Console Output

Objective #1: Compile and execute a simple Java program.

• Memorize the following "Hello World" Java program.
  
  // John Doe
  
  public class HelloWorld
  {
     public static void main(String[] args)
     {
        System.out.println("hello world");
     }
  }

• The name of the class is HelloWorld and can be chosen by the program. Except for the string "Hello World", the rest of this program consists of Java keywords and operators. A string is a set of characters that are enclosed in quotation marks.
• A Java source code file must be named to match the name of the class that it contains. The code above must be saved in a file named HelloWorld.java (using a 4-letter extension even if you're going to compile the program on a Windows computer.)
• Here is an interesting look at "hello world" programs in other computer languages.
• The System class is part of the API (application programming interface) provided with the Java environment. It gives you system-independent access to system-dependent functionality. In other words, it allows you to take advantage of graphic methods with a Java program even if you aren't sure of the platform that your Java program is going to be executed on.
• System.out is the full name of the out object in the System class. Notice that there is no instantiated System object. Rather, out is referred to directly from the class name. This is because out is a class variable rather than an instance variable. When you execute a program, one class variable is created per class, no matter how many instances exist of that class. So even if there is another System.out statement in the main function, the same out variable would be used.
• You can also associate methods with a class which are called class methods which should not be confused with instance methods.
• An escape sequence can be used inside of a string to print special characters that would otherwise be impossible to print due to the special role of certain characters. For example, because double quotes are used to delimit strings, how would you print a double quote if you actually wanted to? The statement
  
  System.out.println("He said, "Come over here!"");
  
  causes compiler errors since the double quote before the capital C is interpreted as the end of the string. In order to print the desired quotation above, you would need to use escape sequences to "escape" the inside double quotes. The statement
  
  System.out.println("He said, \"Come over here!\"");
  
  would work correctly because it uses the escape sequence \" in two places to indicate to the compiler that the inside double quotes should be displayed as double quotes. In Java (and many other computer languages), the backslash is used as the escape operator. By preceding certain characters with a backslash, you are indicating to the compiler to do something special. In the case of the escape sequence \", you are telling the compiler to display a double quote rather than to interpret the double quote as the beginning or end of a string literal.
  
  Another escape sequence is \n which is used to create a new line in console output.
  
  System.out.print("hello world\n");
  
  is more or less equivalent to
  
  System.out.println("hello world");
  
  since the \n causes a new line after the phrase "hello world" prints just as the println method causes a new line to print. Note that the print method does NOT cause a new line to follow the phrase "hello world". The statement
  
  System.out.println("Sunday\nMonday\nTuesday\nWednesday\nThursday\nFriday\nSaturday");
  
  would cause the days of the week to display on separate lines of output.
  
  The escape sequence \\ causes a single backslash to be displayed. The statement
  
  System.out.println("some\\none");
  
  would cause the string literal      some\none      to display since the escape sequence \\ causes a backslash to appear. Notice that the two consecutive characters \n are not treated as an escape sequence in this example.
  
  
• See pp. 23+ for some other examples of useful escape sequences. However, only the three escape sequences \", \n, and \\ are tested on the AP exam.
  
  
• Long statements can be broken up into multiple lines of code. For example
  
  int bigNum = 1 + 2 + 3 + 4 + 5 + 6 +
                      7 + 8 + 9 + 10;
  
  will compile. You do not need to use a line continuation operator like you do in Visual Basic. But you cannot break up a string literal in this way
  
  System.out.println("Four score and seven years ago
       stuff happened");
  
  However that statement could be written as
  
  System.out.println("Four score and seven years ago " +
     "stuff happened");
  
  since the string literal was broken up into two string literals.
  
  
• There are three methods of creating comments in Java.
  • You can begin a comment with // such as
    
    // John Doe
    
  • You can enclose a comment within the matching symbols /* .... */ such as
    
    /*
       Mr. Minich
       Best Java Programmer in the world
    */
    
  • You can enclose a javadoc comment within the symbols /** .... */ as in
    
    /**
       The class Fish can be used to manipulate
       a virtual fish.
    */
    
    Notice that there are two asterisks in the opening comment tag. Javadoc comments are used to create HTML help pages for the code that you create. By enclosing javadoc comments along with your source code in the appropriate places, a utility can be used that automatically creates HTML Web pages that explain your code and serve as online help. In fact, the online Java API was built from javadoc comments.