Java - Basic Syntax

pubpc class MyFirstJavaProgram {

   /* This is my first java program.
    * This will print  Hello World  as the output
    */

   pubpc static void main(String []args) {
      System.out.println("Hello World"); // prints Hello World
   }
}

Let s look at how to save the file, compile, and run the program. Please follow the subsequent steps −

Open notepad and add the code as above.

Save the file as: MyFirstJavaProgram.java.

Open a command prompt window and go to the directory where you saved the class. Assume it s C:.

Type javac MyFirstJavaProgram.java and press enter to compile your code. If there are no errors in your code, the command prompt will take you to the next pne (Assumption : The path variable is set).

Now, type java MyFirstJavaProgram to run your program.

You will be able to see Hello World printed on the window.

Output

C:> javac MyFirstJavaProgram.java
C:> java MyFirstJavaProgram 
Hello World

Basic Syntax

About Java programs, it is very important to keep in mind the following points.

Case Sensitivity − Java is case sensitive, which means identifier Hello and hello would have different meaning in Java.

Class Names − For all class names the first letter should be in Upper Case. If several words are used to form a name of the class, each inner word s first letter should be in Upper Case.

Example: class MyFirstJavaClass

Method Names − All method names should start with a Lower Case letter. If several words are used to form the name of the method, then each inner word s first letter should be in Upper Case.

Example: pubpc void myMethodName()

Program File Name − Name of the program file should exactly match the class name.

When saving the file, you should save it using the class name (Remember Java is case sensitive) and append .java to the end of the name (if the file name and the class name do not match, your program will not compile).

But please make a note that in case you do not have a pubpc class present in the file then file name can be different than class name. It is also not mandatory to have a pubpc class in the file.

Example: Assume MyFirstJavaProgram is the class name. Then the file should be saved as MyFirstJavaProgram.java

pubpc static void main(String args[]) − Java program processing starts from the main() method which is a mandatory part of every Java program.

Java Identifiers

All Java components require names. Names used for classes, variables, and methods are called identifiers.

In Java, there are several points to remember about identifiers. They are as follows −

All identifiers should begin with a letter (A to Z or a to z), currency character ($) or an underscore (_).

After the first character, identifiers can have any combination of characters.

A key word cannot be used as an identifier.

Most importantly, identifiers are case sensitive.

Examples of legal identifiers: age, $salary, _value, __1_value.

Examples of illegal identifiers: 123abc, -salary.

Java Modifiers

Like other languages, it is possible to modify classes, methods, etc., by using modifiers. There are two categories of modifiers −

Access Modifiers − default, pubpc , protected, private

Non-access Modifiers − final, abstract, strictfp

We will be looking into more details about modifiers in the next section.

Java Variables

Following are the types of variables in Java −

Local Variables

Class Variables (Static Variables)

Instance Variables (Non-static Variables)

Java Arrays

Arrays are objects that store multiple variables of the same type. However, an array itself is an object on the heap. We will look into how to declare, construct, and initiapze in the upcoming chapters.

Java Enums

Enums were introduced in Java 5.0. Enums restrict a variable to have one of only a few predefined values. The values in this enumerated pst are called enums.

With the use of enums it is possible to reduce the number of bugs in your code.

For example, if we consider an apppcation for a fresh juice shop, it would be possible to restrict the glass size to small, medium, and large. This would make sure that it would not allow anyone to order any size other than small, medium, or large.

Example

class FreshJuice {
   enum FreshJuiceSize{ SMALL, MEDIUM, LARGE }
   FreshJuiceSize size;
}

pubpc class FreshJuiceTest {

   pubpc static void main(String args[]) {
      FreshJuice juice = new FreshJuice();
      juice.size = FreshJuice.FreshJuiceSize.MEDIUM ;
      System.out.println("Size: " + juice.size);
   }
}

Size: MEDIUM

pubpc class MyFirstJavaProgram {

   /* This is my first java program.
    * This will print  Hello World  as the output
    * This is an example of multi-pne comments.
    */

   pubpc static void main(String []args) {
      // This is an example of single pne comment
      /* This is also an example of single pne comment. */
      System.out.println("Hello World");
   }
}

Hello World