Input and Output in Java

Monday, February 10, 2014

Nancy L. Harris

+ Reference for this topic

Java Tutorials I/O

1/20/2012CS239 – Spring 2012

+ Pictorial view of data streams (from the Java tutorial)

+What can we read and write?

Bytes – used for binary data, sounds, pictures

Characters – used for textual data

We will focus on character data

+ What does a “stream” look like

It is not organized as we are used to looking at a “file”.

It is conceptually an infinitely long series of bytes.

Some readers deal with those bytes as text characters.

And each format item (new lines, tabs, spaces) have a corresponding character representation.

+ What’s a file?

A “file” can be thought of as a named bunch of data.

That data can be binary (like executable programs) or it can be textual (like the source files you make with JGrasp).

Text files are still binary, but their data can be directly interpreted as characters from the Unicode character set.

+ Processing a file

To read from a file Open the file Read its data Close the file

To write to a file Open the file Write its data Close the file

+Java classes (note: there are other classes that deal with file IO, but these will serve our purposes in 159)

Input File class Scanner class

Output File class PrintWriter class

1/20/2011CS239 – Spring 2011 8

File: “An abstract representation of file and directory pathnames.” (java api)

Instantiating a File does not “open” the file nor automatically check its existence.

Instantiating a Scanner or PrintWriter object does open the underlying file. FileNotFoundException if the file cannot be found or opened.

+ Making a copy of a file

Involves reading from a source and writing to a target.

Demo

Note:

File I/O requires the handling of “checked” exceptions.

These exceptions must be handled or re-thrown.

+ Exception Classes

Starting Out With JavaControl Structures to ObjectsBy Tony Gaddis

Copyright © 2005,

Pearson Addison-Wesley. All

rights

reserved

.

Chapter 12Slide #10

Object

Throwable

ExceptionError

RuntimeExceptionIOException

FileNotFoundExceptionEOFException

…

…

…

…

+A note about whitespace

Scanner automatically uses “whitespace” to parse input.

We can force it to use something else by the useDelimiter() method.

Scanner can process:

standard input (System.in)

files

lines of text

Its process of reading individual elements is also called parsing or tokenizing.

CS239 – Spring 2011 11

+What is EOF

EOF stands for End Of File and lets the input processor know that we have no more data.

Scanner’s hasNext method returns true if we have not yet reached EOF. False means we have reached the end of the file.

You might see EOF in submit when there is more output expected, but your file ends or vice versa.

1/20/2011

+Today’s Lab

You will build a “main” to read data from a file and use it to build a TextAnalyzer object. You will then call the TextAnalyzer methods to analyze the text you have read in.

The text may have multiple lines. You should read in all lines and build a String that you can send to the TextAnalyzer. To preserve the new lines in the original text, you will need to add them back in as you build the String.

See solution posted to Canvas if you do not have a TextAnalyzer class that you can use.

1/20/2011