Console Input and Output in Java

Console Input and Output in Java
An Introduction

Computer Science Department

bernstdh@jmu.edu

Understanding Streams

What Are They:
- A sequence of bytes with indeterminate length that ends with an END-OF-STREAM character (Ctrl-D in Unix and Ctrl-Z in Windows)
Why They Are Used:
- The way you work with a stream is independent of its channel (e.g., keyboard, console, network connection, storage devices)

"Standard" Streams

What Are They?
- Streams that are associated with (what used to be) the standard channels for input and output
What Does This Mean Now?
- The terminal window (sometimes called the console) has standard streams associated with it
- Most IDEs have standard streams associated with one or more windows/panes/views

"Standard" Streams in Java

A Brief Quiz

Some Questions (Related to Output):
- What is System (and how can you tell)?
- What is System.out (and how can you tell)?
- What is System.err (and how can you tell)?
Some Questions (Related to Input):
- What is System (and how can you tell)?
- What is System.in (and how can you tell)?
- What methods does System.in have (and not have)?
Some Questions Related to Both:
- How would you use System.out, System.err, and/or System.in?

Encapsulation of Standard Streams

Standard Output/Error:
- System.out and System.err are PrintStream objects
Standard Input:
- System.in is an InputStream object

Using a PrintStream Object

Unformatted Output:
- PrintStream.print(java.lang.String) prints the String (i.e., sends it to the stream)
- PrintStream.println(java.lang.String) prints the String followed by the line separator String (which is "\n" in Linux, "\r" in OSX and "\r\n" in MSW)
An Important Point:
- Text sent to an output stream may be buffered
- To force text through the buffer use PrintStream.flush()
- This is particularly important to be aware of when debugging

Using a PrintStream Object (cont.)

An Observation:
- Output often needs to be formatted in a very particular way
Format Strings:
- Encapsulated in the Formatter class
Two Approaches to Using Format Strings:
- Use the String.format(java.lang.String, java.lang.Object...) method to create a formatted String and then call print() or println()
- Use the PrintStream.printf(java.lang.String, java.lang.Object...) convenience method to perform both steps in one call

The Format String

Using a PrintStream Object (cont.)

An Example that uses printf()

javaexamples/iobasics/PrintfExample.java

Using an InputStream Object

Raw Input:
- The read() methods can be used to read one or more bytes
An Observation:
- We don't want to have to work with the binary representation of char, double, or int
A Better Approach:
- "Wrap" the InputStream in a Scanner object and use its next___() methods

Using a Scanner Object

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

Using a Scanner Object (cont.)

An Example that uses a Scanner

javaexamples/iobasics/ScannerExample.java

Processing Input with a Scanner (cont.)

An Inconvenience when Using Scanner Objects

javaexamples/basics/ScannerInconvenienceExample.java

Using a Scanner Object (cont.)

The Issue:
- The methods that read "up to" a delimiter do not consume newline characters
A "Work Around":
- Invoke nextLine() immediately after invoking one of these methods

The "Modern" Approach to Console I/O

A Recent Addition:
- A Console object can be retrieved using the System.console() method and it has both readLine() and printf() methods
Problems with this Approach:
- The console() method returns null in IDEs
- The streams can't be re-directed within a program making it inappropriate for many testing environments

There's Always More to Learn