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You can write data to a file instead of the computer screen. You can write certain data to a file while still putting other data on the screen. Or you may need access to multiple files simultaneously. Or you may want to query the user for input rather than accepting it all on the command line. Or maybe you want to read data out of a file that's in a particular format. In Java all these methods take place as streams. < > Using File I/O streams. The
System.out.println() statement we've been using all along is an implementation of Streams.A program that writes a string to a file
In order to use the Java file classes, we must import the Java input/output package (java.io) in the following manner
import java.io.*;
Inside the main method of our program, we must declare a FileOutputStream object. In this case, we wish to write a string to the file, and so we create a new PrintStream object that takes as its constructor the existing FileOutputStream. Any data we send from PrintStream will now be passed to the FileOutputStream, and ultimately to disk. We then make a call to the println method, passing it a string, and then close the connection.
import java.io.*;
Inside the main method of our program, we must declare a FileOutputStream object. In this case, we wish to write a string to the file, and so we create a new PrintStream object that takes as its constructor the existing FileOutputStream. Any data we send from PrintStream will now be passed to the FileOutputStream, and ultimately to disk. We then make a call to the println method, passing it a string, and then close the connection.
Source Code
/*
* FileOutput
* Demonstration of FileOutputStream and PrintStream classes
*/
import java.io.*;
class FileOutput
{
public static void main(String args[])
{
FileOutputStream out; // declare a file output object
PrintStream p; // declare a print stream object
try
{
// Create a new file output stream connected to "myfile.txt"
out = new FileOutputStream("myfile.txt");
// Connect print stream to the output stream
p = new PrintStream( out );
p.println ("This is written to a file myFile.txt");
p.close();
}
catch (Exception e)
{
System.err.println ("Error writing to the file myFile.txt");
}
}
}
* FileOutput
* Demonstration of FileOutputStream and PrintStream classes
*/
import java.io.*;
class FileOutput
{
public static void main(String args[])
{
FileOutputStream out; // declare a file output object
PrintStream p; // declare a print stream object
try
{
// Create a new file output stream connected to "myfile.txt"
out = new FileOutputStream("myfile.txt");
// Connect print stream to the output stream
p = new PrintStream( out );
p.println ("This is written to a file myFile.txt");
p.close();
}
catch (Exception e)
{
System.err.println ("Error writing to the file myFile.txt");
}
}
}
Interactively communicating with the user
Source Code
import java.io.*;
class PersonalHello {
public static void main (String args[])
{
byte name[] = new byte[100];
int nr_read = 0;
System.out.println("Your name Please?");
try {
nr_read = System.in.read(name);
System.out.print("Hello ");
System.out.write(name,0,nr_read);
}
catch (IOException e) {
System.out.print("I did not get your name.");
}
}
}
First we print a query requesting the user's name. Then we read the user's name using the
System.in.read() method. This method takes a byte array as an argument, and places whatever the user types in that byte array. Then, like before, we print "Hello." Finally we print the user's name.
The program doesn't actually see what the user types until he or she types a carriage return. This gives the user the chance to backspace over and delete any mistakes. Once the return key is pressed, everything in the line is placed in the array.
Reading Numbers
The
getNextInteger() method that will accept an integer from the user. Here it is: static int getNextInteger() {
String line;
DataInputStream in = new DataInputStream(System.in);
try {
line = in.readLine();
int i = Integer.valueOf(line).intValue();
return i;
}
catch (Exception e) {
return -1;
}
} // getNextInteger ends hereReading Formatted Data
Writing a text file
Sometimes you want to save your output in a file. To do this we'll need to learn how to write data to a file.
Source Code
// Write the Fahrenheit to Celsius table in a file
import java.io.*;
class FahrToCelsius {
public static void main (String args[]) {
double fahr, celsius;
double lower, upper, step;
lower = 0.0; // lower limit of temperature table
upper = 300.0; // upper limit of temperature table
step = 20.0; // step size
fahr = lower;
try {
FileOutputStream fout = new FileOutputStream("test.out");
// now to the FileOutputStream into a PrintStream
PrintStream myOutput = new PrintStream(fout);
while (fahr <= upper) { // while loop begins here
celsius = 5.0 * (fahr-32.0) / 9.0;
myOutput.println(fahr + " " + celsius);
fahr = fahr + step;
} // while loop ends here
} // try ends here
catch (IOException e) {
System.out.println("Error: " + e);
System.exit(1);
}
} // main ends here
}- Open a FileOutputStream using a line likeThis line initializes the FileOutputStream with the name of the file you want to write into.
FileOutputStream fout = new FileOutputStream("test.out"); - Convert the FileOutputStream into a PrintStream using a statement like
PrintStream myOutput = new PrintStream(fout);The PrintStream is passed the FileOutputStream from step 1. - Instead of using
System.out.println()usemyOutput.println().System.outandmyOutputare just different instances of thePrintStreamclass. To print to a differentPrintStreamwe keep the syntax the same but change the name of thePrintStream.
Reading a text file
// Imitate the Unix cat utility
import java.io.*;
class cat {
public static void main (String args[]) {
String thisLine;
//Loop across the arguments
for (int i=0; i < args.length; i++) {
//Open the file for reading
try {
FileInputStream fin = new FileInputStream(args[i]);
// now turn the FileInputStream into a DataInputStream
try {
DataInputStream myInput = new DataInputStream(fin);
try {
while ((thisLine = myInput.readLine()) != null) { // while loop begins here
System.out.println(thisLine);
} // while loop ends here
}
catch (Exception e) {
System.out.println("Error: " + e);
}
} // end try
catch (Exception e) {
System.out.println("Error: " + e);
}
} // end try
catch (Exception e) {
System.out.println("failed to open file " + args[i]);
System.out.println("Error: " + e);
}
} // for end here
} // main ends here
} 
