Java Lesson 1: Basic Concepts in Java

• Concepts of Java
  • Java is a byte-compiled language. You compile source code into byte code, which is then interpreted by the Java Virtual Machine.
  • In theory, a Java application can be ported to any platform that has a Java Virtual Machine.
  • Java is built on classes and objects. You cannot have a java program without a class.
  • Source files have the extension .java , and object code (compiled code) has the extension .class .
  • Java is case sensitive. CapitalIZatIon CoUnTs!
  • When first learning Java, there are some things you will have to just type in faith. The language requires some complexities that beginning programmers will not understand for a while.
• Building a Java Program
  1. Begin by defining the class.
     • Identify the class as "public", "private" or "protected".
     • Use the keyword "class".
     • Give the class a name. Note that the class name must be the same as the file name.
     • Use curly braces {} to show beginning and end of the class definition.
  2. Within the braces, define at least one method
     • Indentify the method as "public", "private" or "protected"
     • Use the keyword "static".
     • Identify the return type. Use "void" if no value is passed from this method to another part of the program.
     • Name your method. Use the name "main" if there is only one method.
     • In parentheses list the parameters used by the method.
     • Enclose the code for the method in more curly braces {}.
  3. Within the method braces, write code to do something
     • End each line of code with a semi-colon ;
     • Remember, capitalization counts
  4. Save the file.
     • The file name should be the same as the class name.
     • Use the extension .java when saving code.
  5. Compile the code
     • Using the JDK compiler, type javac filename.java
     • The compiled code will be named filename.class
     • If the compiler displays error messages, edit the code as directed and try to compile again.
  6. Run the code by typing java filename
• Example
  

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

  • Save the above code as "HelloWorld.java"
  • At a terminal, type javac HelloWorld.java to compile.
  • At a terminal, type java HelloWorld to run the program.