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: public 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).

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

• public 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, public , 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 initialize 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 list 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 application 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

Live Demo

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

public class FreshJuiceTest {

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

The above example will produce the following result −

Output

Size: MEDIUM

Note − Enums can be declared as their own or inside a class. Methods, variables, constructors can be defined inside enums as well.

Java Keywords

The following list shows the reserved words in Java. These reserved words may not be used as constant or variable or any other identifier names.

Comments in Java

Java supports single-line and multi-line comments very similar to C and C++. All characters available inside any comment are ignored by Java compiler.

Example

Live Demo

public class MyFirstJavaProgram {

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

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

Output

Hello World