Transcript Classes and Methods

Title Slid
CSC 444
Java Programming
Input & Output
By
Ralph B. Bisland, Jr.
Warning
Output in Java is UGLY
Input in Java Is Very,
Very UGLY
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Stream Classes
• Stream: Any input source or output
destination.
• Java provides the Stream classes:
BufferedReader and PrintWriter for
input and output respectively.
• Constructor in BufferedReader
public BufferedReader (Reader in);
• Constructor in PrintWriter
public PrintWriter (OutputStream out,
boolean autoFlush);
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Buffered Reader Hierarchy Diagram
Object
Reader
BufferedReader
InputStreamReader
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BufferedReader
public BufferedReader (Reader in);
To instantiate an object of type
BufferedReader it is necessary to supply the
constructor with the correct type
(Reader).
Note that Reader is a superclass of
InputStreamReader
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Buffered Reader (ctd)
Remember that an object lower down a class
hierarchy to be passed as an argument to a
method that requires a parameter of a class
further up the class hierarchy.
Can substitute type Reader with the type
InputStreamReader.
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Buffered Reader (ctd)
Constructor from the class
InputStreamReader:
public InputStreamReader
(InputStream in);
Now we need to find a constant that
represents the standard input stream.
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Buffered Reader (ctd)
Hooray!!!!
The class java.lang.System contains the
constant in of type InputStream that
represents the standard input stream and
corresponds to the keyboard.
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Buffered Reader (ctd)
Instantiate an object of type
InputStreamReader as follows:
InputStreamReader stream =
new InputStreamReader (System.in);
The object stream may be used to instantiate
the object keyboard as follows:
BufferedReader keyboard =
new BufferedReader (stream);
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Buffered Reader (ctd)
Combining the two statements into one,
we get:
BufferedReader keyboard = new
BufferedReader(new
InputStreamReader(System.in));
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Print Writer Hierarchy Diagram
Object
OutputStream
Writer
FilterOutputStream
PrintWriter
PrintStream
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Print Writer (ctd)
public PrintWriter
(OutputStream out,
boolean autoFlush);
Looking at the hierarchy diagram, notice that
a class of type OutputStream may be
substituted for a class of type PrintStream.
Why make the substitution?
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Print Writer (ctd)
The class java.lang.System contains a
constant out of type PrintStream that
represents the standard output stream, and
corresponds to the screen.
The boolean value autoFlush causes the
output buffer to be flushed if it is set to true.
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Print Writer (ctd)
Instantiate an object of type PrintWriter as
follows:
PrintWriter screen =
new PrintWriter(System.out, true);
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Input and Output
• The class BufferedReader contains a
method readLine that will allow the
reading of data type String from the
keyboard, by using
keyboard.readLine().
• You may only read data of type String
using readLine.
• How do you read data if type int, float,
long, etc.?
• Input the number as a string and use the
appropriate wrapper class to do the
conversion.
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Reading String Data
• Since all input is read as a string, reading
string data is fairly simple.
• Before reading data into a string we must
set up for a buffered read.
BufferedReader keyboard = new
BufferedReader (new
inputStreamReader (System.in));
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Reading String Data (ctd)
• When we want to read into a string
variable we use the readLine
method.
String name;
name = keyboard.readLine();
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Reading Numeric Data
To read a value and convert it we must use
the Wrapper class.
Integer class has a constructor method,
Integer (String S)that converts a String
to an Integer.
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Reading Numeric Data (ctd)
Example: new Integer
(keyboard.readLine()) creates an object
of type Integer.
Before converting we must set up for a
buffered read.
BufferedReader keyboard =
new BufferedReader
(new inputStreamReader (System.in))
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Reading Numeric Data
To convert read an input string and convert
it an int.
BufferedReader keyboard =
new BufferedReader
(new inputStreamReader(System.in));
int number;
number = new Integer
(keyboard.readline()).intValue();
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Reading Numeric Data (ctd)
To read a string and convert it to a floating
point:
fltnumber = new Float
(keyboard.readLine()).floatValue();
Remember all primitives have wrapper
classes.
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Outputting Data
Outputting data is much simpler than
inputting data.
The class PrintWriter contains methods
for displaying data as strings.
The methods are: print, println, and
flush.
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Outputting Data (ctd)
Before anything is displayed the screen
Display must be set up.
PrintWriter screen =
new PrintWriter
(System.out, true);
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Outputting Data (ctd)
To display a string on the screen and then
move the cursor to the next output line.
screen.println (“Guardez loo”);
To display a string on the screen and leave
the cursor at the end of the line.
screen.print (“Hold ye hand”);
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Outputting Data (ctd)
To display the current contents of the
buffer (flush the buffer).
screen.flush();
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Outputting Data (ctd)
Strings may be output using Unicode format:
screen.println
(“\u0041\u0042\u0043”);
Escape sequences may be used to format
data:
\t = Display data at the next tab stop.
\n= New line
screen.println (“Ralph\nBisland”);26
Outputting Data (ctd)
To display string variables:
String name = “Ginny Fleck”;
screen.println (name);
Strings can be concatenated:
screen.print(“Hi there: “
+ name); screen.printLn();
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Outputting Data (ctd)
Primitives can be converted to strings by
concatenation;
int number = 5;
screen.println (+ number);
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Outputting Data (ctd)
Primitives may be concatenated with strings
to produce output.
float netPay = 1000.0f;
screen.println (“Net pay = $” +
netPay);
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A Quick and Dirty Print
Can still use the print or println method from
the System package to write data out.
System.out.println (“Guardez des loo!!”);
System.out.print (“Hold “)
System.out.print (“ye “);
System.out.println (“hand!”);
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Simple Problem
Write a Java program to read a string of characters from the
Keyboard, print out the input string, convert the string to an
array of characters, print out the array of characters, then convert
The array of characters back to a string and display the new string.
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SimpleStringRead.java
orca% cat SimpleStringRead.java
import java.io.*;
class SimpleStringRead
{
static PrintWriter scr =
new PrintWriter(System.out,true);
static BufferedReader kb = new
BufferedReader
(new InputStreamReader(System.in));
public static void main (String [] args)
throws IOException
{
String str;
scr.print ("Enter a string: ");
scr.flush();
str = kb.readLine();
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SimpleStringRead.java (ctd)
scr.println ("String: " + str);
//
// Convert string to an array of characters
//
char[] aoc = str.toCharArray();
for (int i = 0; i < aoc.length; i++)
scr.println ("aoc[" + i + "] = " + aoc[i])
//
// Convert array back to a string
//
String str2 = String.valueOf(aoc);
scr.println ("Converted string: " + str2);
}
}
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SimpleStringRead.java (ctd)
orca% javac SimpleStringRead.java
orca% java SimpleStringRead
Enter a string: abcde
String: abcde
aoc[0] = a
aoc[1] = b
aoc[2] = c
aoc[3] = d
aoc[4] = e
Converted string: abcde
orca%
34
Reading From Files
• Instead of reading from the keyboard we
now want to be able to read from an
ASCII text file.
• A file must be “opened” before it can be
read from.
• The class FileReader contains a
constructor that requires the pathname
(path and filename) of an input file.
FileReader file = new FileReader
(“indata.dat”);
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Notes On UNIX Filenames
• UNIX filenames must be specified in the
proper case.
• UNIX filenames may be specified in
another directory.
(“~bisland/java/data/myfile.da
t”)
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Notes On PC File Names
• PC files may be specified in other
subdirectories and/or disks.
(“a:\\java\\data\\myfile.dat”)
• Note the use of double backslashes to
avoid confusion with escape sequences.
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Accessing Data From A File
• To provide the same method that was used
with keyboard input (readLine) it is
necessary to use the object file in the
constructor of the BufferedReader; thus
creating a stream object inputFile that is
associated with reading from a disk file
with the pathname (“mydata.dat”).
BufferedReader inputFile = new
BufferedReader (file);
• The instance method to close a file defined
in the class BufferedReader is close().
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Sample Program
• Write a Java program to read in
student’s names and GPA’s, stored in
the UNIX file called stu.dat and
display them on the screen.
• The data file is formatted as follows:
GPA of student1
name of student 1
GPA of student 2
name of student 2
. . .
0
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The Solution
// Documentation goes here
import java.io.*;
class Gpas
{
static PrintWriter scr = new PrintWriter
(System.out, true);
public static void main (String [] args)
throws IOException
{
FileReader file = new FileReader
(“mydata.dat”);
BufferedReader inputFile = new
BufferedReader (file);
String name;
float gpa;
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Program (ctd)
gpa = new
Float(inputFile.readLine()).floatValue();
while (gpa != 0.0f)
{
name = inputFile.readLine();
scr.printLn(gpa + “\t” + name);
gpa = new
Float(inputFile.readLine()).floatValue();
}
inputFile.close();
}
}
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Writing Data Out To Files
• The class FileWriter contains a constructor
that requires the pathname of an output
file.
FileWriter file2 = new
FileWriter (“outfile.dat”);
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Writing Data Out to Files
• To provide the same methods that were
used with screen output (print, println,
flush) it is necessary to use the object
file2 in the constructor of the
PrintWriter; thus creating a stream
object outputFile that is associated
with the writing to the disk file with the
pathname “outfile.dat”.
PrintWriter outputFile = new
PrintWriter (file2);
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Sample Program
• Write a Java program to read a UNIX text
file and copy it’s contents to another UNIX
text file.
• The input file is called “input.dat”.
• The output file is called “output.dat”.
• The last line of the input file contains the
word STOP.
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The Program
// Insert documentation here
import java.io.*;
class ReadWrite
{
public static void main (String [] args)
throws IOException;
{
FileReader file1 = new FileReader
(“input.dat”);
BufferedReader inputFile = new
BufferedReader (file1);
FileWriter file2 = new FileWriter
(“output.dat”);
PrintWriter outfile = new
PrintWriter (file2);
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The Program (ctd)
String inline;
inline = inputFile.readLine();
while (!inline.equals(“STOP”)
{
outputFile.println (inline);
inline = inputFile.readLine();
}
inputFile.close();
outputFile.close();
}
}
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Tokenizing
• Text files with one data item per input line
are generally inefficient and useless.
• A token is a data item delimiter by some
character(s). Examples: Space(s), comma,
End of line marker, etc.
• Whitespace is defines as either: a space
(\0020), horizontal tab (\u0009), new line
(\u000A), vertical tab (\u000B), or form
feed (\u000C).
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Tokenizing (ctd)
• To read multiple data items from a
single line, we can use methods from
the class StringTokenizer in the
package java.util.
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StringTokenizer Class
public class StringTokenizer implements
enumeration
{
public StringTokenizer (String str,
String delim, boolen returnTokens);
public StringTokenizer (String str,
String delim);
public Stringtokenizer (String str);
public boolean hasMoretokens();
public String nextToken();
public String nexttoken(String delim);
public boolean hasMoreElements();
public Object nextElement();
public int countTokens();
}
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Using StringTokenizer
• To use StringTokenizer, it must be
instantiated from the input string.
StringTokenizer data = new
StringTokenizer(inputFile.readLine());
• A line can only be split into tokens if we
know what the delimiting character is for
each token.
• Example: Lines of text are delimited by a
return character and data items are
separated by a blank space.
• If the token delimiter is not specified, the
delimiter is assumed to be whitespace. 50
Using StringTokenizer (ctd)
• Both nextToken methods return a token
of type String.
• The method nextToken() assumes a
delimiter of whitespace.
String token = data.nextToken();
• The method nextToken (String delim)
allows the user to specify the delimiter.
String token = data.nextToken(“\u000A”);
• The method countTokens() returns the
number of tokens in a string that are
delimited by whitespace.
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Example Program
• Read in two tokens from the keyboard.
• The tokens are separated by whitespace.
• Print out the original string containing the
input line.
• Print out each taken on a separate output
line.
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StringTokenizerRead.java
orca% cat StringTokenizerRead.java
import java.io.*;
import java.util.*;
class StringTokenizerRead
{
static PrintWriter scr =
new PrintWriter(System.out,true);
static BufferedReader kb = new
BufferedReader
(new InputStreamReader(System.in));
//---------------------------------------------
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StringTokenizerRead.java (ctd)
public static void main (String [] args)
throws IOException
{
String data = kb.readLine();// Read the line
scr.println (data);
// Print the line
StringTokenizer st =
new StringTokenizer (data); // Tokenize
String buf = st.nextToken();// Get 1st token
scr.println (buf); // Print 1st token
buf = st.nextToken(); // Get next token
scr.println (buf);
// Print next token
}
}
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StringTokenizerRead.java(ctd)
orca% javac StringTokenizerRead.java
orca% java StringTokenizerRead
one two
one two
one
two
orca%
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Example Program
Write a Java program to read a text file and
count the number of words in file.
A word is a group of characters delimited by
whitespace.
End of file is indicated by two return
characters.
Upon reading the first return, the number of
tokens will be zero.
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Program
//Documentation goes here
import java.io.*;
import java.util.*;
class CountWords
{
static PrintWriter scr =
new PrintWriter (System.out, true);
public static void main (String[] args
throws IOException
{
FileReader file = new FileReader(“indata.dat”);
BufferedReader inputFile = new
BufferedReader (file);
StringTokenizer data;
int numberOfTokens;
int numberOfWords = 0;
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Program (ctd)
data = new
StringTokenizer(inputFile.readLine());
numberOfTokens = data.countTokens();
while (numberOfTokens != 0)
{
for (int words=1; words <= numberOfTokens;
words++)
{
scr.print(data.nextToken() + “ “);
scr.flush();
}
scr.printLn();
numberOfwords=numberOfWords+numberOfTokens;;
data = new
StringTokenizer(inputFile.readLine());
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Program (ctd)
numberOfTokens = data.countTokens()
}
scr.println(“\nThere are “+numberOfWords+
” words in the file.”);
}
}
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