Basic Language Elements in Java

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.
To Understand the Introduction part read this post.


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
}}

• Integer: byte, short, int, and long.
• Floating Point: float and double
• Character: char
• Boolean: variable with a value of true or false.

/** 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());
  }

}