(Identifiers, keywords, literals, white spaces and comments)

This part of the java tutorial teaches you the basic language elements and syntax for the java programming language. Once you get these basic language concepts you can continue with the other object oriented programming language concepts.

KEYWORDS

There are certain words with a specific meaning in java which tell (help) the compiler what the program is supposed to do. These Keywords cannot be used as variable names, class names, or method names. Keywords in java are case sensitive, all characters being lower case.

Keywords are reserved words that are predefined in the language; see the table below (Taken from Sun Java Site). All the keywords are in lowercase.

	   abstract    default    if            private      this
	boolean     do         implements    protected    throw
	break       double     import        public       throws
	byte        else       instanceof    return       transient
	case        extends    int           short        try
	catch       final      interface     static       void
	char        finally    long          strictfp     volatile
	class       float      native        super        while
	const       for        new           switch
	continue    goto       package       synchronized

Keywords are marked in yellow as shown in the sample code below

/** This class is a Hello World Program used to introduce the Java Language*/
    public class HelloWorld {
       public static void main(String[] args) {
       System.out.println(“Hello World”); //Prints output to console

For more information on different Keywords –    Java Keywords

Some Tricky Observations: The words virtual, ifdef, typedef, friend, struct and union are all words related to

the C programming language. const and goto are Java keywords. The word finalize is the name of a method

of the Object class and hence not a keyword. enum and label are not keywords.

COMMENTS

Comments are descriptions that are added to a program to make code easier to understand. The compiler ignores comments and hence its only for documentation of the program.

Java supports three comment styles.

     Block style comments begin with /* and terminate with */ that spans multiple lines.

     Line style comments begin with // and terminate at the end of the line. (Shown in the above program)

    Documentation style comments begin with /** and terminate with */ that spans multiple lines. They are generally created using the automatic documentation generation tool, such as javadoc. (Shown in the above program)

name of this compiled file is comprised of the name of the class with .class as an extension.

VARIABLE, IDENTIFIERS AND DATA TYPES

Variables are used for data that change during program execution. All variables have a name, a type, and a scope. The programmer assigns the names to variables, known as identifiers. An Identifier must be unique within a scope of the Java program. Variables have a data type, that indicates the kind of value they can store. Variables declared inside of a block or method are called local variables; They are not automatically initialized. The compiler will generate an error as a result of the attempt to access the local variables before a value has been assigned.

public class localVariableEx {
  public static int a;
  public static void main(String[] args) {
    int b;
    System.out.println("a : "+a);
    System.out.println("b : "+b); 	//Compilation error
}}

Note in the above example, a compilation error results in where the variable is tried to be accessed and not at the place where its declared without any value.

data type indicates the attributes of the variable, such as the range of values that can be stored and the operators that can be used to manipulate the variable. Java has four main primitive data types built into the language. You can also create your own composite data types.

Java has four main primitive data types built into the language. We can also create our own data types.

/** Comment
 * Displays "Hello World!" to the standard output. 

 */
public class HelloWorld {
      String output = "";
      static HelloWorld helloObj;  //Line 1 

      public HelloWorld(){
            output = "Hello World";
      } 

      public String printMessage(){
            return output;
      } 

      public static void main (String args[]) {
            helloObj = new HelloWorld();  //Line 2
            System.out.println(helloObj.printMessage());
  }

}