Question: What is Java?

Java is a widely used object-oriented programming language and software platform that runs on billions of devices, including notebook computers, mobile devices, gaming consoles, medical devices and many others. The rules and syntax of Java are based on the C and C++ languages.

Question: How Java works?

Java is a technology consisting of both a programming language and a software platform. To create an application using Java, you need to download the Java Development Kit (JDK), which is available for Windows, macOS, and Linux. You write the program in the Java programming language, then a compiler turns the program into Java bytecode—the instruction set for the Java Virtual Machine (JVM) that is a part of the Java runtime environment (JRE). Java bytecode runs without modification on any system that supports JVMs, allowing your Java code to be run anywhere.

The Java software platform consists of the JVM, the Java API, and a complete development environment. The JVM parses and runs (interprets) the Java bytecode. The Java API consists of an extensive set of libraries including basic objects, networking and security functions; Extensible Markup Language (XML) generation; and web services. Taken together, the Java language and the Java software platform create a powerful, proven technology for enterprise software development.

Question: Why Java matters?

Java technology is an ideal framework for developing web applications, the foundation for a digital business in any industry. Java application servers are web containers for Java components, XML, and web services, which interact with databases and provide dynamic web content. Java application servers form a stable deployment environment for enterprise applications with capabilities such as transaction management, security, clustering, performance, availability, connectivity, and scalability.

How: Java Is Platform-Independent?

Platform independence is one of the most significant advantages that Java has over other programming languages, particularly for systems that need to work on many different platforms. Java is platform-independent at both the source and the binary level.

At the source level, Java’s primitive data types have consistent sizes across all development platforms. Java’s foundation class libraries make it easy to write code that can be moved from platform to platform without the need to rewrite it to work with that platform. Platform-independence doesn’t stop at the source level, however. Java binary files are also platform-independent and can run on multiple problems without the need to recompile the source. How does this work? Java binary files are actually in a form called bytecodes.

Normally, when you compile a program written in C or in most other languages, the compiler translates your program into machine codes or processor instructions. Those instructions are specific to the processor your computer is running—so, for example, if you compile your code on a Pentium system, the resulting program will run only on other Pentium systems. If you want to use the same program on another system, you have to go back to your original source, get a compiler for that system, and recompile your code Section 1.1 shows the result of this system: multiple executable programs for multiple systems.

Section 1.1

Things are different when you write code in Java. The Java development environment has two parts: a Java compiler and a Java interpreter. The Java compiler takes your Java program and instead of generating machine codes from your source files, it generates bytecodes.

To run a Java program, you run a program called a bytecode interpreter, which in turn executes your Java program Section 1.2 . You can either run the interpreter by itself, or—for applets— there is a bytecode interpreter built into HotJava and other Java-capable browsers that runs the applet for you.

Section 1.2

Getting Started with Programming in Java

In order to write Java programs, you will, of course, need a Java development environment. At the time this book is being written, Sun’s Java Development Kit provides everything you need to start writing Java programs. The JDK is available for Sun SPARC systems running Solaris 2.2 or higher and for Windows NT and Windows 95.

Applets and Applications

Java applications fall into two main groups: applets and applications.

Applet

Applet is a special type of program that is embedded in the webpage to generate the dynamic content. It runs inside the browser and works at client side.

Advantage of Applet

• It works at client side so less response time.
• Secured
• It can be executed by browsers running under many plateforms, including Linux, Windows, Mac Os etc.

Disadvantage of Applet

• Plugin is required at client browser to execute applet.

Applications

Java applications are more general programs written in the Java language. Java applications don’t require a browser to run, and in fact, Java can be used to create most other kinds of applications that you would normally use a more conventional programming language to create. HotJava itself is a Java application.

1. "Hello, World!" Program

This is the simplest Java program, typically used to test if the Java setup is working correctly.

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Use:

• Prints "Hello, World!" to the console.
• Used as an introductory program for beginners to understand the basic structure of a Java program.

public class VariablesExample {
    public static void main(String[] args) {
        int age = 25;
        double salary = 55000.50;
        char grade = 'A';
        boolean isActive = true;

        System.out.println("Age: " + age);
        System.out.println("Salary: " + salary);
        System.out.println("Grade: " + grade);
        System.out.println("Is Active: " + isActive);
    }
}

public class ConditionalExample {
    public static void main(String[] args) {
        int age = 18;

        if (age >= 18) {
            System.out.println("You are eligible to vote.");
        } else {
            System.out.println("You are not eligible to vote.");
        }
    }
}

public class LoopExample {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            System.out.println("Count: " + i);
        }
    }
}

public class MethodExample {
    public static void main(String[] args) {
        int result = addNumbers(5, 10);
        System.out.println("Sum: " + result);
    }

    public static int addNumbers(int a, int b) {
        return a + b;
    }
}

class Person {
    String name;
    int age;

    public void displayInfo() {
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
    }
}

public class Main {
    public static void main(String[] args) {
        Person person1 = new Person();
        person1.name = "Alice";
        person1.age = 30;
        person1.displayInfo();
    }
}