Transcript Input/Output

COMP201 Java Programming
Topic 5: Input and Output
Reading: Chapter 12
COMP201 Topic 6 / Slide 2
Objectives and Outline
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Objectives:
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Overall view i/o in java
Reading and writing local files
Outline
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Introduction and overview
Reading and writing local files
– Connecting to files
– Reading and writing characters
– Reading and writing objects
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File management
COMP201 Topic 6 / Slide 3
Introduction and Overview
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When talking about IO, we need to consider files
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At different locations:
– blocks of main memory
– local file system
– over the net
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In different formats:
– text or binary
– zipped or not zipped
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With different access modes:
–
–
–
–
plain sequential,
buffered,
pushback,
Random
COMP201 Topic 6 / Slide 4
Introduction and Overview
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Java streams
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Input streams:
– Objects from where we read input sequences
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Output stream:
– Objects where we write output sequences
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Java has stream classes allow us to deal with all possible combinations
of location, format, and access mode.
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In particular, Java streams provide an abstraction of files at different
locations
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Local files and files from internet can be handled the same way.
We discuss only local files in this lecture.
In the next few slides, we give some of the stream classes
COMP201 Topic 6 / Slide 5
Introduction and Overview
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All classes for reading from character streams inherit from
abstact class Reader
Reader has one abstract method read, which returns the next
unicode character or –1 (EOF)
Reader
CharArray
Reader
InputStream
Reader
File
Reader
Piped
Reader
Filter
Reader
Pushback
Reader
String
Reader
Buffered
Reader
LineNumber
Reader
COMP201 Topic 6 / Slide 6
Introduction and Overview
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All classes for writing character streams inherit from abstact
class Writer
Writer has one abstract method write(int b), which writes
a unicode charater to an output
Writer
CharArray
Filter
Writer
Writer
OutputStream
Writer
File
Writer
String
Writer
Print
Writer
Piped
Writer
Buffered
Writer
COMP201 Topic 6 / Slide 7
Introduction and Overview
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All classes for reading from byte streams inherit from abstact
class InputStream
InputStream has one abstract method read, which returns
the next byte character or –1 (EOF)
InputStream
ByteArray
Object
Piped
StringBuffer
InputStream InputStream InputStream
InputStream
File
Filter
Sequence
InputStream
InputStream
InputStream
....
Buffered
InputStream
Data
InputStream
LineNumber
InputStream
Pushback
InputStream
COMP201 Topic 6 / Slide 8
Introduction and Overview
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All classes for writing to byte streams inherit from abstact class
OutputStream
OutputStream has one abstract method write(int b),
which writes one byte to an output
OutputStream
ByteArray
File
Piped
OutputStream
OutputStream
OutputStream
Object
Filter
OutputStream
OutputStream
....
Buffered
Data
OutputStream OutputStream
PrintStream
Checked
OutputStream
COMP201 Topic 6 / Slide 9
Reading and Writing local files
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Plan
– Connecting to files: open files
– Reading and writing characters
 Parsing
– Reading and writing objects
COMP201 Topic 6 / Slide 10
Connecting to Files
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Open a file for writing bytes
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FileOutputStream out = new FileOutputStream(“employee.dat”);
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FileOutputStream has method for writing bytes: out.write(int b);
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Seldom write individual bytes
write method used by higher level streams
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Open a file for reading bytes
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FileInputStream in = new FileInputStream(“employee.dat”);
FileInputStream has method for reading bytes in.read();
Seldom read individual bytes
read method used by higher level streams
COMP201 Topic 6 / Slide 11
Writing Characters
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An OutputStreamWriter is a bridge from character streams to byte
streams
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Has method for writing character: void write(int c)
Nesting OutputStreamWriter with FileOutputStream allows us
to write individual characters to files
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OutputStreamWriter out = new OutputStreamWriter (new
FileOutputStream(“employee.dat”) );
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If out.write(‘a’), out.flush() then ‘a’ goes to the file (“employee.dat”).
– Two classes in action here
 One converts ‘a’ into bytes
 One write bytes to file
COMP201 Topic 6 / Slide 12
Writing Characters
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The combination of OutputStreamWriter and
FileOutputStream is commonly used.
A convenience class FileWriter is hence introduced
FileWriter f = new FileWriter(“employee.dat”);
is equivalent to
OutputStreamWriter f = new OutputStreamWriter( new
FileOutputStream(“employee.dat”));
COMP201 Topic 6 / Slide 13
Writing Charaters
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Seldom write characters one by one
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PrintWriter prints formatted representations of objects to a text-output
stream
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Usually write strings. How to write Strings?
Has methods print and println.
Example
PrintWriter out = new PrintWriter( new
FileWriter(“employee.dat”));
 If out.println(“this is a test”),
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–
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then the string “this is a test” goes to file “employee.dat”
Three classes in action here
– PrintWriter breaks the string into characters
– OutputStreamWriter converts characters into bytes
– FileOutputStream writes bytes to file
WriteTextTest.java
COMP201 Topic 6 / Slide 14
Reading Characters
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For writing we have
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FileOutputStream, OutputStreamWriter, FileWriter,
PrintWriter
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For reading we have
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FileInputStream, InputStreamReader, FileReader,
BufferedReader
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BufferedReader has method readLine for reading one line
ReadTextTest.java
COMP201 Topic 6 / Slide 15
Read and Write Standard IO
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System.out predefined PrintStream, stands for screen.
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Print to screen:
l
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System.in predefined InputStream, stands for keyboard.
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System.out.print(); System.out.println();
Nest System.in with BufferedReader
Use method readLine
Example: Echo.java
COMP201 Topic 6 / Slide 16
Parsing
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Reading a text file / Parsing input
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s=readLine() gives you a long string.
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Need to break the string into individual strings and
convert them into proper type.
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To do this, use the StringTokenizer class in
java.util.
Create a StringTokenizer object for the
delimiter “|”
StringTokenizer t = new StringTokenizer(s, “|”);
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COMP201 Topic 6 / Slide 17
Parsing
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Reading a text file / Parsing input
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Use the nextToken method to extract the next piece from
string.
t.nextToken() --- name.
t.nextToken() --- salary as string. Need Double.parseDouble
t.nextToken() --- year as string. Need Integer.parseInt
t.nextToken() --- month as string. Need Integer.parseInt
t.nextToken() --- day as string. Need Integer.parseInt
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Here we know that there are 5 tokens on each line.
In general, call t.hasMoreTokens before each
t.nextToken
Use StreamTokenizer to parse a file
DataFileTest.java
COMP201 Topic 6 / Slide 18
Reading and Writing Objects
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When useful
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Need to save some information and retrieve it later.
Saved information does not have to be human readable.
Why bother (since we can write and read text files)
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Much easier than writing and reading text files
COMP201 Topic 6 / Slide 19
Writing Objects
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To save an object, open an ObjectOutputStream
ObjectOutputStream out =
new ObjectOutputStream (
new FileOutputStream(“employee.dat”));
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Simply use writeObject method to save an object
Employee harry = new Employee("Harry Hacker", 35000,
new Day(1989,10,1));
Manager carl = new Manager("Carl Cracker", 75000,
new Day(1987,12,15));
out.writeObject(harry);
out.writeObject(carl);
COMP201 Topic 6 / Slide 20
Reading Objects
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To get back an object, open an ObjectInputStream
ObjectInputStream in = new ObjectInputStream (
new FileInputStream(“employee.dat”));
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Use readObject method to retrieve objects in the same order in
which they were written
Employee e1 = (Employee) in.readObject();
Employee e2 = (Employee) in.readObject();
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Cast necessary because readObject returns an object of class
Object.
COMP201 Topic 6 / Slide 21
Serializable Interface
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For object writing/reading to work, the class must
implement the serializable interface
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Class Employee implements Serializable(){…}
Since Arrays, one can write/read an array of any objects in
one sentence
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Employee[] staff; …
out.writeObject( staff );
(Employee[])in.readObject();
ObjectFileTest.java
COMP201 Topic 6 / Slide 22
File Management
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Use java.io.File
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Creating a new directory:
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Create a File object
File tempDir = new File( “temp”);
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Create directory
tempDir.mkdir();
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Creating a new file
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Create a File object:
File foo = new File(“dirName”+File.separator + “data.txt”);
File foo = new File(“dirName”, “data.txt”);
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Create file
foo.createNewFile();
COMP201 Topic 6 / Slide 23
File Management
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Inspecting contents of a directory
someDir.list() returns an array of file names under the directory
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Deleting files and directories
someDir.delete();
someFile.delete();
FindDirectories.java