Chapter 415 min read

Java Basic Syntax & Core Concepts

Mastering Java syntax rules, identifiers, modifiers, variables, enums, and keywords.

Released1995

Core MottoStrict Conventions & Object-Oriented Blueprint

CreatorsOracle / Sun Microsystems

1. Core Object-Oriented Terms

A Java program is a collection of objects communicating by invoking each other's methods.

Object

States & Behaviors:An object has states (e.g., color, name, breed) and behaviors (e.g., barking, eating).

Class Instance:An object is a concrete instance created from a class template.

Class

Blueprint Template:A class defines the state attributes and behavior methods supported by objects of its type.

Methods & Instance Variables

Methods:Represent behaviors where logic is executed and data is manipulated.

Instance Variables:Unique sets of variables assigned to each object instance, defining its individual state.

2. Essential Syntax Rules

Following strict naming and structural conventions is required for Java code to compile.

Case Sensitivity & File Naming

Case Sensitivity:Identifiers like Hello and hello are distinct and treated differently.

Program File Name:The file name must match the public class name exactly, appending .java (e.g., MyFirstJavaProgram.java).

Class Names:PascalCase — first letter of every word should be capitalized (e.g., MyFirstJavaClass).

Method Names:camelCase — starts with a lowercase letter, inner words capitalized (e.g., myMethodName()).

Main Method Entry Point

Execution Point:Java processing always starts from public static void main(String args[]), which is mandatory for executable programs.

java

public static void main(String args[]) {
    // Execution starts here
}

3. Identifiers & Modifiers

Rules for naming components and controlling access levels across Java classes.

Java Identifiers

Naming Rules:Must begin with a letter (A-Z or a-z), currency symbol ($), or underscore (_).

Subsequent Characters:Can contain any combination of letters, numbers, $, or _.

Restrictions:Reserved keywords cannot be used as identifiers.

Legal Examples:age, $salary, _value, __1_value

Illegal Examples:123abc, -salary

Java Modifiers

Access Modifiers:default, public, protected, private

Non-Access Modifiers:final, abstract, strictfp

Classroom Discussion Starter

Why does Java enforce strict case sensitivity and class-to-filename matching?

Takeaway Goal: Matching filenames and public class names allows the compiler and JVM to locate compiled bytecode files efficiently across different operating systems.

4. Variables & Java Enums

Java supports local, instance, and class variables, alongside restricted value lists using Enums.

Types of Variables

Local Variables:Declared inside methods or block scopes.

Instance Variables:Non-static variables bound to specific object instances.

Class Variables:Static variables shared across all instances of a class.

Java Enums

Restricted Values:Introduced in Java 5.0 to restrict variables to predefined constants, reducing bugs.

Declaration:Can be defined independently or nested inside classes.

java

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);
   }
}
// Output: Size: MEDIUM

5. Reserved Java Keywords

Reserved words cannot be used as identifiers (variables, classes, or method names).

Key Reserved Keywords

Primitive Types:boolean, byte, char, double, float, int, long, short

Control Flow:break, case, continue, default, do, else, for, if, switch, while

Access & Inheritance:class, enum, extends, implements, interface, package, private, protected, public

Exception Handling:catch, finally, throw, throws, try

Modifiers & Scope:abstract, final, native, static, strictfp, synchronized, transient, volatile

Unused Reserved Words:const, goto

6. Comments & Blank Lines

Documentation techniques used to improve code readability while being ignored by the compiler.

Single-line & Multi-line Comments

Single-line Comments:Defined using //

Multi-line Comments:Wrapped between /* and */

Blank Lines:Lines containing only whitespace are completely ignored by the compiler.

java

public class CommentExample {
   /* This is a multi-line comment.
    * It spans across multiple lines.
    */
   public static void main(String []args) {
      // This is a single-line comment
      System.out.println("Comments Example");
   }
}

7. Introduction to Inheritance & Interfaces

Core OOP principles for code reusability and establishing structural contracts.

Inheritance

Code Reusability:Derive a subclass from an existing superclass using the extends keyword to reuse fields and methods.

Interfaces

Communication Contract:An interface defines method signatures that implementing subclasses must fulfill using the implements keyword.