Chapter 9-Text File I/O

Overview

Text File I/O and Streams Writing to a file. Reading from a file. Parsing and tokenizing. Random Access Review.

Streams/ Text File I/O

Why File I/O?

Users rarely do all of their computing in one sitting. Even if they do finish, they want to save the results.

In all of our programs so far, the results are lost when our program ends.

If we use file I/O, we can save information for future use, read in setup information, etc.

What are streams?

Streams are basically just data that is waiting for processing.

They can be used for file I/O, network I/O, and any other way that two programs might want to communicate with each other.

We have been using Streams whenever we output to the screen or take in from the keyboard.

Using streams.

When we System.out.println a string, that string is written to an output stream. When the computer is ready, that stream is flushed out to the monitor.

When we type things as input into the keyboard, it goes into an input stream. SavitchIn then reads from this stream when we want input from the user.

Text I/O vs. Binary I/O

The book describes two kinds of I/O, only one of which we will be covering.

Text I/O writes out information to a file in such a way that we can read it with a standard text editor like Notepad or Word.

Binary I/O writes things out in merely 1’s and 0’s and is usually not easily readable by humans.

When using I/O

When using any of the file I/O methods or classes you will want to include the java.io library.

import java.io.*;

Writing to a file.

Writing to a file

We will first look at writing information to a text file, since it is easier than reading.

The first thing that we need to do is open a file. We do this by creating a PrintWriter object.PrintWriter output = new PrintWriter(

new FileOutputStream(“Somefile.txt”));

Streams and output files

Java has a bunch of I/O streams, and we often need to layer these for doing I/O operations. You can do it in separate steps or in a single one.

FileOutputStream a = new FileOutputStream(“Somefile.txt”);

PrintWriter output = new PrintWriter(a);

Or

PrintWriter output = new PrintWriter(new FileOutputStream(“Somefile.txt”));

More on opening files

The PrintWriter is an output stream that allows for the usual kind of output we are used to doing with a console.

When opening any file(reading or writing), you will need to be able to catch the FileNotFoundException that can be thrown if the file requested does not exist. Use try and catch blocks.

The delicate exposition of nonexistence... Now that we have the file open, we can

write to it at our leisure. Instead of writing strings to the console

with System.out.println(“Hello”), we now replace System.out with “output” (or the name of our stream).

output.println(“Hello”);

Writing other types

We can write anything we could write to the screen. Integers, doubles, anything.

int I = 5;double j = 4.0;output.println(“This is a number: “ + I); //Writes “This is a number: 5\n” to Somefile.txtoutput.println(“So is this: “ +4.0); //Writes “So is this: 4.0\n” to Somefile.txt

Always put things back the way you found them. When you are finished writing to your

file, you want to close it. If you don’t close it, someone else that needs the file may not be able to open it.

If you don’t close it, Java will close it for you when your program ends normally(if it ends normally).

output.close();

Notes about basic writing.

Can also use the print(String) function to print without an automatic new line.

When you open a file for writing, any current contents of the file are erased.

You can also open a file in append mode if you would like.

Opening in append mode

Append mode just means the file opens pointing at the end of the file as opposed to the beginning.

PrintWriter a = new PrintWriter(new FileOutputStream(“Somefile”, true));

Says to append(omit or usefalse to just overwrite)

Reading from a file.

Reading from a file.

Writing to a file is nice, but eventually we will probably want to retrieve that information back into the Java program.

So we need the ability to read from files. Reading strings is easy. Reading other

types is hard (just like with console input, except there is no SavitchIn to use as a shortcut).

Opening a file for reading.

Again we will layer streams, but this time we use input readers.

BufferedReader input = new BufferedReader(new FileReader(“Somefile.txt”));

Reading Strings from files...

Once you have opened the file, you can use readLine() to read in the next new-line delimited string.

String a = input.readLine();

When there are no strings left.

When you have already read all of the strings in the file, readLine() will return the value null, so you should always test for this before using the string.

String a = input.readLine();if(a != null)

System.out.println(a);

Close the file

Just like when writing, we should close the file when we are done with it. Looks the same too.

input.close();

That’s great, but what about non-strings? When we use readLine(), we are returned a

line of information as a String. This line can have all kinds of info inside of it:

integers, doubles, characters, booleans, objects(possibly).

To retrieve this information, we need to parse the string given to us by readLine().

Parsing and tokenizing

Parsing- What is it?

The general idea of parsing is to find all the useful information that we want that is contained in a String.

Thus in the string:

we might want the 3, the 4, and the + operator, but not the spaces and the new line.

3 + 4 \n

Parsing- the simple case

If we have a string that merely has a single piece of data in it, then we can use the parse method of the wrapper class.

String s = “3.40”;int I = Integer.parseInt(“5”); //I=5double D = Double.parseDouble(s);//D=3.40

Parsing exceptions

Any time you use one of the number parsing methods, you will need to be ready to catch a NumberFormatException.

Parsing types

Integers- Integer.parseInt(String);Doubles- Double.parseDouble(String);Floats- Float.parseFloat(String);Longs- Long.parseLong(String);Booleans- Boolean.valueOf(String);

//true if “true” //false otherwise.

Characters- String.charAt(int);//”Hello”.charAt(0) = ‘H’

It’s not always that simple...

Unfortunately, all of those parsing methods required that the data be the only thing in the string.

Often the Strings returned by readLine() have more info than we need.

We need a way to break the big string returned by readLine() into smaller strings that each have just one piece of data in them.

This is tokenizing.

StringTokenizer class

The StringTokenizer class is located in java.util, so you will need to import java.util.* or java.util.StringTokenizer.

When you create a StringTokenizer object, you give it the string you want to break up into pieces.

StringTokenizer blar = new StringTokenizer(“This is some stuff”);

StringTokenizer

The default way for a StringTokenizer to break up a word is by whitespace.

We can tell how many pieces are left in the string by calling countTokens() or just see if there are more by using hasMoreTokens().

We get the next chunk by calling nextToken().

while(blar.hasMoreTokens()){

System.out.println(blar.nextToken())}

Getting tokenswhile(blar.hasMoreTokens()){

System.out.println(blar.nextToken());}

Thisissomestuff

StringTokenizer details

You should always make sure there are more tokens available before trying to get one. If there aren’t any left, the method will throw an exception and likely crash your program.

You can use different delimiters than whitespace if you want to. See the Java documentation on how to do that.

You can also have the delimiters themselves be returned as separate tokens. Again see the documentation.

Random Access

RandomAccessFile

If you are used to the old way of dealing with files (read, write, seek), you can still do this with RandomAccessFile.

RandomAccessFile allows you to both read and write inside a file. You can also skip back and forth through the file to find what you want.

Opening and closing RandomAccessFilesRandomAccessFile file = new RandomAccessFile(“Something.txt”, “rw”);

//opened for read-write access.//could also just use “r” for read access.

...file.close();

Other useful functions inside of RandomAccessFile You can read(), readLine(), readInt(),

readDouble(),readChar(),readBoolean(). You can get the length() of a file, or

seek(long) to a position. Writing is not as easy:

writeDouble(double), writeInt(int), writeBoolean(boolean), writeBytes(String), writeChars(String).

When to use RandomAccessFile

We don’t usually want to use this class, unless you just need to slightly modify a file.

Usually want to use two separate files, one for reading, and one for writing. Most examples I have seen in the books are used this way.

Review

File I/O Review

What do we have to import to be able to do file I/O?

What do we have to import to do tokenizing? What do we have to import to use the parse

methods of the primitive wrappers (Double.parseDouble(String)…)?