1.2 A First Program

We begin our study of Java with a program that prints a message greeting the world.

Example 1

class Greet
{
   // This program prints a simple message
   public static void main (String[] args)
   {
      System.out.println("Hello, world");
   }
}

Let us look in detail at the components of the program in Example 1.

class
All Java programs are contained in a class. The start of a class is indicated by the reserved word class. Java contains about fifty reserved words, all of which are listed later in this chapter.

Greet
This is the name that identifies the class. The choice of a name is, within limits, up to the programmer but, customarily, classes have names that start with an upper case letter. We chose the name Greet because the program greets the world but we could have called it FirstProgram if we had chosen to do so. In fact, as far as the computer is concerned, we could have called the class Athena or Saskatoon but it is better, for the benefit of your human readers, to use a meaningful name.

{ }
The beginning and end of any section of a Java program are indicated by brace brackets, { and }. The beginning and end of the class are indicated by the { in the second line and the } in the last line of the program.

// This program ...
The two adjacent slashes indicate a comment in a Java program. Anything on a line after // is ignored by the compiler. Although they do not affect the way that the program works, comments can be very useful to any person trying to understand a program. A comment can also be indicated in a Java program by using /* to start the comment and */ to end it. Using this style, we could have written our comment as /* This program prints a simple message */ Usually, comments that appear entirely on one line are written using the // style while those that occupy multiple lines are written using the /* ... */ style.

public static void main (String[] args)
Java programs consist of one or more methods, (sometimes called functions or procedures in other programming languages). If the program is run directly by the Java interpreter, then exactly one of its methods must be called main. (Such programs are sometimes called application programs. As we will see later when we study applets, Java has other types of programs that do not have a main method.) Our program has only one method so it must be called main. This line is the header of the main method. The other words and symbols in the header will be explained later. For now, just use them, exactly as shown.

{ }
The inner pair of brace brackets defines the beginning and end of the definition of the main method, just as the outer pair of brace brackets defines the limits of the definition of the class.

System.out.println("Hello, world");
This statement calls on a method called println to send the string of characters Hello world contained in double quotes to some output device (probably your computer's screen). Whatever the output device is, we can think of it as a page of paper. The println method, given a string of characters, writes the string on the page and then moves on to the next line of the page, ready to print the next item, if there is one. As we suggested in discussing the header of the method, the details of the use of println will be made clear later. For now, just use it exactly as shown, with an upper case S on System, dots between System, out, and println, and including both the parentheses and the terminating semi-colon.

You should notice in Example 1 the way in which the parts of the program have been indented. Everything within the brace brackets that enclose the body of the class is indented two spaces. Similarly, everything within the brace brackets that enclose the body of the main method is indented two more spaces past the header of the method. Indentation is not necessary for the computer but it can improve the readability for a human. Since your programs should be clear as well as correct, you should always indent them. Your instructor might suggest an indentation style that is slightly different from the one that we use. The important thing is to use a style that is both clear and consistent.

Example 2

As far as a computer is concerned, the program of the previous example is exactly equivalent to either of the following:

class Greet{public static void main(String[]
args){System.out.println("Hello, world");}}

    class             Greet          {
        public
static

                                              void
   main(              String           [
         ]              args       )
   {
                 System.out.println
      (                              "Hello, world"
                  )     ;
         }  
                              }

Notice the ways in which blanks are used in Example 2.

At least one blank must be used to separate two adjacent words. Thus, classGreet or voidmain would be incorrect.

If a symbol appears between two words, it can be used as a separator so that a blank is no longer required. For example, main(String and main ( String are equivalent.

Extra blanks, even entire blank lines, do not usually change the meaning of the surrounding symbols. Blanks can even be inserted before or after dots so that System . out . println would be correct. (It is, however, considered bad style to have blanks around dots; you should avoid it.) Finally, extra blanks inside a string (inside double quotes) will not be ignored so that the output produced by printing the strings "Hello , world" and "Hello, world" would be different.

A number of variations can be made to our basic printing technique.

To print a blank line, we can simply write
```
   System.out.println();
```
If we want to print a string that is longer than we have room for on one line of our program, we cannot simply continue the string onto a second line. If we were to do so, the compiler would get upset and refuse to compile our program. Instead, to print a very long string, we can write something like the following:
```
   System.out.println("This will print a verrrrrry long"
                    + " string on one line by joining"
                    + " these strings into one string");
```
The process of joining strings is called concatenation. The plus sign used with strings acts as the concatenation operator in Java.
If we do not wish to jump to a new line after printing a string, we can use the method print instead of println as illustrated in the next example.

Example 3

The following program illustrates some uses of print and println.

class PrintDemo
{
   public static void main (String[] args)
   {
      System.out.println("Never put off till tomorrow");
      System.out.println();
      System.out.print("what you can do");
      System.out.println(" the day");
      System.out.println();
      System.out.println("after" + " " + "tomorrow.");
   }
}

This will produce the following output.

Never put off till tomorrow

what you can do the day

after tomorrow.

Since the beginning and end of a string to be written by print or println are marked by double quotes, how do we print a string that contains double quotes? Java's solution is to use a backslash character, \, immediately in front of a double quote to signal the computer that it should print the character ".

Example 4

The statement

System.out.println("Angela said, "
                 + "\"That's ridiculous, Bram.\"");

would print

Angela said, "That's ridiculous, Bram."

The technique used to print a double quote can be used in a number of other ways. For example, to print a backslash, you should write \\. To jump to a new line, you should write \n. (The n indicates a new line.) These pairs are called escape sequences. A complete list of escape sequences can be found in Appendix C.

Example 5

The statement

System.out.println("A backslash: \\\na double qute: \"");

would produce the output

A backslash:  \
a double quote:  "

Exercises 1.2

What does this program print?

class Advice
{
   public static void main (String[] args)
   {
      System.out.print("If at first ");
      System.out.println("you don't succeed" + ",");
      System.out.print("failure may be ");
      System.out.println("your style");
   }
}

Rewrite the following program using the indentation style shown in the text.

   class Quote{public static void main(String[]args){
   System.out.print("The unexamined life");
   System.out.println(" is not worth living.");}}

Write a single Java statement that would print the following, exactly as shown.
A slash is "/" while a backslash is "\"

The following program contains a number of errors. Rewrite the program with the errors corrected.

class BadForm
 public void main (string() args);
 {
  System.Out.Println('What's wrong with this?')
 }

Write a Java program to print your name in giant letters as shown in the following example.

   BBBB    EEEEE     A     TTTTT   RRRR    III   X   X
   B   B   E        A A      T     R   R    I    X   X
   B   B   E       A   A     T     R   R    I     X X
   BBBB    EEE     AAAAA     T     RRRR     I      X
   B   B   E       A   A     T     R   R    I     X X
   B   B   E       A   A     T     R   R    I    X   X
   BBBB    EEEEE   A   A     T     R   R   III   X   X

Write Java programs to print each design. You might find it helpful to use squared paper in planning the appearance of your output.

PARALLELOG                   APEZO                     D
 A        R                 R     I                   I I
  R        A               TRAPEZOID                 A   A
   ALLELOGRAM                                       M     M
                                                     O   O
                                                      N N
                                                       D

Write a complete Java program to print your name and address as they would appear on the envelope of a letter written to you at your home.