Transcript File

Chapter 16 Simple Input and Output
Prerequisites for Part IV
Chapter 8 Inheritance and Polymorphism
Chapter 15 Exceptions and Assertions
Chapter 16 Simple Input and Output
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Objectives
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To discover file properties, delete and rename files using the File class (§16.2).
To understand how I/O is processed in Java (§16.3).
To distinguish between text I/O and binary I/O (§16.3).
To read and write characters using FileReader and FileWriter (§16.4).
To improve the performance of text I/O using BufferedReader and
BufferedWriter (§16.4).
To write primitive values, strings, and objects as text using PrintWriter and
PrintStream (§16.4).
To read and write bytes using FileInputStream and FileOutputStream (§16.6).
To read and write primitive values and strings using
DataInputStream/DataOutputStream (§16.6).
To store and restore objects using ObjectOutputStream and ObjectInputStream,
and to understand how objects are serialized and what kind of objects can be
serialized (§16.9 Optional).
To use the Serializable interface to enable objects to be serializable (§16.9
Optional).
To use RandomAccessFile for both read and write. (§16.10 Optional)
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The File Class
The File class is intended to provide an abstraction that
deals with most of the machine-dependent complexities
of files and path names in a machine-independent
fashion. The filename is a string. The File class is a
wrapper class for the file name and its directory path.
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Obtaining file
properties and
manipulating
file
java.io.File
+File(pathname: String)
Creates a File object for the specified pathname. The pathname may be a
directory or a file.
+File(parent: String, child: String) Creates a File object for the child under the directory parent. child may be a
filename or a subdirectory.
+File(parent: File, child: String) Creates a File object for the child under the directory parent. parent is a File
object. In the preceding constructor, the parent is a string.
+exists(): boolean
Returns true if the file or the directory represented by the File object exists.
+canRead(): boolean
Returns true if the file represented by the File object exists and can be read.
+canWrite(): boolean
Returns true if the file represented by the File object exists and can be written.
+isDirectory(): boolean
Returns true if the File object represents a directory.
+isFile(): boolean
Returns true if the File object represents a file.
+isAbsolute(): boolean
Returns true if the File object is created using an absolute path name.
+isHidden(): boolean
Returns true if the file represented in the File object is hidden. The exact
definition of hidden is system-dependent. On Windows, you can mark a file
hidden in the File Properties dialog box. On Unix systems, a file is hidden if
its name begins with a period character '.'.
+getAbsolutePath(): String
Returns the complete absolute file or directory name represented by the File
object.
+getCanonicalPath(): String
Returns the same as getAbsolutePath() except that it removes redundant
names, such as "." and "..", from the pathname, resolves symbolic links (on
Unix platforms), and converts drive letters to standard uppercase (on Win32
platforms).
+getName(): String
Returns the last name of the complete directory and file name represented by
the File object. For example, new File("c:\\book\\test.dat").getName() returns
test.dat.
+getPath(): String
Returns the complete directory and file name represented by the File object.
For example, new File("c:\\book\\test.dat").getPath() returns c:\book\test.dat.
+getParent(): String
Returns the complete parent directory of the current directory or the file
represented by the File object. For example, new
File("c:\\book\\test.dat").getParent() returns c:\book.
+lastModified(): long
Returns the time that the file was last modified.
+delete(): boolean
Deletes this file. The method returns true if the deletion succeeds.
+renameTo(dest: File): Boolean
Renames this file. The method returns true if the operation succeeds.
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Example 16.1 Using the File Class
Objective: Write a program that demonstrates how to
create files in a platform-independent way and use the
methods in the File class to obtain their properties. Figure
16.1 shows a sample run of the program on Windows, and
Figure 16.2 a sample run on Unix.
TestFileClass
Run
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How is I/O Handled in Java?
A File object encapsulates the properties of a file or a path, but does not
contain the methods for reading/writing data from/to a file. In order to
perform I/O, you need to create objects using appropriate Java I/O classes.
FileReader input = new FileReader("temp.txt");
int code = input.read();
System.out.println((char)code);
Program
Input stream
Input object
created from an
input class
File
Output object
created from an
output class
File
Output stream
FileWriter output = new FileWriter("temp.txt");
output.write("Java 101");
output.close();
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Coding Essentials
Declaring exception in the method
public static void main(String[] args)
throws IOException {
FileWriter output =
new FileWriter("temp.txt");
output.write("Java 101");
output.close();
Using try-catch block
public static void main(String[] args) {
try {
FileWriter output =
new FileWriter("temp.txt");
output.write("Java 101");
output.close();
FileReader input =
new FileReader("temp.txt");
int code = input.read();
System.out.println((char)code);
input.close();
FileReader input =
new FileReader("temp.txt");
int code = input.read();
System.out.println((char)code);
input.close();
}
catch (IOException ex) {
ex.printStackTrace();
}
}
}
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Text File vs. Binary File

Data stored in a text file are represented in human-readable
form. Data stored in a binary file are represented in binary form.
You cannot read binary files. Binary files are designed to be
read by programs. For example, the Java source programs are
stored in text files and can be read by a text editor, but the Java
classes are stored in binary files and are read by the JVM. The
advantage of binary files is that they are more efficient to
process than text files.

Although it is not technically precise and correct, you can
imagine that a text file consists of a sequence of characters and a
binary file consists of a sequence of bits. For example, the
decimal integer 199 is stored as the sequence of three
characters: '1', '9', '9' in a text file and the same integer is stored
as a byte-type value C7 in a binary file, because decimal 199
equals to hex C7 ().
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Text I/O Classes
InputStreamReader
FileReader
Reader
BufferedReader
Object
BufferedWriter
Writer
OutputStreamWriter
FileWriter
PrintWriter
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Reader
The value is returned as a Unicode.
java.io.Reader
+read(): int
Reads the next character from the input stream. The value returned is an int in
the range from 0 to 65535, which represents a Unicode character. Returns -1
at the end of the stream.
+read(cbuf: char[]): int
Reads characters from the input stream into an array. Returns the actual
number of characters read. Returns -1 at the end of the stream.
+read(cbuf: char[], off:
int, len: int): int
Reads characters from the input stream and stores into cbuf[off], cbuf[off+1],
…, cbuf[off+len-1]. The actual number of bytes read is returned. Returns -1
at the end of the stream.
+close(): void
Closes this input stream and releases any system resources associated with the
stream.
+skip(n: long): long
Skips over and discards n characters of data from this input stream. The actual
number of characters skipped is returned.
+markSupported(): boolean Tests if this input stream supports the mark and reset methods.
+mark(readlimit: int): void Marks the current position in this input stream.
+reset(): void
Repositions this stream to the position at the time the mark method was last
called on this input stream.
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Writer
The Unicode value.
java.io.Writer
+write(int c): void
Writes the specified character to this output stream. The parameter c is an int
value. (char)c is written to the output stream.
+write(cbuf: byte[]): void Writes all the characters in array cbuf to the output stream.
+write(cbuf: char[], off: Writes cbuf[off], cbuf[off+1], …, cbuf[off+len-1] into the output stream.
int, len: int): void
+write(str: String): void
Writes the characters from the string into the output stream.
+write(str: String, off: int, Writes a portion of the string characters into the output stream.
len: int): void
Closes this input stream and releases any system resources associated with the
+close(): void
stream.
+flush(): void
Flushes this output stream and forces any buffered output characters to be written
out.
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FileReader/FileWriter
InputStreamReader
FileReader
Reader
BufferedReader
Object
BufferedWriter
Writer
OutputStreamWriter
FileWriter
PrintWriter
FileReader/FileWriter associates an
input/output stream with an external file.
All the methods in FileReader/FileWriter
are inherited from its superclasses.
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FileReader
To construct a FileReader, use the following constructors:
public FileReader(String filename)
public FileReader(File file)
A java.io.FileNotFoundException would occur if you attempt to
create a FileReader with a nonexistent file.
TestFileReader
Run
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FileWriter
To construct a FileWriter, use the following constructors:
public FileWriter(String filename)
public FileWriter(File file)
public FileWriter(String filename, boolean append)
public FileWriter(File file, boolean append)
If the file does not exist, a new file would be created. If the file already
exists, the first two constructors would delete the current contents in
the file. To retain the current content and append new data into the file,
use the last two constructors by passing true to the append parameter.
TestFileWriter
Run
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Optional
InputStreamReader/OutputStreamWriter
InputStreamReader
FileReader
Reader
BufferedReader
Object
BufferedWriter
Writer
OutputStreamWriter
FileWriter
PrintWriter
All the methods in InputStreamReader/OutputStreamWriter
are inherited from Reader/Writer except getEncoding(), which
returns the name of encoding being used by this stream.
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Optional
InputStreamReader/OutputStreamWriter
InputStreamReader/OutputStreamWriter are used to convert between
bytes and characters. Characters written to an OutputStreamWriter are
encoded into bytes using a specified encoding scheme. Bytes read
from an InputStreamReader are decoded into characters using a
specified encoding scheme. You can specify an encoding scheme using
a constructor of InputStreamReader/OutputStreamWriter. If no
encoding scheme is specified, the system’s default encoding scheme is
used.
Program
A character is converted
into the Unicode
The Unicode of
the character is
returned
A character stored in
a specified encoding
The Unicode of
the character is
sent out
A character is converted into the
code for the specified encoding
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BufferedReader/BufferedWriter
InputStreamReader
FileReader
Reader
BufferedReader
Object
BufferedWriter
Writer
OutputStreamWriter
FileWriter
PrintWriter
The buffered stream classes inherit methods from their superclasses. In
addition to using the methods from their superclasses, BufferedReader
has a readLine() method to read a line, and BufferedWriter has a
newLine() method to write a line separator. If the end of stream is
reached, readLine() returns null.
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PrintWriter/PrintStream
InputStreamReader
FileReader
Reader
BufferedReader
Object
BufferedWriter
Writer
OutputStreamWriter
FileWriter
PrintWriter
BufferedWriter is used to output characters and strings. PrintWriter and
PrintStream can be used to output objects, strings and numeric values as text.
PrintWriter was introduced in JDK 1.2 to replace PrintStream. Both classes are
almost identical in the sense that they provide the same function and same
methods for outputting strings and numeric values as text. PrintWriter is more
efficient than PrintStream. So, you use PrintWriter rather than PrintStream.
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Methods in PrintWriter/PrintStream
public void print(Object o)
public void println(Object o)
public void print(String s)
public void print(char c)
public void println(String s)
public void println(char c)
public void print(char[] cArray)
public void print(int i)
public void println(char[] cArray)
public void println(int i)
public void print(long l)
public void print(float f)
public void println(long l)
public void println(float f)
public void print(double d)
public void print(boolean b)
public void println(double d)
public void println(boolean b)
PrintWriter and PrintStream also contain the JDK 1.5 printf
method for printing formatted output, which was introduced
in Section 2.17, “Formatted Output.”
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Constructing PrintWriter
This section introduces PrintWriter, but PrintStream can be used in the
same way. To construct a PrintWriter, use the following constructors:
public PrintWriter(Writer out)
public PrintWriter(Writer out, boolean autoFlush)
If autoFlush is true, the println methods will cause the buffer to be
flushed.
The constructors and methods in PrintWriter and PrintStream do not
throw an IOException. So you don’t need to invoke them from a trycatch block.
TestPrintWriter
Run
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Case Studies: Text Viewer
This case study writes a program that views a text file in a text
area. The user enters a filename in a text field and clicks the View
button; the file is then displayed in a text area.
FileViewer
Run
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Binary I/O
Text I/O requires encoding and decoding. The JVM converts a
Unicode to a file specific encoding when writing a character and
coverts a file specific encoding to a Unicode when reading a
character. Binary I/O does not require conversions. When you write a
byte to a file, the original byte is copied into the file. When you read a
byte from a file, the exact byte in the file is returned.
Text I/O
The Unicode of
the character
Encoding/
Decoding
The encoding of the character is
stored in the file
Binary I/O
A byte is
read/written
The same byte in the file
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Binary I/O Classes
FileInputStream
DataInputStream
InputStream
FilterInputStream
BufferedInputStream
ObjectInputStream
Object
OutputStream
FileOutputStream
BufferedOutputStream
FilterOutputStream
DataOutputStream
ObjectOutputStream
PrintStream
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InputStream
The value returned is a byte as an int type.
java.io.InputStream
+read(): int
Reads the next byte of data from the input stream. The value byte is returned as
an int value in the range 0 to 255. If no byte is available because the end of
the stream has been reached, the value –1 is returned.
+read(b: byte[]): int
Reads up to b.length bytes into array b from the input stream and returns the
actual number of bytes read. Returns -1 at the end of the stream.
+read(b: byte[], off: int,
len: int): int
Reads bytes from the input stream and stores into b[off], b[off+1], …,
b[off+len-1]. The actual number of bytes read is returned. Returns -1 at the
end of the stream.
+available(): int
Returns the number of bytes that can be read from the input stream.
+close(): void
Closes this input stream and releases any system resources associated with the
stream.
+skip(n: long): long
Skips over and discards n bytes of data from this input stream. The actual
number of bytes skipped is returned.
+markSupported(): boolean Tests if this input stream supports the mark and reset methods.
+mark(readlimit: int): void Marks the current position in this input stream.
+reset(): void
Repositions this stream to the position at the time the mark method was last
called on this input stream.
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OutputStream
The value is a byte as an int type.
java.io.OutputStream
+write(int b): void
Writes the specified byte to this output stream. The parameter b is an int value.
(byte)b is written to the output stream.
+write(b: byte[]): void
Writes all the bytes in array b to the output stream.
+write(b: byte[], off: int, Writes b[off], b[off+1], …, b[off+len-1] into the output stream.
len: int): void
+close(): void
Closes this input stream and releases any system resources associated with the
stream.
+flush(): void
Flushes this output stream and forces any buffered output bytes to be written out.
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FileInputStream/FileOutputStream
FileInputStream
DataInputStream
InputStream
FilterInputStream
BufferedInputStream
ObjectInputStream
Object
OutputStream
FileOutputStream
BufferedOutputStream
FilterOutputStream
DataOutputStream
ObjectOutputStream
PrintStream
FileInputStream/FileOutputStream
associates a binary input/output stream with
an external file. All the methods in
FileInputStream/FileOuptputStream are
inherited from its superclasses.
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FileInputStream
To construct a FileInputStream, use the following
constructors:
public FileInputStream(String filename)
public FileInputStream(File file)
A java.io.FileNotFoundException would occur if you attempt to
create a FileInputStream with a nonexistent file.
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FileOutputStream
To construct a FileOutputStream, use the following constructors:
public FileOutputStream(String filename)
public FileOutputStream(File file)
public FileOutputStream(String filename, boolean append)
public FileOutputStream(File file, boolean append)
If the file does not exist, a new file would be created. If the file already
exists, the first two constructors would delete the current contents in
the file. To retain the current content and append new data into the file,
use the last two constructors by passing true to the append parameter.
TestFileStream
Run
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FilterInputStream/FilterOutputStream
FileInputStream
DataInputStream
InputStream
FilterInputStream
BufferedInputStream
ObjectInputStream
Object
OutputStream
FileOutputStream
BufferedOutputStream
FilterOutputStream
DataOutputStream
ObjectOutputStream
PrintStream
Filter streams are streams that filter bytes for some purpose. The basic byte input
stream provides a read method that can only be used for reading bytes. If you want to
read integers, doubles, or strings, you need a filter class to wrap the byte input stream.
Using a filter class enables you to read integers, doubles, and strings instead of bytes
and characters. FilterInputStream and FilterOutputStream are the base classes for
filtering data. When you need to process primitive numeric types, use DatInputStream
and DataOutputStream to filter bytes.
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DataInputStream/DataOutputStream
DataInputStream reads bytes from the stream
and converts them into appropriate primitive
type values or strings.
FileInputStream
DataInputStream
InputStream
FilterInputStream
BufferedInputStream
ObjectInputStream
Object
OutputStream
FileOutputStream
BufferedOutputStream
FilterOutputStream
DataOutputStream
ObjectOutputStream
PrintStream
DataOutputStream converts primitive type values
or strings into bytes and output the bytes to the
stream.
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DataInputStream
DataInputStream extends FilterInputStream
and implements the DataInput interface.
InputStream
FilterInputStream
DataInputStream
+DataInputStream(
in: InputStream)
java.io.DataInput
+readBoolean(): boolean Reads a Boolean from the input stream.
+readByte(): byte
Reads a byte from the input stream.
+readChar(): char
Reads a character from the input stream.
+readFloat(): float
Reads a float from the input stream.
+readDouble(): float
Reads a double from the input stream.
+readInt(): int
Reads an int from the input stream.
+readLong(): long
Reads a long from the input stream.
+readShort(): short
Reads a short from the input stream.
+readLine(): String
Reads a line of characters from input.
+readUTF(): String
Reads a string in UTF format.
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DataOutputStream
DataOutputStream extends FilterOutputStream and implements the
DataOutput interface.
OutputStream
FilterOutputStream
DataOutputStream
+DataOutputStream(
in: InputStream)
java.io.DataOutput
+writeBoolean(b: Boolean): void Writes a Boolean to the output stream.
+writeByte(v: int): void
Writes to the output stream the eight low-order bits
of the argument v.
+writeBytes(s: String): void
Writes the lower byte of the characters in a string to
the output stream.
+writeChar(c: char): void
Writes a character (composed of two bytes) to the
output stream.
+writeChars(s: String): void
Writes every character in the string s, to the output
stream, in order, two bytes per character.
+writeFloat(v: float): void
Writes a float value to the output stream.
+writeDouble(v: float): void
Writes a double value to the output stream.
+writeInt(v: int): void
Writes an int value to the output stream.
+writeLong(v: long): void
Writes a long value to the output stream.
+writeShort(v: short): void
Writes a short value to the output stream.
+writeUTF(s: String): void
Writes two bytes of length information to the output
stream, followed by the UTF representation of
every character in the string s.
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Characters and Strings in Binary I/O
A Unicode consists of two bytes. The writeChar(char c) method
writes the Unicode of character c to the output. The
writeChars(String s) method writes the Unicode for each character in
the string s to the output.
Why UTF? What is UTF?
UTF is a coding scheme that allows systems to operate with both
ASCII and Unicode efficiently. Most operating systems use ASCII.
Java uses Unicode. The ASCII character set is a subset of the
Unicode character set. Since most applications need only the ASCII
character set, it is a waste to represent an 8-bit ASCII character as a
16-bit Unicode character. The UTF is an alternative scheme that
stores a character using 1, 2, or 3 bytes. ASCII values less than 0x7F
are coded in one byte. Unicode values less than 0x7FF are coded in
two bytes. Other Unicode values are coded in three bytes.
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Using DataInputStream/DataOutputStream
Data streams are used as wrappers on existing input and output
streams to filter data in the original stream. They are created using the
following constructors:
public DataInputStream(InputStream instream)
public DataOutputStream(OutputStream outstream)
The statements given below create data streams. The first statement
creates an input stream for file in.dat; the second statement creates an
output stream for file out.dat.
DataInputStream infile =
new DataInputStream(new FileInputStream("in.dat"));
DataOutputStream outfile =
new DataOutputStream(new FileOutputStream("out.dat"));
TestDataStream
Run
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Order and Format
CAUTION: You have read the data in the same order and same format
in which they are stored. For example, since names are written in UTF
using writeUTF, you must read names using readUTF.
Checking End of File
TIP: If you keep reading data at the end of a stream, an EOFException
would occur. So how do you check the end of a file? You can use
input.available() to check it. input.available() == 0 indicates that it is
the end of a file.
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BufferedInputStream/
BufferedOutputStream
Using buffers to speed up I/O
FileInputStream
DataInputStream
InputStream
FilterInputStream
BufferedInputStream
ObjectInputStream
Object
OutputStream
FileOutputStream
BufferedOutputStream
FilterOutputStream
DataOutputStream
ObjectOutputStream
PrintStream
BufferedInputStream/BufferedOutputStream does not contain new
methods. All the methods BufferedInputStream/BufferedOutputStream are
inherited from the InputStream/OutputStream classes.
36
Constructing
BufferedInputStream/BufferedOutputStream
// Create a BufferedInputStream
public BufferedInputStream(InputStream in)
public BufferedInputStream(InputStream in, int bufferSize)
// Create a BufferedOutputStream
public BufferedOutputStream(OutputStream out)
public BufferedOutputStream(OutputStreamr out, int bufferSize)
37
Case Studies: Copy File
This case study develops a program that copies files. The user needs
to provide a source file and a target file as command-line arguments
using the following command:
java Copy source target
The program copies a source file to a target file and displays the
number of bytes in the file. If the source does not exist, tell the user
the file is not found. If the target file already exists, tell the user the
file already exists.
Copy
Run
38
More on Text Files and Binary Files
Now it is time to tell the complete story and set the record straight.
Computers do not differentiate between a binary file and a text file. All
files are stored in binary format. So, all files are essentially binary
files. Text I/O is built upon binary I/O to provide a level of abstraction
for character encoding and decoding. Encoding and decoding are
automatically performed by text I/O. In general, you should use text
input to read a file created by a text editor or a text output program,
and use binary input to read a file created by a Java binary output
program. For binary input, you need to know exactly how data were
written in order to read them in correct type and order. Binary I/O also
contains methods to read/writer a character and string.
39
Write a byte 199 as a numeric value
import java.io.*;
Binary stream
Character stream
Output 199
Output "199"
FileOutputStream
FileWriter
0xC7 (199 in decimal)
out.dat
1 9
9
0x31 0x39 0x39
public class Test {
public static void main(String[] args) throws IOException {
FileOutputStream output = new FileOutputStream("out.dat");
output.write(199); // Output byte 199 to the stream
output.close();
out.txt
FileInputStream input = new FileInputStream("out.dat");
System.out.println(input.read()); // Read and display a byte
input.close();
}
}
40
Write a byte 199 as characters
import java.io.*;
Binary stream
Character stream
Output 199
Output "199"
FileOutputStream
0xC7 (199 in decimal)
FileWriter
out.dat
1 9
9
0x31 0x39 0x39
out.txt
public class Test {
public static void main(String[] args) throws IOException {
FileWriter output = new FileWriter("out.txt");
output.write("199"); // Output string "199" to the stream
output.close();
// Read and display three characters
FileReader input = new FileReader("out.txt");
System.out.print((char)input.read());
System.out.print((char)input.read());
System.out.println((char)input.read());
input.close();
}
}
41
Optional
Object I/O
DataInputStream/DataOutputStream enables you to perform I/O for
primitive type values and strings.
ObjectInputStream/ObjectOutputStream enables you to perform I/O
for objects in addition for primitive type values and strings.
FileInputStream
DataInputStream
InputStream
FilterInputStream
BufferedInputStream
ObjectInputStream
Object
OutputStream
FileOutputStream
BufferedOutputStream
FilterOutputStream
DataOutputStream
ObjectOutputStream
PrintStream
42
ObjectInputStream
ObjectInputStream extends InputStream and
implements ObjectInput and ObjectStreamConstants.
java.io.InputStream
ObjectStreamConstants
java.io.DataInput
java.io.ObjectInputStream
+ObjectInputStream(in: InputStream)
java.io.ObjectInput
+readObject(): Object
Reads an object.
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ObjectOutputStream
ObjectOutputStream extends OutputStream and
implements ObjectOutput and ObjectStreamConstants.
java.io.OutputStream
ObjectStreamConstants
java.io.DataOutput
java.io.ObjectOutputStream
+ObjectOutputStream(out: OutputStream)
java.io.ObjectOutput
+writeObject(o: Object): void Writes an object.
44
Using Object Streams
You may wrap an ObjectInputStream/ObjectOutputStream on any
InputStream/OutputStream using the following constructors:
// Create an ObjectInputStream
public ObjectInputStream(InputStream in)
// Create an ObjectOutputStream
public ObjectOutputStream(OutputStream out)
TestObjectOutputStream
Run
TestObjectInputStream
Run
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The Serializable Interface
Not all objects can be written to an output stream. Objects that can be
written to an object stream is said to be serializable. A serializable
object is an instance of the java.io.Serializable interface. So the class
of a serializable object must implement Serializable.
The Serializable interface is a marker interface. It has no methods, so
you don't need to add additional code in your class that implements
Serializable.
Implementing this interface enables the Java serialization mechanism
to automate the process of storing the objects and arrays.
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The transient Keyword
If an object is an instance of Serializable, but it contains
non-serializable instance data fields, can the object be
serialized? The answer is no. To enable the object to be
serialized, you can use the transient keyword to mark these
data fields to tell the JVM to ignore these fields when
writing the object to an object stream.
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The transient Keyword, cont.
Consider the following class:
public class Foo implements java.io.Serializable {
private int v1;
private static double v2;
private transient A v3 = new A();
}
class A { } // A is not serializable
When an object of the Foo class is serialized, only variable v1 is
serialized. Variable v2 is not serialized because it is a static variable,
and variable v3 is not serialized because it is marked transient. If v3
were not marked transient, a java.io.NotSerializableException would
occur.
48
Serializing Arrays
An array is serializable if all its elements are serializable.
So an entire array can be saved using writeObject into a file
and later restored using readObject. Listing 16.12 stores an
array of five int values an array of three strings, and an
array of two JButton objects, and reads them back to
display on the console.
TestObjectStreamForArray
Run
49
Random Access Files
All of the streams you have used so far are known as
read-only or write-only streams. The external files of
these streams are sequential files that cannot be updated
without creating a new file. It is often necessary to
modify files or to insert new records into files. Java
provides the RandomAccessFile class to allow a file to be
read from and write to at random locations.
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RandomAccessFile
DataInput
DataInput
java.io.RandomAccessFile
+RandomAccessFile(file: File, mode:
String)
Creates a RandomAccessFile stream with the specified File object and
mode.
+RandomAccessFile(name: String,
mode: String)
Creates a RandomAccessFile stream with the specified file name
string and mode.
+close(): void
Closes the stream and releases the resource associated with the stream.
+getFilePointer(): long
Returns the offset, in bytes, from the beginning of the file to where the
next read or write occurs.
+length(): long
Returns the length of this file.
+read(): int
Reads a byte of data from this file and returns –1 an the end of stream.
+read(b: byte[]): int
Reads up to b.length bytes of data from this file into an array of bytes.
+read(b: byte[], off: int, len: int) : int
Reads up to len bytes of data from this file into an array of bytes.
+seek(long pos): void
Sets the offset (in bytes specified in pos) from the beginning of the
stream to where the next read or write occurs.
+setLength(newLength: long): void
Sets a new length of this file.
+skipBytes(int n): int
Skips over n bytes of input discarding the skipped bytes.
+write(b: byte[]): void
+write(byte b[], int off, int len)
Writes b.length bytes from the specified byte array to this file, starting
at the current file pointer.
+write(b: byte[], off: int, len: int):
void
Writes len bytes from the specified byte array starting at offset off to
this file.
51
File Pointer
A random access file consists of a sequence of bytes. There is a
special marker called file pointer that is positioned at one of these
bytes. A read or write operation takes place at the location of the file
pointer. When a file is opened, the file pointer sets at the beginning of
the file. When you read or write data to the file, the file pointer moves
forward to the next data. For example, if you read an int value using
readInt(), the JVM reads four bytes from the file pointer and now the
file pointer is four bytes ahead of the previous location.
file pointer
file
byte byte …
byte byte byte byte byte
…
byte byte byte byte byte
(A) Before readInt()
…
byte byte byte byte byte
(B) Before readInt()
file pointer
file
byte byte …
byte byte byte byte byte
52
RandomAccessFile Methods
Many methods in RandomAccessFile are the same as
those in DataInputStream and DataOutputStream.
For example, readInt(), readLong(),
writeDouble(), readLine(), writeInt(), and
writeLong() can be used in data input stream or data
output stream as well as in RandomAccessFile
streams.
53
RandomAccessFile Methods, cont.

void seek(long pos) throws IOException;
Sets the offset from the beginning of the
RandomAccessFile stream to where the next read
or write occurs.

long getFilePointer() IOException;
Returns the current offset, in bytes, from the
beginning of the file to where the next read
or write occurs.
54
RandomAccessFile Methods, cont.

long length()IOException
Returns the length of the file.

final void writeChar(int v) throws
IOException
Writes a character to the file as a two-byte Unicode,
with the high byte written first.

final void writeChars(String s)
throws IOException
Writes a string to the file as a sequence of
characters.
55
RandomAccessFile Constructor
RandomAccessFile raf =
new RandomAccessFile("test.dat",
"rw"); //allows read and write
RandomAccessFile raf =
new RandomAccessFile("test.dat",
"r"); //read only
56
A Short Example on
RandomAccessFile
TestRandomAccessFile
Run
57
Optional
Case Studies: Address Book
Now let us use RandomAccessFile to create a useful
project for storing and viewing and address book. The user
interface of the program is shown in Figure 16.24. The
Add button stores a new address to the end of the file. The
First, Next, Previous, and Last buttons retrieve the first,
next, previous, and last addresses from the file,
respectively.
58
Fixed Length String I/O
Random access files are often used to process files of records. For
convenience, fixed-length records are used in random access files
so that a record can be located easily. A record consists of a fixed
number of fields. A field can be a string or a primitive data type. A
string in a fixed-length record has a maximum size. If a string is
smaller than the maximum size, the rest of the string is padded with
blanks.
file
Record 1
Record 2
Record n
Student 1
Student 2
Student n
e.g.,
Field1 Field 2 … Field k
name street city state zip
FixedLengthStringIO
59
Address Implementation
The rest of the work can be summarized in the following steps:
Create the user interface.
Add a record to the file.
Read a record from the file.
Write the code to implement the button actions.
AddressBook
Run
60