Java Basic - Basic Syntax

A Java program it can be defined as a collection of objects that communicate via invoking each other’s methods.

• Object - Objects have states and behaviors.
• Class - A class can be defined as a template/blue print that describles the behaviors/states that object of its type support.
• Methods - A methods is basically a behavior. A class can contain many methods.
• Instance Variables - Each object has its unique set of instance variables. An object’s state is create by the values assigned to these instance variables.

Tables

• Example First Java Program
• Basic Syntax
• Java Identifiers
• Java Modifiers
• Java Variables
• Java Arrays
• Java Enums
• Inheritance
• Interfaces

Example First Java Program ⤴

public class MyFirstJavaProgram {
    /* This is my first java program. 
    * This will print 'Hello World' as the output
    */
    public static void main(String []args) {
    System.out.println("Hello World"); // prints Hello World
    }
} 

Put this code in a file’s name “MyFirstJavaProgram.java” (must exactly typo). Compile and run with:

$javac MyFirstJavaProgram.java
$java MyFirstJavaProgram
Hello World

Basic Syntax ⤴

With Java programs, it is very important to keep in mind the following points:

1. Case Sensitivity - Java is case sensitive, so Hello and hello have different meaning in Java.
2. Class Names - For all class names, the first letter should be in Upper Case. Ex: MyFirstJavaClass.
3. Method Names - All method names should start with a Lower Case letter. Ex: public void myMethodName().
4. Program File Name - Name of the program file should exactly match the (only) public class name in this file. Ex: assume ‘MyFirstJavaProgram’ is the class name. Then the file should be saved as ‘MyFirstJavaProgram.java’.
5. public static void main(String args[]) - Java program processing starts from the main() methods which is a mandatory part of every Java program.
6. a single line comment (//), a multiline comment (/* */), and a Javadoc comment (/** */).
7. Java code is organized into folders called packages. To reference classes in other packages, you use an import statement. A wildcard (*) ending an import statement means you want to import all classes in that package. It does not include packages that are inside that one. java.lang is a special package that does not need to be imported. Note: importing by classname takes precedence over wildcards.
8. Constructors create Java objects. A constructor is a method matching the class name and omitting the return type. When an object is instantiated, fields and blocks of code are initialized first. Then the constructor is run.

Java Identifiers ⤴

In Java, we have several points that will need to remember about identifiers:

1. All identifiers should begin with a letter (A-Z or a-z), currency character $ or underscore _.
2. After the first character, identifiers can have any combination of character.
3. Identifier cannot have the same name with any keyword.
4. Most importantly identifiers are case sensitive.
5. Example of legal identifiers: age, $salary, _value, __1_value.
6. Example 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.

Java Variables ⤴

Types of variables in Java are:

• Local Variables
• Class Variables (Static Variables)
• Instance Variables (Non-static variables)

Declaring a variable involves stating the data type and giving the variable a name. Variables that represent fields in a class are automatically initialized to their corresponding “zero” or null value during object instantiation. Local variables must be specically initialized. Identifiers may contain letters, numbers, $, or _. Identifiers may not begin with numbers.

Java Arrays ⤴

Arrays are objects that store multiple variables of the same type. However, an array itself is an object on the heap.

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.

Note:

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

Inheritance ⤴

In Java, classes can be derived from classes.

This concept allows you to reuse the fields and methods of the existing class without having to rewrite this code in a new class. In this scenario:

• The existing class is called the superclass.
• The derived class is called the subclass.

Interfaces ⤴

In Java language, an interface can by defined as a contract between objects on how to communicate with each other. Interfaces play a vital role when it comes to the concept of inheritance.

An interface defines the methods, a deriving class (subclass) should use. But the implementation of the methods is totally up to the subclass.