Character Input and Output in Java

Character Input and Output in Java
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Review of Console I/O

Standard Output:
- System.out and System.err are PrintStream objects that have print(), println(), and printf() methods
Standard Input:
- System.in is an InputStream object that can be used to read bytes

General Character-Based I/O

Input:
- Wrap any InputStream in a Scanner object that can be used to read double and int values and String objects or a BufferedReader that can be used to read String objects
Output:
- Wrap any output stream in a PrintWriter
Both:
- Call close() when done

Obtaining Other Input/Output Streams

The Most Common Channel:
- Files
Other Important Channels:
- Network connections
- Memory
- String objects

Input with a Scanner

Important Constructors:
- Scanner(java.io.InputStream)
- Scanner(java.io.File)
Some Relevant Methods:
- Scanner.useDelimiter(java.lang.String)
- Scanner.hasNextDouble() , Scanner.hasNextInt() , and Scanner.hasNextLine()
- Scanner.nextDouble() , Scanner.nextInt() , and Scanner.nextLine()

Input with a Scanner (cont.)

An Important Issue:
- The methods that read "up to" a delimiter do not consume newline characters
A "Work Around":
- In some circumstances (when an int is the only thing on a line), you will need to invoke nextLine() immediately after invoking one of these methods

Input with a BufferedReader

The Steps::
1. Construct a BufferedReader from an InputStreamReader or FileReader
2. Read using the readLine() method (which throws IOException)
3. Break the String object into parts (i.e., tokenize)
4. Convert the parts (as necessary)
Two Approaches to Tokenizing:
- StringTokenizer
- String.split(java.lang.String)
Converting:
- Double.parseDouble(java.lang.String) and Integer.parseInt(java.lang.String)

Tokenizing Input

Tokenizing with split():
- Is very convenient when every record contains every field
- The length of the array can be used to handle situations with too few fields (but can't be used to identify which is missing)
Tokenizing with a StringTokenizer:
- Is somewhat less convenient when every record contains every field (because it requries multiple calls to nextToken()
- nextToken() throws NoSuchElementException which can be used to handle situations with too few fields
- Can be told to return the delimiters so can be used to detect specific missing fields

An Example that Uses a Scanner (cont.)

javaexamples/iobasics/ScannerExample.java

An Example that Uses a BufferedReader (cont.)

javaexamples/iobasics/BufferedReaderExample.java

Reading Until End-of-Stream

Using a while Loop

javaexamples/iobasics/EndOfStreamExample.java (Fragment: while)

Reading Until End-of-Stream (cont.)

Using a do-while Loop

javaexamples/iobasics/EndOfStreamExample.java (Fragment: do)

Reading Until End-of-Stream (cont.)

Using an Elegant Idiom

javaexamples/iobasics/EndOfStreamExample.java (Fragment: elegant)

Output with a PrintWriter

Unformatted Output:
- PrintWriter.print(java.lang.String) prints the String (i.e., sends it to the underlying OutpuStream)
- PrintWriter.println(java.lang.String) prints the String followed by a '\n'
An Important Point:
- Text sent to an output stream may be buffered
- To force text through the buffer use PrintWriter.flush()
- This is particularly important to be aware of when debugging

Output with a PrintWriter (cont.)

Format Strings:
- Encapsulated in the Formatter class
One Way to use Format Strings:
- Use the PrintWriter.printf(java.lang.String, java.lang.Object...) method to format the output

Formatting Output (cont.)

Another Way to Use Format Strings:
- Convert everything into a formatted String before output
An Important Method:
- The String.format(java.lang.String, java.lang.Object...) method will do most of what you need (and works like printf())

Going Further

: Other Ways to Format Output

Some Useful Classes:
- NumberFormat
- ChoiceFormat
- MessageFormat
An Example:
- javaexamples/iobasics/ChoiceFormatExample.java

Going Further

: Internationalizing/Localizing Output

Internationalization:
- Numbers, dates, currencies, etc... are often formatted diffferently in different countries
The Locale Class:
- An encpasulation of a geographical, political or cultural region that can be used for locale-sensitive tasks
International Standards:
- ISO Language Codes
- ISO Country Code

Going Further

: Internationalizing/Localizing Output (cont.)

An Example that Formats Numbers

javaexamples/iobasics/NumberFormatExample.java

Streams from Files

Reminder:
- Files (or, really, file handles) are encapsulated in the File class
Constructing Streams from File Objects:
- FileInputStream(java.io.File)
- FileOutputStream(java.io.File)
Convenience Constructors:
- FileReader(java.io.File)
- PrintWriter(java.io.File)
- Scanner(java.io.File)
Note:
- These constructors throw the checked exception java.io.IOException (they actually throw a more "specific" exception -- we'll learn what this means later)

Streams from Files (cont.)

An Example of File Input Using a BufferedReader

javaexamples/iobasics/FacultyRecordTokenizer.java

Streams from Files (cont.)

An Example of File Input Using a Scanner

javaexamples/iobasics/FacultyRecordScanner.java

Streams from Files (cont.)

An Example of File Output

javaexamples/iobasics/Quizzer.java

There's Always More to Learn