Introduction to Java

Java was designed to fix C++'s biggest pains — manual memory management, header files, fragile cross-platform builds — while keeping a familiar C-style syntax. Trade-off: a fatter runtime and slower startup. Worth it for most server/Android workloads.

A Brief History

Use Java 21 (LTS) for new projects. Java 17 is also widely deployed.

How Java Code Becomes Output

.java  →  javac  →  .class  →  JVM (interprets / JIT-compiles)  →  CPU instructions

The JVM uses a JIT (Just-In-Time) compiler to convert hot bytecode into native code at runtime — so long-running Java apps approach C++ speed.

Anatomy of a Java File

package com.example.demo;        // optional but recommended
import java.util.List;            // standard library import
public class Hello {              // class name == filename
    public static void main(String[] args) {
        System.out.println("Hello, Java!");
    }
}

Rules

• Filename must match the public class name: Hello.java → public class Hello.
• The entry point is exactly public static void main(String[] args).
• Statements end with semicolons ;.
• Blocks use braces { }.
• Strings use double quotes ". Single quotes ' are only for char.

Installing the JDK

Recommended: Eclipse Temurin (Adoptium)

# Windows (winget)
winget install EclipseAdoptium.Temurin.21.JDK
# macOS (Homebrew)
brew install --cask temurin@21
# Linux (Debian/Ubuntu)
sudo apt install openjdk-21-jdk

Verify:

java --version
javac --version

Both should report 21.x.

Running Java

The Long Way (any version)

javac Hello.java
java  Hello

Single-File (Java 11+)

java Hello.java

JShell — Java's REPL (Java 9+)

$ jshell
jshell> int x = 5;
jshell> System.out.println(x * x);
25
jshell> /exit

Great for trying snippets without writing a full class.

A Slightly Bigger Example

public class Greet {
    public static void main(String[] args) {
        String name = args.length > 0 ? args[0] : "world";
        System.out.println("Hello, " + name + "!");
    }
}

$ java Greet.java Asha
Hello, Asha!

String[] args is the array of command-line arguments — note it's a real String array, not argc/argv.

Build Tools You'll Meet

For real projects you'll use a build tool, not raw javac:

For learning, plain javac/java or an IDE is enough.

IDEs

• IntelliJ IDEA Community — free; the best Java experience by a wide margin.
• VS Code + Extension Pack for Java — lightweight, great for small projects.
• Eclipse — older, still common in enterprises.

Java vs C++ vs Python (Quick Map)

What's Next

You now know how to write, compile and run a Java program. Next: variables and the type system that catches most errors before the program even starts.