- The structure/organization of a
computer program resembles
the structure of
a text document, like
a book
You all know what a book look like and I will use the structure/organization of a book to illustrate the structure/organization of a (Java) program
- Quick review of what a book look like:
- A book consists of a number of chapters
- Each chapter consists of a number of paragraphs
- Each paragraph consists of a number of sentences
- Each sentence must obey the syntax rules in the English language
- Quick overview of what a Java program
look like:
- A Java program consists of a number of classes
- Each class consists of a number of methods (and some variables)
- Each method consists of a number of (program) statements
- Each statement must obey the syntax rules in the Java programming language
- The following picture shows what a
Java program look like
in its entirety:
Explanation:
- The yellow rectangle represents
a Java program
- The blue rectangles represent
Java constructs that
are called classes
- One Java program is
made up of
one or more classes
- Each class is usually stored in a separate (UNIX) file with the extension .java
- One Java program is
made up of
one or more classes
- The beige rectangles represent
Java constructs that
are called methods
- One Java class
usually contains
one or more methods
(It is possible to have no methods defined inside a class, but such a class is extremely rare)
- One Java method contains
one or more
(program) statements
(A single statement does not do much useful work. So a Java method usually contains many (program) statements)
- One Java class
usually contains
one or more methods
- A program statement
(or statement for short),
is the smallest execution unit in
a Java program.
(A statement is like a sentence in a book)
- The yellow rectangle represents
a Java program
- Fact:
- The execution of a Java program begins with the method with the name main()
This practice originates in the C programming language and has been "inherited" by C++ and Java.
- Consequence:
- One of the methods in every
Java program must be named
main().
(Otherwise, you will get a error when you run your program.)
- One of the methods in every
Java program must be named
main().
- The simplest Java program
consists of 1 class
and the class consists of
1 method called main.
Example:
- Remember that:
- You must have the method main in every Java program
Because you only have 1 method... so the method must be named main --- (in this case, you have no choice...)
- This Java program is written as follows:
Note:
- In the figure, I used different color
to delimit the different
concepts
- Pay attention to the
headings preceding the braces and
braces themselves.
- The heading public class Hello
followed by a
pair of corresponding braces
delimits a
class
(Each class has a name, the name of this class is Hello)
- The heading
public static void main
followed by a
pair of corresponding braces
delimits a
method
(Each method has a name, the name of this method is main)
- The heading public class Hello
followed by a
pair of corresponding braces
delimits a
class
On the next webpage, we will use the above simple program to illustrate some concepts of the Java programming language
- In the figure, I used different color
to delimit the different
concepts
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link and
save in a scratch directory
- To compile: javac Hello.java
- To run: java Hello