Class 04: An Introduction to Java (2)

Held: Wednesday, 1 September 2004

Summary: Today we continue our initial investigations into the Java programming language, focusing primarily on input and output.

Related Pages:

• EBoard.
• Third.java.

Notes:

• Letter case (lowercase vs. uppercase) is generally important in the commands you type.
• No quiz today, sorry.
• Office hours are currently M-F 10:00-11:00.

Overview:

• Your first Java program, revisited.
• String-based input in Java.
• Reading numbers.

Daily Configuration for Working with Java

• Open a terminal window by clicking on the computer screen.
• Type cd CS152
• Type mkdir some-name-for-todays-class (for today, mkdir class04)
• Type cd some-name-for-todays-class (for today, mkdir class04)
• Type kate, which starts the Kate editor. (Warning! For some reason, on our current system, Kate spews a ton of error messages. Ignore them.)
• Enter your Java code.

Your First Java Program, Revisited

• Enter the sample program (Second.java) in Kate as specified above.
• Replace the line that says introjava with class04.
• Ask Kate to open a terminal pane by selecting View:Tool Views:Show Terminal.
• Type jc Second.java
• Correct any errors.
• Type ji class04.Second to confirm that it runs.
• Make some change to the output.
• Recompile.
• Type ji introjava.Second. Why haven't you seen changes?
• Type ji class04.Second.

Input in Java

• Just as we must create a PrintWriter (or, more precisely, a java.io.PrintWriter) in order to write output, so must we create a particular object to read input.
• It turns out to be a little more difficult to create an input object. Here's some sample code.

  	// Set up input using keyboard.
  	BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));
  

• Of course, we must also do appropriate imports

  import java.io.InputStreamReader;
  import java.io.BufferedReader;
  

• To use the keyboard, we call the readLine() method and store it in a string.

  	String color = keyboard.readLine();
  

Numeric Input

• Unfortunately (or fortunately, depending on your perspective), BufferedReader objects can only read strings.
• How do we read other values, such as numbers? We first read them as strings and then convert to numbers.
• We use the Integer.parseInt(String s) method to convert strings to integers.
• Example forthcoming.