normally has a table of these tokens and a piece of code that moves through the states in the table as new tokens are read.

You should think of the StringTokenizer only as shorthand for a simple and specific kind of StreamTokenizer. However, if you have a String that you want to tokenize and StringTokenizer is too limited, all you have to do is turn it into a stream with StringBufferInputStream and then use that to create a much more powerful StreamTokenizer.

At this point you might be scratching your head, wondering if there is another design for IO streams that could require more typing. Could someone have come up with an odder design?” Prepare yourself: Java 1.1 makes some significant modifications to the IO stream library. When you see the Reader and Writer classes your first thought (like mine) might be that these were meant to replace the InputStream and OutputStream classes. But that’s not the case. Although some aspects of the original streams library are deprecated (if you use them you will receive a warning from the compiler), the old streams have been left in for backwards compatibility and:

As a result there are situations in which you have more layers of wrapping with the new IO stream library than with the old. Again, this is a drawback of the decorator pattern – the price you pay for added flexibility.

The most important reason for adding the Reader and Writer hierarchies in Java 1.1 is for internationalization. The old IO stream hierarchy supports only 8-bit byte streams and doesn’t handle the 16-bit Unicode characters well. Since Unicode is used for internationalization (and Java’s native char is 16-bit Unicode), the Reader and Writer hierarchies were added to support Unicode in all IO operations. In addition, the new libraries are designed for faster operations than the old.

As is the practice in this book, I will attempt to provide an overview of the classes but assume that you will use online documentation to determine all the details, such as the exhaustive list of methods.

Almost all of the Java 1.0 IO stream classes have corresponding Java 1.1 classes to provide native Unicode manipulation. It would be easiest to say “Always use the new classes, never use the old ones,” but things are not that simple. Sometimes you are forced into using the Java 1.0 IO stream classes because of the library design; in particular, the java.util.zip libraries are new additions to the old stream library and they rely on old stream components. So the most sensible approach to take is to try to use the Reader and Writer classes whenever you can, and you’ll discover the situations when you have to drop back into the old libraries because your code won’t compile.

Here is a table that shows the correspondence between the sources and sinks of information (that is, where the data physically comes from or goes to) in the old and new libraries.

Sources & Sinks: Java 1.0 class	Corresponding Java 1.1 class
InputStream	Reader converter: InputStreamReader
OutputStream	Writer converter: OutputStreamWriter
FileInputStream	FileReader
FileOutputStream	FileWriter
StringBufferInputStream	StringReader
(no corresponding class)	StringWriter
ByteArrayInputStream	CharArrayReader
ByteArrayOutputStream	CharArrayWriter
PipedInputStream	PipedReader
PipedOutputStream	PipedWriter

In general, you’ll find that the interfaces in the old library components and the new ones are similar if not identical.

In Java 1.0, streams were adapted for particular needs using “decorator” subclasses of FilterInputStream and FilterOutputStream. Java 1.1 IO streams continues the use of this idea, but the model of deriving all of the decorators from the same “filter” base class is not followed. This can make it a bit confusing if you’re trying to understand it by looking at the class hierarchy.

In the following table, the correspondence is a rougher approximation than in the previous table. The difference is because of the class organization: while BufferedOutputStream is a subclass of FilterOutputStream, BufferedWriter is not a subclass of FilterWriter (which, even though it is abstract, has no subclasses and so appears to have been put in either as a placeholder or simply so you wouldn’t wonder where it was). However, the interfaces to the classes are quite a close match and it’s apparent that you’re supposed to use the new versions instead of the old whenever possible (that is, except in cases where you’re forced to produce a Stream instead of a Reader or Writer).

Filters: Java 1.0 class	Corresponding Java 1.1 class
FilterInputStream	FilterReader
FilterOutputStream	FilterWriter (abstract class with no subclasses)
BufferedInputStream	BufferedReader (also has readLine( ))
BufferedOutputStream	BufferedWriter
DataInputStream	use DataInputStream (Except when you need to use readLine( ), when you should use a BufferedReader)
PrintStream	PrintWriter
LineNumberInputStream	LineNumberReader
StreamTokenizer	StreamTokenizer (use constructor that takes a Reader instead)
PushBackInputStream	PushBackReader

There’s one direction that’s quite clear: Whenever you want to use readLine( ), you shouldn’t do it with a DataInputStream any more (this is met with a deprecation message at compile time), but instead use a BufferedReader. Other than this, DataInputStream is still a “preferred” member of the Java 1.1 IO library.

To make the transition to using a PrintWriter easier, it has constructors that take any OutputStream object. However, PrintWriter has no more support for formatting than PrintStream does; the interfaces are virtually the same.

Apparently, the Java library designers felt that they got some of the classes right the first time so there were no changes to these and you can go on using them as they are:

Java 1.0 classes without corresponding Java 1.1 classes
DataOutputStream
File
RandomAccessFile
SequenceInputStream

The DataOutputStream, in particular, is used without change, so for storing and retrieving data in a transportable format you’re forced to stay in the InputStream and OutputStream hierarchies.

To see the effect of the new classes, let’s look at the appropriate portion of the IOStreamDemo.java example modified to use the Reader and Writer classes:

In general, you’ll see that the conversion is fairly straightforward and the code looks quite similar. There are some important differences, though. First of all, since random access files have not changed, section 6 is not repeated.

Section 1 shrinks a bit because if all you’re doing is reading line input you need only to wrap a BufferedReader around a FileReader. Section 1b shows the new way to wrap System.in for reading console input, and this expands because System.in is a DataInputStream and BufferedReader needs a Reader argument, so InputStreamReader is brought in to perform the translation.

In section 2 you can see that if you have a String and want to read from it you just use a StringReader instead of a StringBufferInputStream and the rest of the code is identical.

Section 3 shows a bug in the design of the new IO stream library. If you have a String and you want to read from it, you’re not supposed to use a StringBufferInputStream any more. When you compile code involving a StringBufferInputStream constructor, you get a deprecation message telling you to not use it. Instead, you’re supposed to use a StringReader. However, if you want to do formatted memory input as in section 3, you’re forced to use a DataInputStream – there is no “DataReader” to replace it – and a DataInputStream constructor requires an InputStream argument. So you have no choice but to use the deprecated StringBufferInputStream class. The compiler will give you a deprecation message but there’s nothing you can do about it.[45]

Section 4 is a reasonably straightforward translation from the old streams to the new, with no surprises. In section 5, you’re forced to use all the old streams classes because DataOutputStream and DataInputStream require them and there are no alternatives. However, you don’t get any deprecation messages at compile time. If a stream is deprecated, typically its constructor produces a deprecation message to prevent you from using the entire class, but in the case of DataInputStream only the readLine( ) method is deprecated since you’re supposed to use a BufferedReader for readLine( ) (but a DataInputStream for all other formatted input).

If you compare section 5 with that section in IOStreamDemo.java, you’ll notice that in this version, the data is written before the text. That’s because a bug was introduced in Java 1.1, which is shown in the following code: