Transcript 04-Filesx

Files and I/O Streams in Java
Written by Amir Kirsh, Edited by Liron Blecher
© Amir Kirsh
Agenda
• System Parameters
• File Class
• I/O Streams
• Reading from the standard input
• Scanner Class
• Binary files
• Text files and character encoding
System Parameters
• Even though Java works on different platforms
and is designed to be Platform Neutral, there
might be cases when you need to get specific
environment parameters
• Use System.getProperties() to get all available
properties
• Use System.getProperty() to get a specific
property (you must provide the property name)
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examples.systemproperties
DEMO
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Agenda
• System Parameters
• File Class
• I/O Streams
• Reading from the standard input
• Scanner Class
• Binary files
• Text files and character encoding
File Class
• The File class represents a file or directory
• Supports inquiries on the file or directory, such as:
• Rename
• Open (if it’s a file) to get a stream of bytes for read / write
• List all of its child files (if it’s a directory)
• Delete it
• etc.
• Useful method to get all Root files (or directories) of a
machine is the static method: File.listRoots()
• In Java 7 – the entire File API was re-factored:
http://docs.oracle.com/javase/tutorial/essential/io/index.html
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Agenda
• System Parameters
• File Class
• I/O Streams
• Reading from the standard input
• Scanner Class
• Binary files
• Text files and character encoding
I/O Streams
• In Java reading and writing data between different sources
are all done using the same API – I/O Streams
• I/O Stream classes represents a stream of bytes / chars
• InputStream
• InputStreamReader
• BufferedReader
• DataInputStream
• I/O Streams are not related only to files, we can have a
stream of bytes for network sockets, Byte Array or even for
a String
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I/O Streams
• There are two major families of I/O Streams:
• Input / Output streams – used for reading / writing one byte at a time from/to
a resource
• Reader / Writer – used to convert binary data to characters (and again,
read/write them one character at a time)
Reader and Writer streams wrap regular input/output streams
• Buffered streams (used in both families) are used to read data
(either bytes or characters) into an internal buffer.
This makes them more efficient and also allows to read
characters in lines (and not just one by one)
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I/O Streams – Input / Output
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I/O Streams – Reader / Writer
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Streams - Closing
•
After finishing reading / writing from a stream (any
stream), the stream needs to be closed (this is
especially true for files, network, hardware, etc.
•
Use the close method to close the stream; since the
method declared that it throws an IOException, it is
usually called inside a try / catch block
•
In Java 7, there’s a shorter way to open a stream and
close it using the try-with-resource statement
http://java.dzone.com/articles/java-7-new-feature%E2%80%93
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Streams
Tip!
When sending an existing object over and over again
over an output stream, it might stay in memory on the
other side (input stream). In order to be able to send the
entire object again call the method reset before sending
the object again.
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examples.files.TextFile
DEMO
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Agenda
• System Parameters
• File Class
• Streams
• Reading from the standard input
• Scanner Class
• Binary files
• Text files and character encoding
Reading from the standard input
try {
BufferedReader in =
new BufferedReader(
new InputStreamReader(System.in) );
String str = "";
while (str != null) {
str = in.readLine();
System.out.println(str.toUpperCase());
}
} catch (IOException e) { }
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Reading from the standard input
• System.in an object of type InputStream
• A stream of bytes (NOT chars!)
• InputStreamReader
• A bridge from byte stream to character stream, can read single chars
• BufferedReader
• adds the method “readLine”
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Writing to the console
•
The method System.out.println() is a wrapper for this
code:
BufferedWriter consoleWriter =
new BufferedWriter(
new OutputStreamWriter(System.out));
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Agenda
• System Parameters
• File Class
• I/O Streams
• Reading from the standard input
• Scanner Class
• Binary files
• Text files and character encoding
Scanner
Scanner is a helper class for getting input
Scanner s = new Scanner(System.in);
System.out.println("Please insert a string: ");
String str = s.nextLine();
System.out.println("Please insert a number: ");
int i = s.nextInt(); // may throw InputMismatchException
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Scanner
•
The Scanner class is a wrapper for this code:
•
new BufferedReader(new
InputStreamReader(System.in));
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Agenda
• System Parameters
• File Class
• I/O Streams
• Reading from the standard input
• Scanner Class
• Binary files
• Text files and character encoding
Binary Files
• Binary Files hold data in binary format
• DataInputStream / DataOutputStream -- for primitive types
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examples.files
DEMO
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Serialization
Serialization is a mechanism that enables to read/write entire
objects over streams (to a file or a server socket).
Using the marker interface Serializable interface and the
ObjectInputStream / ObjectOutputStream to:
• readObject()
• writeObject()
• When writing Object, you can set a version for a
Class, thus enabling changing the class without the
need to change the files it is written on:
http://www.mkyong.com/java-bestpractices/understand-the-serialversionuid/
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Agenda
• System Parameters
• File Class
• I/O Streams
• Reading from the standard input
• Scanner Class
• Binary files
• Text files and character encoding
Text Files and Character Encoding
Joel On SW about character encoding:
http://www.joelonsoftware.com/articles/Unicode.html – a must read!
• InputStream
• An I/O stream of bytes (NOT chars!)
• InputStreamReader
• A bridge from byte stream to character stream, can read single chars
• An important parameter is the Charset
• Also when constructing a String out of bytes, it’s important to
provide the Charset used on the byte array
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