comp201 java programming topic 5: input and output reading: chapter 12
TRANSCRIPT
COMP201 Java Programming
Topic 5: Input and Output
Reading: Chapter 12
COMP201 Topic 6 / Slide 2
Objectives and Outline Objectives:
Overall view i/o in java Reading and writing local files
Outline Introduction and overview Reading and writing local files
– Connecting to files
– Reading and writing characters
– Reading and writing objects File management
COMP201 Topic 6 / Slide 3
Introduction and Overview When talking about IO, we need to consider files
At different locations:– blocks of main memory– local file system– over the net
In different formats:– text or binary– zipped or not zipped
With different access modes:– plain sequential, – buffered, – pushback, – Random
COMP201 Topic 6 / Slide 4
Java streams Input streams:
– Objects from where we read input sequences Output stream:
– Objects where we write output sequences
Java has stream classes allow us to deal with all possible combinations of location, format, and access mode.
In particular, Java streams provide an abstraction of files at different locations 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
Introduction and Overview
COMP201 Topic 6 / Slide 5
Introduction and Overview 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
LineNumberReader
PushbackReader
InputStreamReader
CharArrayReader
FilterReader
BufferedReader
StringReader
PipedReader
FileReader
COMP201 Topic 6 / Slide 6
Introduction and Overview 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
PrintWriter
FileWriter
BufferedWriter
OutputStreamWriter
PipedWriter
FilterWriter
CharArrayWriter
StringWriter
COMP201 Topic 6 / Slide 7
Introduction and Overview 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
PushbackInputStream
LineNumberInputStream
DataInputStream
BufferedInputStream
FileInputStream
ByteArrayInputStream
FilterInputStream
SequenceInputStream
StringBufferInputStream
ObjectInputStream
PipedInputStream
. . . .
COMP201 Topic 6 / Slide 8
Introduction and Overview 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
CheckedOutputStream
PrintStreamData
OutputStreamBuffered
OutputStream
ByteArrayOutputStream
FilterOutputStream
ObjectOutputStream
PipedOutputStream
. . . .
FileOutputStream
COMP201 Topic 6 / Slide 9
Reading and Writing local files Plan
– Connecting to files: open files
– Reading and writing characters Parsing
– Reading and writing objects
COMP201 Topic 6 / Slide 10
Connecting to Files
Open a file for writing bytes FileOutputStream out = new FileOutputStream(“employee.dat”);
FileOutputStream has method for writing bytes: out.write(int b); Seldom write individual bytes write method used by higher level streams
Open a file for reading bytes 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 An OutputStreamWriter is a bridge from character streams to byte
streams Has method for writing character: void write(int c)
Nesting OutputStreamWriter with FileOutputStream allows us to write individual characters to files OutputStreamWriter out = new OutputStreamWriter (new
FileOutputStream(“employee.dat”) ); 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 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 Seldom write characters one by one
Usually write strings. How to write Strings?
PrintWriter prints formatted representations of objects to a text-output stream Has methods print and println.
Example PrintWriter out = new PrintWriter( new
FileWriter(“employee.dat”)); If out.println(“this is a test”),
– 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 For writing we have
FileOutputStream, OutputStreamWriter, FileWriter, PrintWriter
For reading we have FileInputStream, InputStreamReader, FileReader,
BufferedReader
BufferedReader has method readLine for reading one line
ReadTextTest.java
COMP201 Topic 6 / Slide 15
Read and Write Standard IO System.out predefined PrintStream, stands for screen.
Print to screen:l System.out.print(); System.out.println();
System.in predefined InputStream, stands for keyboard. Nest System.in with BufferedReader Use method readLine
Example: Echo.java
COMP201 Topic 6 / Slide 16
Parsing Reading a text file / Parsing input
s=readLine() gives you a long string.
Need to break the string into individual strings and convert them into proper type.
To do this, use the StringTokenizer class in java.util.
Create a StringTokenizer object for the delimiter “|”
StringTokenizer t = new StringTokenizer(s, “|”);
COMP201 Topic 6 / Slide 17
Parsing Reading a text file / Parsing input
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
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 When useful
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) Much easier than writing and reading text files
COMP201 Topic 6 / Slide 19
Writing Objects To save an object, open an ObjectOutputStream ObjectOutputStream out =
new ObjectOutputStream ( new FileOutputStream(“employee.dat”));
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 To get back an object, open an ObjectInputStream ObjectInputStream in = new ObjectInputStream ( new FileInputStream(“employee.dat”));
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();
Cast necessary because readObject returns an object of class Object.
COMP201 Topic 6 / Slide 21
Serializable Interface For object writing/reading to work, the class must
implement the serializable interface Class Employee implements Serializable(){…}
Since Arrays, one can write/read an array of any objects in one sentence Employee[] staff; … out.writeObject( staff ); (Employee[])in.readObject();
ObjectFileTest.java
COMP201 Topic 6 / Slide 22
File Management Use java.io.File Creating a new directory:
Create a File objectFile tempDir = new File( “temp”);
Create directorytempDir.mkdir();
Creating a new file Create a File object:
File foo = new File(“dirName”+File.separator + “data.txt”);
File foo = new File(“dirName”, “data.txt”);
Create filefoo.createNewFile();
COMP201 Topic 6 / Slide 23
File Management Inspecting contents of a directory
someDir.list() returns an array of file names under the directory
Deleting files and directoriessomeDir.delete();
someFile.delete();
FindDirectories.java