Structure of a Java Program

  • Structure of a Java program:

  • A Java program consists of a number of classes
  • A Java class contains a number of (1) class vars, (2) instance vars and/or (3) methods
  • A Java class contains a number of (1) param vars, (2) local vars and/or (3) statements
  • There are 4 kinds of statements: (1) assignment, (2) cond., (3) loop and (4) method call

  Your first Java program  

  • The "classic" Hello World program in Java: 

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

    Notice:

    • The Java program consists of one or more classes

    • A class contains 0 or more (class or instance) variables and 0 or more methods/functions

    • A method contains parameters, local variables and program statements

DEMO: 01-basics/02-hello/hello.java (in ~/c/OutSchool/CS/AP-CS/demo/)

  How to define a class  

  • A Java class is defined with the keyword class: 

    public class hello    // "hello" is the name of the class
{
    public static void main(String[] args)
    {
        System.out.println("Hello World");
    }
}

  • The matching curly braces { ... } denotes the boundaries of a class

  • Inside a class, you can define:

    • Class variables (later)

    • Instance variables (later)

    • Methods (e.g.: main( ))

Show the structure of the hello class with slide 1

Adding comments to the codes in your Java program

  • Comment:

    • A comment is a part/portion of a program that is ignored by the compiler

    • A comment is intended to be read by humans --- e.g., the clarify the program code

  • Java has 2 ways to specify comments: 

        // The rest of the line is considered a comment

  /* Every between the "comment quotes"
       are considered as comment         */

  • Comments are ignored by the Java program

    (The purpose of comments is to make a computer program more readable to humans)

Example comments in a Java program

  

/* ----------------------------------------------------
   This is your first Java program
   ---------------------------------------------------- */

public class hello     // The name of this class is hello
{
    // There are no variables defined in the class

    /* ------------------------------------------------------
       A Java program starts executing with the method "main"
       ------------------------------------------------------ */
    public static void main(String[] args)
    {
        System.out.println("Hello World");  // Prints "Hello World"
    }
}

DEMO: demo/01-basics/03-comment/hello.java