text-file i/o

Text-File I/O

Text-File Output with PrintWriter

The class PrintWriter is the preferred stream class for writing a text file.

PrintWriter contains a method called println, like the method of the same name we used as System.out.println except it sends output to a text file instead of the screen.

A text file is opened with a statement like:outputStream =

new PrintWriter(new FileOutputStream(“out.txt”));

Text-File Output with PrintWriter (cont’d)

The above statement connects the stream named outputStream to the file named out.txt

Making this connection is referred to as opening the file.

When you connect a file to a stream in this way, your program always starts with an empty file.

If the file already exists, the old contents are lost.

Example of File I/Oimport java.io.*;import java.util.*;

public class TextFileOutputDemo{ public static void main(String[] args) { PrintWriter outputStream = null; try { outputStream = new PrintWriter(new FileOutputStream("out.txt")); } catch(FileNotFoundException e) { System.out.println("Error opening the file out.txt."); System.exit(0); }

System.out.println("Enter three lines of text:"); String line = null; Scanner keyboard = new Scanner(System.in); int count; for (count = 1; count <= 3; count++) { line = keyboard.nextLine( ); outputStream.println(count + " " + line); } outputStream.close( ); System.out.println("Those lines were written to out.txt."); }}

A File Has Two Names

One name is the real name of the file that is used by the operating system.

The other name is the name of the stream that is connected to the file. This is the name your program uses when it refers to the file.

How you can name the real file depends on your operating system. The name must be a legal name in that system.

FileNotFoundException

This is a poorly named exception. When it is thrown when you are trying to

open a file for output, it doesn’t really mean it could find the file.

It means that for some reason it could not open the file. It might mean that the name was already used for a directory (folder) name, for example.

Closing a File

When your program is finished writing to a file, it should close the stream connected to it, e.g., outputStream.close();

The class PrintWriter, and every other class for file output or file input streams, has a method named close.

When this method is invoked, the system releases any resources used to connect the stream to the file.

Closing a File (cont’d)

If your program does not close a file before the program ends, Java will close it for your as long as your program ends normally.

If your program ends abnormally with a file still open, the file could be damaged.

It is best to close files in your program as soon as they are no longer needed.

Overwriting a File

When you connect a stream to a text file by using the class PrintWriter in the following way, you always produce an empty file:PrintWriter outputStream =

new PrintWriter(new FileOutputStream(“out.txt”));

If there is no file named out.txt, one will be created.

If out.txt already exists, the existing file will be eliminated and a new empty file will be created.

Appending to a File

If, however, you want append to an existing file, you can connect the file to the output stream as follows:outputStream =

new PrintWriter(new FileOutputStream(“out.txt”, true));

If the file out.txt does not already exist, Java will create an empty file of that name and append the output to the end of this empty file.

If the file out.txt already exists, the old contents will remain and the program’s output will be placed after those contents.

Use toString for Text-File Output

It is common to include a method toString() in classes.

This method should produce a string value that intuitively represents the data in an object.

The method toString has a special property. If you do not include it in an invocation of System.out.println, it is invoked automatically.

Example using a toString Method

import java.io.*;import java.util.*;

/** Class for data on endangered species. This class is serialized.*/public class Species implements Serializable{ private String name; private int population; private double growthRate;

public Species( ) { name = null; population = 0; growthRate = 0; }

public Species(String initialName, int initialPopulation, double initialGrowthRate) { name = initialName; if (initialPopulation >= 0) population = initialPopulation; else { System.out.println("ERROR: Negative population."); System.exit(0); } growthRate = initialGrowthRate; }

Example using a toString Method (cont’d)

public String toString() { return ("Name = " + name + "\n" + "Population = " + population + "\n" + "Growth rate = " + growthRate + "%"); }

public void readInput( ) { Scanner keyboard = new Scanner(System.in); System.out.println("What is the species' name?"); name = keyboard.nextLine( );

System.out.println( "What is the population of the species?"); population = keyboard.nextInt( ); while (population < 0) { System.out.println("Population cannot be negative."); System.out.println("Reenter population:"); population = keyboard.nextInt( ); }

System.out.println( "Enter growth rate (percent increase per year):"); growthRate = keyboard.nextDouble( ); }


public void writeOutput( ) { System.out.println("Name = " + name); System.out.println("Population = " + population); System.out.println("Growth rate = " + growthRate + "%"); }

/** Precondition: years is a nonnegative number. Returns the projected population of the calling object after the specified number of years. */ public int projectedPopulation(int years) { double populationAmount = population; int count = years; while ((count > 0) && (populationAmount > 0)) { populationAmount = (populationAmount + (growthRate/100) * populationAmount); count--; } if (populationAmount > 0) return (int)populationAmount; else return 0; }

}


public void set(String newName, int newPopulation, double newGrowthRate) { name = newName; if (newPopulation >= 0) population = newPopulation; else { System.out.println("ERROR: using a negative population."); System.exit(0); } growthRate = newGrowthRate; }

public String getName( ) { return name; }

public int getPopulation( ) { return population; }

public double getGrowthRate( ) { return growthRate; }

public boolean equals(Species otherObject) { return ((name.equalsIgnoreCase(otherObject.name)) && (population == otherObject.population) && (growthRate == otherObject.growthRate)); }

Text-File Input with BufferedReader

The class BufferedReader is the preferred stream class for reading from a text file.

To open a file for reading use the statement:BufferedReader Stream_Name =

new BufferedReader(new FileReader(File_Name));

If your program attempts to open a file for reading, but there is no such file, then a FileNotFoundException is thrown

Text-File Input with BufferedReader (cont’d)

BufferedReader has no methods that, like nextInt, can read a number.

The only way that you can use the class BufferedReader to read a number from a text file is to read it as a string and then convert the string to a number.

The class BufferedReader does have a method, named simply read, that will read a single character, however, it read it as an integer. To get the character you should write:

char next = (char)(inputStream.read());

Example Using the Class BufferedReader

import java.io.*;

public class TextFileInputDemo{ public static void main(String[] args) { try { BufferedReader inputStream = new BufferedReader(new FileReader("data.txt")); String line = null; line = inputStream.readLine( ); System.out.println("The first line in data.txt is:"); System.out.println(line); line = inputStream.readLine( ); System.out.println("The second line in data.txt is:"); System.out.println(line); inputStream.close( ); } catch(FileNotFoundException e) { System.out.println("File data.txt was not found"); System.out.println("or could not be opened."); } catch(IOException e) { System.out.println("Error reading from file data.txt."); } }}

Reading a File Name from the Keyboard

import java.io.*;import java.util.*;

public class TextFileInputDemo2{ public static void main(String[] args) { System.out.println("Enter file name:"); Scanner keyboard = new Scanner(System.in); String fileName = keyboard.next( );

try { BufferedReader inputStream = new BufferedReader(new FileReader(fileName)); String line = null; line = inputStream.readLine( ); System.out.println("The first line in " + fileName + " is:"); System.out.println(line); line = inputStream.readLine( ); System.out.println("The second line in " + fileName + " is:"); System.out.println(line); inputStream.close( ); } catch(FileNotFoundException e) { System.out.println("File " + fileName + " not found."); } catch(IOException e) { System.out.println("Error reading from file " + fileName); } }}

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