CS 111A Lecture Notes - Simple I/O in Java - Chapter 2 part 1

Console Output

• System is a class that is part of the java.lang package. "java.lang" is automaticaly imported into every Java program, so its classes are always available. Every time you write a java program, it automatically has access to the members of the System class. Members of a class are either data declarations (also called "fields") or methods. We use the dot operator to link together the class name, System, with an object of the System class called out, and with a method such as print() of that "out" object:
      System.out.print("hello");
• System.out - a pre-defined object to send output to the command-line (console) such as above.
• The System.out methods, print and println (The extra "ln" means add a blank line at the end) can display any string to the console.
• Another class that's part of the "java.lang" package is the String class. We'll learn more about this later, but for now we can use "String" as a datatype to easily store and manipulate a string of characters.
• Escape Sequences: special character combinations to represent special characters:
  • \n    newline
  • \t    tab
  • \\    one backslash
  • \"    quote character
  Example:

      System.out.println("Here is the first line of"
                         + " text.\nHere is the second line.");
      // notice we can write a string expression over several
      // lines of code as long as we break outside a set of quotes.
      // This keeps your program listing from running off the
      // right edge of the page.
    

• Scanner - a pre-defined class that's part of the java.util package. Scanner is used to get console (command-line) input from the user. To use it, you first have to instantiate an object of the Scanner class, then use that object to call one of its methods, such as nextLine(). See the "Echo" example below to see how this works.

Identifiers

• Identifier - the name of a variable (or other item), which is made up by the programmer, and should give the reader some hint of what it is used for. (e.g. number_of_candy_bars is a very good, clear variable name.) Java is case sensitive with all identifiers and keywords.
• Indentifier Rules - Identifiers must follow the following rules: They must start with a letter or underscore, and contain only letters, digits, or underscore. E.g. variables called 3X or %change are not allowed.
• Reserved Words - pre-defined names (keywords) in Java must not be used for your own identifier names (for example you can't name a variable "int".) For a complete list of reserved words, see page 455 (Appendix C).

Variables

• Variable - a named location in memory that can be used to store a specified type of data, and whose value may be changed in the course of the program. Variables must be declared before they are used in Java.
• int - a datatype that stores an integer (no decimal value allowed). Can be positive or negative.
• String - a datatype (class, actually) that stores any sequence of characters, such as a word, a sentence, or a combination of numbers and symbols, etc.
• Variable declaration examples:
    int x = 5;
  String s1;
  int age, height;


In-class exercise: input user's first name, then last name, and store them into two separate variables. Output them in reverse order with a comma between them.

Names.java: Solution to above exercise

Programming Style

• Programs are written to be read by other humans as well as the computer.
• Use my sample programs and those in the book as your guide for style.
• Indentation: always indent consistently. It greatly improves readability
• Indentation reflects the level, or scope, of a line of code.
• The contents of a set of curly braces "{" and "}" are always indented
• You should always have Comments at the top of your program, stating your name, the name of your program source file, the name of the class and assignment, and a brief description of what the program does.
• Comments should also be used to explain hard-to-understand code, and to summarize big steps such as functions and sometimes loops.
• Use helpful variable names (strive for self-commenting code - clear without comments to explain it.)
• Provide a good user interface, including prompting for inputs and explaining outputs.

Using dialog boxes for input and output

• JOptionPane - this is a class that contains methods for producing simple GUI windows. It requires that you import the Swing class JOptionPane with the following line at the beginning of your program:
      import javax.swing.JOptionPane;
• To display a simple dialog box for output, use JOptionPane.showMessageDialog() for example:
      JOptionPane.showMessageDialog(null, "output string");
    // null puts dialog window in center (default parameter value)
• To display a simple dialog box to get input from the user, use JOptionPane.showInputDialog() for example:
      String response; // declare a String object variable to store the user's entry
    response = JOptionPane.showInputDialog("Enter your response");
• If the user's response is expected to be an integer, use Integer.parseInt() to convert from String to int. For example (continuing from above):
      int numericResponse = Integer.parseInt(response);
• GUI elements like those produced by JOptionPane will not work on hills, because hills doesn't have a way to display graphics like a window. To test a program with a GUI, you'll have to use a computer other than hills, such as your PC (running "java myprogram" from the DOS prompt). Or we can use an applet, as you will see soon.
• System.exit() - When you write an application that has a GUI, the program waits for user interaction, so you must terminate the application with a call to another static method from the System class: exit. (This is not necessary for applets, as you will see soon.)
      System.exit(0);
    // argument of 0 indicates successful termination of the program.
• Example: Hello World program with age input/output

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