Java is a long-standing and widely used programming language developed by Sun Microsystems and now owned by Oracle. It is used to create applications and websites using the Java platform. Java programs are known for their portability, robustness, and scalability. This article aims to provide you with an overview of all the popular Java commands, as well as providing a more in-depth look at some of the more complex features that this language has to offer.
What is Java?
Java is a general-purpose programming language originally designed by Sun Microsystems in 1995. It is an object-oriented programming language based on a virtual machine specification and is designed to have fewer implementation dependencies. It is intended to be platform-independent and suitable for all types of development. Java can be used to create stand-alone applications, applets (for websites), and servlets (for web applications). The language is also used to create software that runs on Android devices as well as desktop applications.
Popular Java Commands
The basic commands of Java form the starting point for any programmer who wants to learn the language. Popular commands include the “Println” command, which prints text to the end user; the “If” statement, which allows you to execute certain pieces of code if certain conditions are met; the “For” loop, which allows you to repeat a certain piece of code until a certain condition is met; and the “Switch” statement, which allows you to make decisions based on multiple different conditions. More intermediate commands include the “While” loop, which allows you to repeat a certain piece of code while a certain condition is true; the “Do While” loop, which executes a piece of code at least once before checking a condition; and the “Try…Catch” statement which allows you to execute pieces of code depending on the type of error that occurs.
Running a Java Program
To execute a Java program, you first need to write the code in a text editor and save it with a .java extension. Here is a simple Hello World example:
public class Main {
public static void main(String[] args) {
System.out.println("Hello World!");
}
}
Save the file as Main.java. Then open a terminal, navigate to the directory containing Main.java, and type “javac Main.java” to compile the code. This will generate a .class file containing the bytecode, which the JVM will execute.
Finally, run the compiled program using the “java Main” command. You will see “Hello World!” printed out, demonstrating the basic workflow of creating and executing a Java program using javac and java commands.
Setting up a Java Development Environment
Setting up a development environment for Java will allow you to smoothly write and execute your programs. There are several popular Integrated Development Environments (IDEs) for Java, such as Eclipse and NetBeans, as well as more lightweight alternatives such as IntelliJ IDEA. Your IDE should be configured with your JDK and should include all the necessary libraries and frameworks so that you can begin development straight away.
Basic Syntax and Structure of the Java Language
Java syntax is quite simple when compared to other programming languages such as C# or C++. This is one of the reasons why it is so popular. The three main components of Java programs are classes, objects, and methods. Classes are used to structure your code and methods are used to manipulate information related to objects. Variables are used throughout classes and objects to store data. Block statements and loop structures are also used for control flow, branching logic, and looping logic.
Understanding Classes and Objects in Java
Classes in Java are like blueprints for objects. All objects in a program will be derived from classes. Classes contain information about how an object should look and behave. They also contain methods, which are pieces of code that provide functionality for objects. All objects will have unique properties that define how they look, how they act, and how they interact with other objects in the system.
Creating and Using Variables in Java
Variables allow you to store data within the scope of their containing class or method. Variables have a defined type such as integers, floats, strings, booleans, or arrays. Variables must always be declared with their type before they can be used. Variables will also have a name which must adhere to certain rules. Once a variable has been declared, it can be used in any place where its type is accepted.
Control Flow Statements in Java
Control flow statements are used to define how a program should flow from one block to another. Popular control flow statements include the If-Else statement, which allows you to decide what course of action should be taken depending on whether a certain condition is true or false; the Switch statement, which allows you to decide what course of action should be taken depending on multiple different conditions; and the For loop and While loops which allow you to repeat pieces of code multiple times until a certain condition is met.
Working with Arrays in Java
Arrays allow you to store multiple instances of variables in one place. For example if you wanted to store 5 integers then you would use an array of 5 integers. Arrays are declared by specifying the type of array that you want and then specifying how many elements it should contain for example int array[5]. You can then access individual elements of the array by specifying their index for example array[3] would be the third element of the array.
Exception Handling in Java
Exception handling allows programs to recover from errors that occur during their execution. It works by wrapping code that could result in errors in try/catch blocks known as exceptions. If an error occurs inside of this block then it will be handled by whatever catch block that has been provided. The exception can be handled either by trying to recover from it or by displaying an error message to the user.
Working with Strings in Java
Strings in Java are sequences of characters (letters, numbers, whitespace) that can be manipulated and stored. A programmer can create strings using single or double quotes (eg. “Hello!” or ‘world’). The String class provides much functionality for manipulating strings: substrings can be selected from strings, characters can be added or removed from strings, strings can be compared for equality, and many other tasks.
Working with Regular Expressions in Java
Regular expressions are a powerful way of searching through data for certain patterns or strings. They provide more flexibility than direct search methods such as contains() or startsWith() as they allow for pattern matching which can include multiple options (e.g.: start with either A or B). Making use of regular expressions requires knowledge of their syntax but rewards this effort with more powerful searches.
Working with Files and Streams in Java
Files provide permanent storage for data whereas streams provide temporary or transient storage. Streaming data is usually sent over a network or other connection while files remain resident on disk. Files need to be opened before they can be read or written while streams do not require opening. Files can contain any type of data while streams usually contain textual data. Working with streams avoids complexities associated with file management such as file creation, opening and manipulation.
Working with JDBC for Database Connectivity
JDBC stands for Java Database Connectivity and provides an API for connecting to databases from within Java programs. It enables developers to write database programs that can run on any Java compatible platform with no modifications. JDBC also allows connections from multiple clients at once allowing for increased scalability when compared with direct connections from clients.
Networking and Sockets Programming in Java
Networking and sockets programming allow applications to communicate across networks by sending messages back and forth between them. They provide many benefits such as low latency transmission and reliable transport of data between multiple hosts over long distances. Java provides APIs such as socket programming and network programming that provide functionality for creating networked applications.
Regular Expressions By Example
Here is an example demonstrating regular expressions in Java:
String pattern = "^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,4}$"; //matches email
String input = "john.doe@example.com";
Pattern r = Pattern.compile(pattern);
Matcher m = r.matcher(input);
if (m.find()) {
System.out.println("Valid email");
} else {
System.out.println("Invalid email");
}
This regex validates an email address using quantifiers, character classes, and boundaries. Regular expressions enable powerful string manipulation capabilities.
Debugging and Troubleshooting Techniques for Java Programs
Debugging and troubleshooting can help uncover underlying problems with a program that might not be apparent when looking at source code alone. Techniques such as code commenting, logging, assertions, code refactoring, unit testing and integration testing are all useful ways debugging and troubleshooting programs written in Java which help uncover underlying issues within the codebase.
Writing Secure Code for the JVM
The JVM (Java Virtual Machine) provides safety features that prevent malicious code from being executed within applications written in Java such as bounds checking, stack inspection and memory allocation prevention to avoid buffer overflows. Writing secure code for the JVM requires knowing the library functions that provide protection against known threats as well as understanding the principles behind secure coding practices such as input validation, output validation and authentication.
by the Bito team. 
