Download Chapter 13 Exception Handling

Chapter 18 Exception Handling
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Motivations
When a program runs into a runtime error, the
program terminates abnormally. How can you
handle the runtime error so that the program can
continue to run or terminate gracefully? This is the
subject we will introduce in this chapter.
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Exception-Handling Overview
Quotient
Run
QuotientWithIf
Run
QuotientWithException
Run
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Exception Advantages
QuotientWithMethod
Run
Now you see the advantages of using exception handling.
It enables a method to throw an exception to its caller.
Without this capability, a method must handle the
exception or terminate the program.
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System Errors
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
System errors are thrown by JVM
and represented in the Error class.
The Error class describes internal
system errors. Such errors rarely
occur. If one does, there is little
you can do beyond notifying the
user and trying to terminate the
program gracefully.
LinkageError
Several more classes
VirtualMachineError
Error
AWTError
Several more classes
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Exceptions
Exception describes errors
caused by your program
and external
circumstances. These
errors can be caught and
handled by your program.
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
LinkageError
Several more classes
VirtualMachineError
Error
AWTError
Several more classes
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Runtime Exceptions
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
LinkageError
VirtualMachineError
Error
AWTError
Several more classes
RuntimeException is caused by
programming errors, such as bad
casting, accessing an out-of-bounds
array, and numeric errors.
Several more classes
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Checked Exceptions vs.
Unchecked Exceptions
RuntimeException, Error and their subclasses are
known as unchecked exceptions. All other
exceptions are known as checked exceptions,
meaning that the compiler forces the programmer
to check and deal with the exceptions.
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Unchecked Exceptions
In most cases, unchecked exceptions reflect programming
logic errors that are not recoverable. For example, a
NullPointerException is thrown if you access an object
through a reference variable before an object is assigned to
it; an IndexOutOfBoundsException is thrown if you access
an element in an array outside the bounds of the array.
These are the logic errors that should be corrected in the
program. Unchecked exceptions can occur anywhere in the
program. To avoid cumbersome overuse of try-catch
blocks, Java does not mandate you to write code to catch
unchecked exceptions.
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Checked or Unchecked Exceptions
ClassNotFoundException
IOException
ArithmeticException
Exception
AWTException
NullPointerException
RuntimeException
IndexOutOfBoundsException
Object
Throwable
Several more classes
IllegalArgumentException
LinkageError
Several more classes
VirtualMachineError
Error
AWTError
Unchecked
exception.
Several more classes
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Declaring, Throwing, and
Catching Exceptions
method1() {
try {
invoke method2;
}
catch (Exception ex) {
Process exception;
}
catch exception
declare exception
method2() throws Exception {
if (an error occurs) {
throw new Exception();
}
throw exception
}
}
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Declaring Exceptions
Every method must state the types of checked
exceptions it might throw. This is known as
declaring exceptions.
public void myMethod()
throws IOException
public void myMethod()
throws IOException, OtherException
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Throwing Exceptions Example
/** Set a new radius */
public void setRadius(double newRadius)
throws IllegalArgumentException {
if (newRadius >= 0)
radius = newRadius;
else
throw new IllegalArgumentException(
"Radius cannot be negative");
}
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Catching Exceptions
try {
statements; // Statements that may throw exceptions
}
catch (Exception1 exVar1) {
handler for exception1;
}
catch (Exception2 exVar2) {
handler for exception2;
}
...
catch (ExceptionN exVar3) {
handler for exceptionN;
}
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Catching Exceptions
try
try
try
catch
catch
catch
An exception
is thrown in
method3
Call Stack
method3
main method
method2
method2
method1
method1
method1
main method
main method
main method
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Catch or Declare Checked Exceptions
Java forces you to deal with checked exceptions. If a method declares a
checked exception (i.e., an exception other than Error or
RuntimeException), you must invoke it in a try-catch block or declare to
throw the exception in the calling method. For example, suppose that
method p1 invokes method p2 and p2 may throw a checked exception (e.g.,
IOException), you have to write the code as shown in (a) or (b).
void p1() {
try {
p2();
}
catch (IOException ex) {
...
}
}
(a)
void p1() throws IOException {
p2();
}
(b)
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Example: Declaring, Throwing, and
Catching Exceptions
 Objective:
This example demonstrates
declaring, throwing, and catching exceptions
by modifying the setRadius method in the
Circle class defined in Chapter 6. The new
setRadius method throws an exception if
radius is negative.
TestCircleWithException
CircleWithException
Run
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Example: Exceptions in GUI
Applications

An error message appears on the console, but the GUI
application continues running.

Write a program that creates a user interface to perform
integer divisions. The user enters two numbers in the text
fields Number 1 and Number 2. The division of Number 1
and Number 2 is displayed in the Result field when the
Divide button is clicked.
IntegerDivision
Run
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Rethrowing Exceptions
try {
statements;
}
catch(TheException ex) {
perform operations before exits;
throw ex;
}
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The finally Clause
try {
statements;
}
catch(TheException ex) {
handling ex;
}
finally {
finalStatements;
}
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animation
Trace a Program Execution
Suppose no
exceptions in the
statements
try {
statements;
}
catch(TheException ex) {
handling ex;
}
finally {
finalStatements;
}
Next statement;
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animation
Trace a Program Execution
try {
statements;
}
catch(TheException ex) {
handling ex;
}
finally {
finalStatements;
}
The final block is
always executed
Next statement;
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animation
Trace a Program Execution
try {
statements;
}
catch(TheException ex) {
handling ex;
}
finally {
finalStatements;
}
Next statement in the
method is executed
Next statement;
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animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
finally {
finalStatements;
}
Suppose an exception
of type Exception1 is
thrown in statement2
Next statement;
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animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
finally {
finalStatements;
}
The exception is
handled.
Next statement;
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animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
finally {
finalStatements;
}
The final block is
always executed.
Next statement;
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animation
Trace a Program Execution
try {
statement1;
statement2;
statement3;
}
catch(Exception1 ex) {
handling ex;
}
finally {
finalStatements;
}
The next statement in
the method is now
executed.
Next statement;
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Cautions When Using Exceptions
 Exception
handling separates error-handling
code from normal programming tasks, thus
making programs easier to read and to modify.
Be aware, however, that exception handling
usually requires more time and resources
because it requires instantiating a new exception
object, rolling back the call stack, and
propagating the errors to the calling methods.
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When to Throw Exceptions
 An
exception occurs in a method. If you want
the exception to be processed by its caller, you
should create an exception object and throw it.
If you can handle the exception in the method
where it occurs, there is no need to throw it.
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When to Use Exceptions
When should you use the try-catch block in the code?
You should use it to deal with unexpected error
conditions. Do not use it to deal with simple, expected
situations. For example, the following code
try {
System.out.println(refVar.toString());
}
catch (NullPointerException ex) {
System.out.println("refVar is null");
}
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When to Use Exceptions
is better to be replaced by
if (refVar != null)
System.out.println(refVar.toString());
else
System.out.println("refVar is null");
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Creating Custom Exception Classes

Use the exception classes in the API whenever possible.

Create custom exception classes if the predefined
classes are not sufficient.

Declare custom exception classes by extending
Exception or a subclass of Exception.
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Custom Exception Class Example
In Listing 18.1, the setRadius method throws an exception if the
radius is negative. Suppose you wish to pass the radius to the
handler, you have to create a custom exception class.
InvalidRadiusException
CircleWithRadiusException
TestCircleWithRadiusException
Run
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Companion
Website
Assertions
An assertion is a Java statement that enables
you to assert an assumption about your
program. An assertion contains a Boolean
expression that should be true during
program execution. Assertions can be used to
assure program correctness and avoid logic
errors.
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Companion
Website
Executing Assertions
When an assertion statement is executed, Java evaluates the
assertion. If it is false, an AssertionError will be thrown. The
AssertionError class has a no-arg constructor and seven
overloaded single-argument constructors of type int, long, float,
double, boolean, char, and Object.
For the first assert statement with no detail message, the no-arg
constructor of AssertionError is used. For the second assert
statement with a detail message, an appropriate AssertionError
constructor is used to match the data type of the message. Since
AssertionError is a subclass of Error, when an assertion becomes
false, the program displays a message on the console and exits.
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Companion
Website
Executing Assertions Example
public class AssertionDemo {
public static void main(String[] args) {
int i; int sum = 0;
for (i = 0; i < 10; i++) {
sum += i;
}
assert i == 10;
assert sum > 10 && sum < 5 * 10 : "sum is " + sum;
}
}
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Companion
Website
Running Programs with
Assertions
By default, the assertions are disabled at runtime. To
enable it, use the switch –enableassertions, or –ea for
short, as follows:
java –ea AssertionDemo
Assertions can be selectively enabled or disabled at
class level or package level. The disable switch is –
disableassertions or –da for short. For example, the
following command enables assertions in package
package1 and disables assertions in class Class1.
java –ea:package1 –da:Class1 AssertionDemo
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Companion
Website
Using Exception Handling or
Assertions
Assertion should not be used to replace exception
handling. Exception handling deals with unusual
circumstances during program execution. Assertions are
to assure the correctness of the program. Exception
handling addresses robustness and assertion addresses
correctness. Like exception handling, assertions are not
used for normal tests, but for internal consistency and
validity checks. Assertions are checked at runtime and
can be turned on or off at startup time.
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Companion
Website
Using Exception Handling or
Assertions, cont.
Do not use assertions for argument checking in public
methods. Valid arguments that may be passed to a public
method are considered to be part of the method’s
contract. The contract must always be obeyed whether
assertions are enabled or disabled. For example, the
following code should be rewritten using exception
handling as shown in Lines 28-35 in Circle.java in
Listing 18.1.
public void setRadius(double newRadius) {
assert newRadius >= 0;
radius = newRadius;
}
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Companion
Website
Using Exception Handling or
Assertions, cont.
Use assertions to reaffirm assumptions. This gives you
more confidence to assure correctness of the program. A
common use of assertions is to replace assumptions with
assertions in the code.
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Companion
Website
Using Exception Handling or
Assertions, cont.
Use assertions to reaffirm assumptions. This gives you
more confidence to assure correctness of the program.
A common use of assertions is to replace assumptions
with assertions in the code (only if the assumptions are
enforced by the caller!).
switch (month) {
case 1: ... ; break;
case 2: ... ; break;
...
case 12: ... ; break;
default: assert false : "Invalid month: " + month
}
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Chapter 19 Binary I/O
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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.
Scanner input = new Scanner(new File("temp.txt"));
System.out.println(input.nextLine());
Program
Input object
created from an
input class
Output object
created from an
output class
Input stream
01011…1001
11001…1011
File
File
Output stream
Formatter output = new Formatter("temp.txt");
output.format("%s", "Java 101");
output.close();
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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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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 program
(a)
The Unicode of
the character
e.g. "199"
,
Encoding/
Decoding
The encoding of the character
is stored in the file
00110001 00111001 00111001
0x31
0x39
0x39
Binary I/O program
(b)
A byte is read/written
e.g. 199
,
The same byte in the file
00110111
0xC7
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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
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50
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
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(
out: OutputStream)
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-8? What is UTF-8?
UTF-8 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-8 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
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Order and Format
CAUTION: You have to read the data in the same order and same
format in which they are stored. For example, since names are written
in UTF-8 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.
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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)
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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
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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
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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
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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.
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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.
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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
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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.
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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
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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.
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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.
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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.
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RandomAccessFile Constructor
RandomAccessFile raf =
new RandomAccessFile("test.dat", "rw");
//allows read and write
RandomAccessFile raf =
new RandomAccessFile("test.dat", "r");
//read only
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A Short Example on
RandomAccessFile
TestRandomAccessFile
Run
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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.
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