INTRODUCTION TO JAVA

WEEK 1

Introduction to Java

• Java is originally designed for programming consumer devices , but it was first successfully used to write applets.

• Compilers are programs that translate a program written in a high-level language into an equivalent machine language.
  In case of Java, this machine language is byte code.

• To run a Java program on a computer, the program must first be translated into an intermediate language called
  bytecode and then interpreted into a particular machine language.

• Unlike machine-language instructions, or machine code, Java bytecode is not tied to any particular operating systems or
  CPU. All we need to run the same Java programs on the different operating systems is the Java interpreters for the desired
  operating systems. Currently, there are Java interpreters for Windows, Mac, Unix and other operating systems. A Java
  interpreter is also called a Java Virtual Machine (JVM) because it  is like a virtual machine that executes bytecode, whereas a
  CPU is a real machine that executes machine code.

Programming Process with Problem analysis-Algorithm- Coding and Execution Cycle

1. Analyze the problem and outline the problem and its solution requirements

2. Design an algorithm to solve the problem

3. Implement the algorithm in a programming language, such as Java (create a program.)

4. Verify that the algorithm works. (compile -run- test)

5. Maintain the program by using and improving it , and modifying it if the problem domain changes.

Processing of a Java Program

1. Use a text editor create a  program (source code.)

2. Compile the program - translate the program into the equivalent bytecode and check for syntax errors.

3. If no error is found, the compiler translates the program into bytecode. otherwise, go to step 1 edit the program.
   to fix the error.

4. The loader connects the bytecode for classes used in the program with the program's bytecode. The loader also loaded
   the complete program's bytecode into main memory.

5. The interpreter translates each bytecode instruction into your computer's machine language, and then executes it.

The basic structure of a Java console application

public class test
{
     public static void main (string[] args)
    {
              Write coding       
     }

}

The file name must be identical to the class name.
In this case the file name you type is  test.
The actual file name is test.java. All java files has an extension of java.

A simple Java program

public class Test
{ //This program displays a message
  public static void main(String[] args)
 {
      System.out.println("I love Java!!!");

 }//end main
}//end class

Program analysis

• public class Test is the class definition. Every Java program must contain at least one programmed-defined class definition.

• The term class is a Java keyword and must be followed by the name of the class.

• The Java keyword public will be explained later in the class. For now, we always create our class definitions with the keyword public.

• public static void main (String [] args) is a method definition line for the Test class's main method.

• A method is a block of code that performs certain tasks.

• System.out.println is a Java command that that instructs the computer to write a string of text enclosed in quotation marks to the command window.

• Comment statement is used to explain in more details in the program. The comment statement is not compiled by the compiler. It usually precede by //
  /* comment */
  // one comment per line
  /*   ................*/  comments on multiple lines

From the source code to running the program 

EditoràSource code àCompileràClass files (byte code) machine languageàInterpreter à Output (Running Program)

Compiler:  translates a programs written in a high-level language into machine code and identify the syntax errors.
Java compiler used in the class is called Dr. Java

Edit , Compile and Test Process

1. Create or edit the program

2. Compile the program. If the program has syntax error(s), go to step 2 to edit and fix the error(s).

3. Run, execute or test the program . If the program has logical error(s) or run-time errors, go to step1.