Java核心技术卷II：第9版高级特性概览

Java核心技术

高级特性

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Figure 1.2. Reader and writer hierarchy

abstract int read()

abstract void write(int c)

The read method returns either a Unicode code unit (as an integer between 0 and 65535) or -1 when you have reached the end of the file. The

write method is called with a Unicode code unit. (See Volume I, Chapter 3 for a discussion of Unicode code units.)

There are four additional interfaces: Closeable, Flushable, Readable, and Appendable (see Figure 1.3). The first two interfaces are very

simple, with methods

void close() throws IOException

and

void flush()

Figure 1.3. The Closeable, Flushable, Readable, and Appendable interfaces

respectively. The classes InputStream, OutputStream, Reader, and Writer all implement the Closeable interface.

Note

The java.io.Closeable interface extends the java.lang.AutoCloseable interface. Therefore, you can use the try-with-

resources statement with any Closeable. Why have two interfaces? The close method of the Closeable interface only throws an

IOException, whereas the AutoCloseable.close method may throw any exception.

OutputStream and Writer implement the Flushable interface.

The Readable interface has a single method

int read(CharBuffer cb)

The CharBuffer class has methods for sequential and random read/write access. It represents an in-memory buffer or a memory-mapped file.

(See Section 1.7.1, “The Buffer Data Structure,” on p. 77 for details.)

The Appendable interface has two methods for appending single characters and character sequences:

Appendable append(char c)

Appendable append(CharSequence s)

The CharSequence interface describes basic properties of a sequence of char values. It is implemented by String, CharBuffer,

StringBuilder, and StringBuffer.

Of the stream zoo classes, only Writer implements Appendable.

java.io.Closeable

5.0

• void close()

closes this Closeable. This method may throw an IOException.

java.io.Flushable

5.0

• void flush()

flushes this Flushable.

java.lang.Readable

5.0

• int read(CharBuffer cb)

attempts to read as many char values into cb as it can hold. Returns the number of values read, or -1 if no further values are available

from this Readable.

java.lang.Appendable

5.0

• Appendable append(char c)

• Appendable append(CharSequence cs)

appends the given code unit, or all code units in the given sequence, to this Appendable; returns this.

java.lang.CharSequence

1.4

• char charAt(int index)

returns the code unit at the given index.

• int length()

returns the number of code units in this sequence.

• CharSequence subSequence(int startIndex, int endIndex)

returns a CharSequence consisting of the code units stored at index startIndex to endIndex - 1.

• String toString()

returns a string consisting of the code units of this sequence.

1.1.3. Combining Stream Filters

FileInputStream and FileOutputStream give you input and output streams attached to a disk file. You need to pass the file name or full

path name of the file to the constructor. For example,

FileInputStream fin = new FileInputStream("employee.dat");

looks in the user directory for a file named employee.dat.

Tip

All the classes in java.io interpret relative path names as starting from the user’s working directory. You can get this directory by a call

to System.getProperty("user.dir").

Caution

Since the backslash character is the escape character in Java strings, be sure to use \\ for Windows-style path names (for example,

C:\\Windows\\win.ini). In Windows, you can also use a single forward slash (C:/Windows/win.ini) because most Windows

file-handling system calls will interpret forward slashes as file separators. However, this is not recommended—the behavior of the

Windows system functions is subject to change. Instead, for portable programs, use the file separator character for the platform on which

your program runs. It is available as the constant string java.io.File.separator.

Like the abstract InputStream and OutputStream classes, these classes support only reading and writing at the byte level. That is, we can

only read bytes and byte arrays from the object fin.

byte b = (byte) fin.read();

As you will see in the next section, if we just had a DataInputStream, we could read numeric types:

DataInputStream din = . . .;

double s = din.readDouble();

But just as the FileInputStream has no methods to read numeric types, the DataInputStream has no method to get data from a file.

Java uses a clever mechanism to separate two kinds of responsibilities. Some streams (such as the FileInputStream and the input stream

returned by the openStream method of the URL class) can retrieve bytes from files and other more exotic locations. Other streams (such as the

DataInputStream and the PrintWriter) can assemble bytes into more useful data types. The Java programmer has to combine the two.

For example, to be able to read numbers from a file, first create a FileInputStream and then pass it to the constructor of a

DataInputStream.

Click here to view code image

FileInputStream fin = new FileInputStream("employee.dat");

DataInputStream din = new DataInputStream(fin);

double s = din.readDouble();

If you look at Figure 1.1 again, you can see the classes FilterInputStream and FilterOutputStream. The subclasses of these classes

are used to add capabilities to raw byte streams.

You can add multiple capabilities by nesting the filters. For example, by default streams are not buffered. That is, every call to read asks the

operating system to dole out yet another byte. It is more efficient to request blocks of data instead and store them in a buffer. If you want buffering

and the data input methods for a file, you need to use the following rather monstrous sequence of constructors:

DataInputStream din = new DataInputStream(

new BufferedInputStream(

new FileInputStream("employee.dat")));

Notice that we put the DataInputStream last in the chain of constructors because we want to use the DataInputStream methods, and we

want them to use the buffered read method.

Sometimes you’ll need to keep track of the intermediate streams when chaining them together. For example, when reading input, you often need

to peek at the next byte to see if it is the value that you expect. Java provides the PushbackInputStream for this purpose.

Click here to view code image

PushbackInputStream pbin = new PushbackInputStream(

new BufferedInputStream(

new FileInputStream("employee.dat")));

Now you can speculatively read the next byte

int b = pbin.read();

and throw it back if it isn’t what you wanted.

if (b != '<') pbin.unread(b);

However, reading and unreading are the only methods that apply to a pushback input stream. If you want to look ahead and also read numbers,

then you need both a pushback input stream and a data input stream reference.

DataInputStream din = new DataInputStream(

pbin = new PushbackInputStream(

new BufferedInputStream(

new FileInputStream("employee.dat"))));

Of course, in the stream libraries of other programming languages, niceties such as buffering and lookahead are automatically taken care of, so it

is a bit of a hassle to resort, in Java, to combining stream filters. However, the ability to mix and match filter classes to construct truly useful

sequences of streams does give you an immense amount of flexibility. For example, you can read numbers from a compressed ZIP file by using

the following sequence of streams (see Figure 1.4):

Click here to view code image

ZipInputStream zin = new ZipInputStream(new FileInputStream("employee.zip"));

DataInputStream din = new DataInputStream(zin);

Figure 1.4. A sequence of filtered streams

(See Section 1.4, “ZIP Archives,” on p. 33 for more on Java’s handling of ZIP files.)

java.io.FileInputStream 1.0

• FileInputStream(String name)

• FileInputStream(File file)

creates a new file input stream using the file whose path name is specified by the name string or the file object. (The File class is

described at the end of this chapter.) Path names that are not absolute are resolved relative to the working directory that was set when

the VM started.

java.io.FileOutputStream 1.0

• FileOutputStream(String name)

• FileOutputStream(String name, boolean append)

• FileOutputStream(File file)

• FileOutputStream(File file, boolean append)

creates a new file output stream specified by the name string or the file object. (The File class is described at the end of this

chapter.) If the append parameter is true, an existing file with the same name will not be deleted and data will be added at the end of

the file. Otherwise, this method deletes any existing file with the same name.

java.io.BufferedInputStream 1.0

• BufferedInputStream(InputStream in)

creates a buffered stream. A buffered input stream reads bytes from a stream without causing a device access every time. When the

buffer is empty, a new block of data is read into the buffer.

java.io.BufferedOutputStream 1.0

• BufferedOutputStream(OutputStream out)

creates a buffered stream. A buffered output stream collects bytes to be written without causing a device access every time. When the

buffer fills up or when the stream is flushed, the data are written.

java.io.PushbackInputStream 1.0

• PushbackInputStream(InputStream in)

• PushbackInputStream(InputStream in, int size)

constructs a stream with one-byte lookahead or a pushback buffer of specified size.

• void unread(int b)

pushes back a byte, which is retrieved again by the next call to read.

Parameters: b The byte to be read again

1.2. Text Input and Output

When saving data, you have the choice between binary and text formats. For example, if the integer 1234 is saved in binary, it is written as the

sequence of bytes 00 00 04 D2 (in hexadecimal notation). In text format, it is saved as the string "1234". Although binary I/O is fast and

efficient, it is not easily readable by humans. We first discuss text I/O and cover binary I/O in Section 1.3, “Reading and Writing Binary Data,” on p.

25.

When saving text strings, you need to consider the character encoding. In the UTF-16 encoding, the string "1234" is encoded as 00 31 00

When saving text strings, you need to consider the . In the UTF-16 encoding, the string "1234" is encoded as 00 31 00

32 00 33 00 34 (in hex). However, many programs expect that text files are encoded in a different encoding. In ISO 8859-1, the encoding

most commonly used in the United States and Western Europe, the string would be written as 31 32 33 34, without the zero bytes.

The OutputStreamWriter class turns a stream of Unicode code units into a stream of bytes, using a chosen character encoding. Conversely,

the InputStreamReader class turns an input stream that contains bytes (specifying characters in some character encoding) into a reader that

emits Unicode code units.

For example, here is how you make an input reader that reads keystrokes from the console and converts them to Unicode:

InputStreamReader in = new InputStreamReader(System.in);

This input stream reader assumes the default character encoding used by the host system, such as the ISO 8859-1 encoding in Western Europe.

You can choose a different encoding by specifying it in the constructor for the InputStreamReader, for example:

InputStreamReader in = new InputStreamReader(new FileInputStream("kremlin.dat"), "ISO8859_5");

See Section 1.2.4, “Character Sets,” on p. 20 for more information on character encodings.

1.2.1. How to Write Text Output

For text output, use a PrintWriter. That class has methods to print strings and numbers in text format. There is even a convenience constructor

to link a PrintWriter to a FileWriter. The statement

PrintWriter out = new PrintWriter("employee.txt");

is equivalent to

PrintWriter out = new PrintWriter(new FileWriter("employee.txt"));

To write to a print writer, use the same print, println, and printf methods that you used with System.out. You can use these methods to

print numbers (int, short, long, float, double), characters, boolean values, strings, and objects.

For example, consider this code:

String name = "Harry Hacker";

double salary = 75000;

out.print(name);

out.print(' ');

out.println(salary);

This writes the characters

Harry Hacker 75000.0

to the writer out. The characters are then converted to bytes and end up in the file employee.txt.

The println method adds the correct end-of-line character for the target system ("\r\n" on Windows, "\n" on UNIX) to the line. This is the

string obtained by the call System.getProperty("line.separator").

If the writer is set to autoflush mode, all characters in the buffer are sent to their destination whenever println is called. (Print writers are always

buffered.) By default, autoflushing is not enabled. You can enable or disable autoflushing by using the PrintWriter(Writer out,

boolean autoFlush) constructor:

PrintWriter out = new PrintWriter(new FileWriter("employee.txt"), true); // autoflush

The print methods don’t throw exceptions. You can call the checkError method to see if something went wrong with the stream.

Note

Java veterans might wonder whatever happened to the PrintStream class and to System.out. In Java 1.0, the PrintStream class

simply truncated all Unicode characters to ASCII characters by dropping the top byte. (At the time, Unicode was still a 16-bit encoding.)

Clearly, that was not a clean or portable approach, and it was fixed with the introduction of readers and writers in Java 1.1. For

compatibility with existing code, System.in, System.out, and System.err are still streams, not readers and writers. But now the

PrintStream class internally converts Unicode characters to the default host encoding in the same way as the PrintWriter does.

Objects of type PrintStream act exactly like print writers when you use the print and println methods, but unlike print writers they

allow you to output raw bytes with the write(int) and write(byte[]) methods.

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