In this chapter, we are going to talk about Java, one of the pillar languages of object-oriented programming, in which the examples seen in this series of chapters will be presented.

We will talk about its history, understand how it works, why we chose it, and prepare you to write your first program.

# A Bit of History and Philosophy

Java was originally conceived by Sun Microsystems in 1995 and has since been adopted as one of the world's most popular programming languages.

In 2010, Oracle Corporation acquired Sun Microsystems, taking Java to new heights. Its motto "Write once, run anywhere" encapsulates its philosophy of portability and efficiency, allowing the same code to work on different platforms without modifications.

# Why Choose Java?

Java (along with C#) is one of the pioneering and most popular object-oriented programming languages; it continues to be of great importance and is highly used within the industry today.

Java is chosen for several reasons:

• Portability: Thanks to the JVM (Java Virtual Machine), Java code is portable and can run on any operating system.

• Object-Oriented: It promotes a clean and modular design, facilitating software maintenance and scalability.

• Security and Robustness: It manages memory automatically and protects against many common vulnerabilities found in other languages.

• Rich Ecosystem: A wide range of libraries and frameworks for almost any imaginable programming task.

# Compiled vs. Interpreted: The Role of the JVM

Java is both compiled and interpreted; the JVM (Java Virtual Machine) is a virtual machine that allows us to execute Java code. It is a program that is installed on top of the operating system and runs our program.

First, the source code is compiled into bytecode by the Java compiler, and then, at runtime, the JVM interprets and executes this bytecode.

This double stage allows for exhaustive verification and optimization, which contributes to Java's efficiency and portability, as the compiled code is independent of the operating system; it is the same for everyone because it executes on the JVM.

The JVM is responsible for interacting with the Operating System.

# Installing the JDK (Java Development Kit)

The JDK is a necessary development kit for writing and executing Java programs. It includes the JRE (Java Runtime Environment) and a set of tools for Java development.

• Step 1: Download the JDK

  Visit the official website of Oracle or an OpenJDK distribution and download the most recent version of the JDK for your operating system.

• Step 2: Installation

  Run the downloaded installer and follow the instructions. Make sure to select an installation PATH that you remember, as you will need to reference it later.

• Step 3: Configuring the PATH

  To be able to run Java commands from the command line, you need to add the location of the Java binary to the system PATH.

  • On Windows: Go to System Settings > Environment Variables and edit the 'Path' system variable to include the path to the Java binary (e.g., C:\Program Files\Java\jdk-11.0.1\bin).
  • On macOS/Linux: Edit your profile file (.bash_profile, .bashrc, .zprofile, etc.) and add export PATH=/usr/lib/jvm/jdk-11.0.1/bin:$PATH, replacing the path with your specific installation path.

• Step 4: Verification

  Open a terminal or command line and type java -version and javac -version. If the installation was successful, you should see the installed versions of the JRE and the Java compiler, respectively.

# IDE (Integrated Development Environment)

An IDE will provide you with an interface for writing, debugging, and executing your code. Some popular ones include:

• Eclipse: The most widely used IDE by Java developers.

• IntelliJ IDEA: Known for its ergonomics and intelligent features.

• NetBeans: Another free and open-source IDE.

Choose one, download it, and install it from its official website.

# Java Syntax

Java is a rich and powerful programming language with many elements in its syntax. Below, I present the basic elements you will need to know to start programming in Java.

public class MyClass {
   // Attributes, methods, etc.
}

public void myMethod(){
   // Code to execute
}

int myNumber = 5; // Integer variable

if (condition1) {
   // code to execute if condition1 is true
} else if (condition2) {
   // code to execute if condition2 is true
} else if (condition3) {
   // code to execute if condition3 is true
   /// ...
} else {
   // code to execute if no previous condition is true
}

switch (expression) {
   case x: 
         // code to execute if expression is equal to x (expression == x)
         break; // does not continue evaluating other cases
   case y: 
         // code to execute if expression is equal to y (expression == y)
         break; // does not continue evaluating other cases
   default:
      // code to execute if no previous cases are met
      // also executes if it didn't exit via some break
}

for (int i=0; i < 10; i++) {
   // code to execute in each iteration
}

while (condition) {
   // code to execute while the condition is true
}

int[] myArray = {1, 2, 3, 4, 5};

public class MyClass {
   // attributes

   // Constructor
   public MyClass() {
      // code to execute during object creation
   }
}

public class MySubclass extends MyClass {
   // Class that inherits from MyClass
}

interface MyInterface {
   public void myMethod();
}

abstract class MyClass {
   public abstract void myMethod(); // abstract method
}

try {
   // code that may generate an exception
} catch (ExceptionType1 e) {
   // code to handle the exception
} catch (ExceptionType2 e) {
   // code to handle the exception
   // ...
} catch (ExceptionTypeN e) {
   // code to handle the exception
}

// names is of type List (Interface)
// when creating it, I assign a concrete type, ArrayList
List<String> names = new ArrayList<>();
names.add("Ana");
names.add("Luis");
// List allows duplicate elements
names.add("Ana");

for (String name : names) {
      System.out.println(name);
}

Set<String> names = new HashSet<>();
names.add("Ana");
names.add("Luis");
names.add("Ana");

// Set does not allow duplicate elements, it will not add it
for (String name : names) {
      System.out.println(name);
}

Map<String, Integer> ages = new HashMap<>();
ages.put("Ana", 25);
ages.put("Luis", 30);
ages.put("Ana", 28); // Keys are unique. This overwrites "Ana"'s value.

for (Map.Entry<String, Integer> entry : ages.entrySet()) {
      System.out.println(entry.getKey() + ": " + entry.getValue());
}

Queue<String> queue = new LinkedList<>();
queue.offer("First");
queue.offer("Second");
queue.offer("Third");

while (!queue.isEmpty()) {
      System.out.println(queue.poll());
}

The basic elements of Java syntax mentioned here form the backbone of any Java program.

Throughout the chapters, we will see what each one means.

# Your First Java Program: "Hello World"

Once you have configured your environment, it's time to write your first program.

• Step 1: Create a New File

  Open your IDE, create a new project, and then a new file called HelloWorld.java.

• Step 2: Write the Code

  Copy the following code into your file.

  public class HelloWorld { // Class
     // public static void main is the entry point of every program
     public static void main(String[] args) {
           System.out.println("Hello, world!");
     }
  }

• Step 3: Compile and Run

  Use your IDE's tools to compile and run the program. You should see "Hello, world!" printed in the output.

# Understanding the Code

• Class: public class HelloWorld defines a public class named HelloWorld. In Java, all code must be inside a class.
• Main Method: public static void main(String[] args) defines the main method, the entry point for any Java program. When you run the program, the JVM calls this method.
• Print to Console: System.out.println("Hello, world!"); prints a line of text to the console. It is a way to show data to the user.

# Troubleshooting Common Issues

Don't be discouraged if you encounter errors; they are part of learning. Always analyze the error, it's likely one of the following:

• JDK Installed Correctly: Make sure that both java and javac are available on your command line or terminal.

• Code Structure: Verify that your code structure exactly matches the example and that the filename matches the class name.

• IDE Configuration: Make sure your IDE is configured to use the JDK you installed.

• "Google it": "Googling" is what will prevent "reinventing the wheel." The Java community is huge and always ready to help; on sites like Stack Overflow, you will find many common problems.

# Exploring Further

Once you feel comfortable with the fundamentals, don't stop. Java is very powerful.

Experiment with more advanced concepts like threads, collections, and GUIs.

Dive into frameworks like Spring to see how Java is handled in enterprise environments.

# Conclusions

Java is not just a language; it is an adventure in the world of programming. With each line of code, you are participating in a story that has been written by millions of developers over decades.

So go ahead, configure your environment, write your code, and be part of the global community that continues to take Java to new horizons.