Transcript Keyboard input

Lecture 13: Keyboard Input and
Text Files
Yoni Fridman
7/23/01
Outline
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Overview
Input classes
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Reading keyboard input
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InputStreamReader
BufferedReader
Reading Strings
Converting to other types
Reading from text files
Overview
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A stream is a technical term for a whole sequence
of text input. (Think of it as a stream of
characters.)
Whenever Java reads input (from the keyboard,
from a file, etc.), it takes in a stream and stores it
in what’s called a Buffer:
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A Buffer is an object that holds each of the
characters read in from a stream. To be of any use
to us, the Buffer must be converted to a String.
Input Classes
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Java has two classes that we’ll need to use to read
input from the keyboard:
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InputStreamReader is a class that’s used to read streams from the
keyboard and save them as Buffers.
BufferedReader is a class that’s used to convert a Buffer to a
String.
These classes aren’t automatically available for
our use.
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To use them, we need the following command at the very
beginning of our program: import java.io.*;
import is a keyword that tells Java to load certain classes for our
use.
Reading Keyboard Input
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Before we can read input from the keyboard, we
must create a new object of the
InputStreamReader class, like this:
InputStreamReader stream = new InputStreamReader(System.in);
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Then we must create a new object of the
BufferedReader class, like this:
BufferedReader buffer = new BufferedReader(stream);
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Finally, we can read input using the instance
method readLine() of the class BufferedReader:
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String input;
input = buffer.readLine();
Converting to Other Types
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Input is always read into Strings. Even if we enter
the number 3, Java stores the String “3”.
We can convert to an int like this:
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int inputInt;
inputInt = Integer.parseInt(input);
And we can convert to a double like this:
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double inputDouble;
inputDouble = Double.valueOf(input).doubleValue();
Reading from Text Files
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What if we have text (or any data) that has been
stored in a file?
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We can read that as input much like we read keyboard input.
There’s another class that we’ll need to read input
from a text file:
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FileReader is a class that’s very similar to InputStreamReader –
it’s used to read streams from a text file.
Reading from Text Files
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Instead of creating an InputStreamReader
object, we create a FileReader object, like this:
FileReader file = new FileReader(“data.txt”);
 In this case, data.txt is the name of our text file – this can be any
name we want it to be.
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Now, we can use the FileReader object file just
like we used the InputStreamReader object
stream before:
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BufferedReader buffer = new BufferedReader(file);
String input = buffer.readLine();
Homework
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Read: 13.4, to the bottom of page 568.
HW6 out today, due Friday.
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Calculator.