Introduction

In this lesson, you'll learn about introduction in Java. Coming from C#, you already have a foundation for understanding this concept. We'll build on that knowledge while highlighting the key differences.

Mirror Card

From C#:

In C#, you're familiar with introduction.

In Java:

Java has its own approach to introduction, which we'll explore step by step.

The Java Way

Let's see how Java handles this concept. Here's a typical example:

Java Example

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

Comparing to C#

Here's how you might have written similar code in C#:

C# (What you know)

using System;

class Hello {
    static void Main() {
        Console.WriteLine("Hello");
    }
}

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

using vs import for bringing in namespaces/packages

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

Console.WriteLine vs System.out.println for printing

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

static void Main() vs public static void main(String[]) — Java requires public and lowercase main

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

C# runs on .NET/CLR; Java runs on JVM

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

Both require a class as the entry point

Step-by-Step Breakdown

1. Entry Point Differences

In C# the entry point is 'static void Main()'. In Java it must be exactly 'public static void main(String[] args)' — lowercase m, public, and the args parameter is required.

static void Main() {
    Console.WriteLine("Hello");
}

Java

public static void main(String[] args) {
    System.out.println("Hello");
}

Common Pitfall

If you capitalize 'Main' in Java the JVM won't find your entry point and will throw a runtime error.

2. Namespaces vs Packages

C# uses 'using' to import namespaces. Java uses 'import' to import packages. Java's java.lang is always imported automatically — no need to import System equivalent.

using System;
using System.Collections.Generic;

Java

import java.util.ArrayList;
import java.util.List;
// java.lang.* (String, Math, etc.) is auto-imported

3. Print Statements

Console.WriteLine has a direct Java equivalent. System.out is a PrintStream object with println, print, and printf methods.

Console.WriteLine("Hello");
Console.Write("No newline");
Console.WriteLine($"Value: {x}");

Java

System.out.println("Hello");
System.out.print("No newline");
System.out.printf("Value: %d%n", x);

Rule of Thumb

println adds a newline; print does not. Use printf for formatted output just like C#'s string.Format.

4. Runtime Platforms

Both languages compile to bytecode and run on a managed runtime with a JIT compiler and garbage collector. The key difference is ecosystem: .NET for C#, JVM for Java.

Java

// C# compilation chain:
// .cs -> (csc/Roslyn) -> .dll/.exe -> CLR JIT -> machine code

// Java compilation chain:
// .java -> (javac) -> .class -> JVM JIT -> machine code

// Both have GC, reflection, and generics at runtime

Rule of Thumb

Think of the JVM and CLR as siblings. Most performance intuitions transfer directly.

Common Mistakes

When coming from C#, developers often make these mistakes:

using vs import for bringing in namespaces/packages
Console.WriteLine vs System.out.println for printing
static void Main() vs public static void main(String[]) — Java requires public and lowercase main

Common Pitfall

Don't assume Java works exactly like C#. While the concepts may be similar, the syntax and behavior can differ significantly.

Key Takeaways

Java entry point is 'public static void main(String[] args)' — lowercase, public, args required
Use import instead of using; java.lang is always available
System.out.println replaces Console.WriteLine
Both platforms are managed runtimes with GC and JIT

Rule of Thumb

The best way to learn is by doing. Try rewriting some of your C# code in Java to practice these concepts.

Introduction

Mirror Card

From C#:

In C#, you're familiar with introduction.

In Java:

Java has its own approach to introduction, which we'll explore step by step.

The Java Way

Let's see how Java handles this concept. Here's a typical example:

Java Example

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

Comparing to C#

Here's how you might have written similar code in C#:

C# (What you know)

using System;

class Hello {
    static void Main() {
        Console.WriteLine("Hello");
    }
}

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

using vs import for bringing in namespaces/packages

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

Console.WriteLine vs System.out.println for printing

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

static void Main() vs public static void main(String[]) — Java requires public and lowercase main

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

C# runs on .NET/CLR; Java runs on JVM

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

Both require a class as the entry point

Step-by-Step Breakdown

1. Entry Point Differences

In C# the entry point is 'static void Main()'. In Java it must be exactly 'public static void main(String[] args)' — lowercase m, public, and the args parameter is required.

static void Main() {
    Console.WriteLine("Hello");
}

Java

public static void main(String[] args) {
    System.out.println("Hello");
}

Common Pitfall

If you capitalize 'Main' in Java the JVM won't find your entry point and will throw a runtime error.

2. Namespaces vs Packages

C# uses 'using' to import namespaces. Java uses 'import' to import packages. Java's java.lang is always imported automatically — no need to import System equivalent.

using System;
using System.Collections.Generic;

Java

import java.util.ArrayList;
import java.util.List;
// java.lang.* (String, Math, etc.) is auto-imported

3. Print Statements

Console.WriteLine has a direct Java equivalent. System.out is a PrintStream object with println, print, and printf methods.

Console.WriteLine("Hello");
Console.Write("No newline");
Console.WriteLine($"Value: {x}");

Java

System.out.println("Hello");
System.out.print("No newline");
System.out.printf("Value: %d%n", x);

Rule of Thumb

println adds a newline; print does not. Use printf for formatted output just like C#'s string.Format.

4. Runtime Platforms

Both languages compile to bytecode and run on a managed runtime with a JIT compiler and garbage collector. The key difference is ecosystem: .NET for C#, JVM for Java.

Java

// C# compilation chain:
// .cs -> (csc/Roslyn) -> .dll/.exe -> CLR JIT -> machine code

// Java compilation chain:
// .java -> (javac) -> .class -> JVM JIT -> machine code

// Both have GC, reflection, and generics at runtime

Rule of Thumb

Think of the JVM and CLR as siblings. Most performance intuitions transfer directly.

Common Mistakes

When coming from C#, developers often make these mistakes:

using vs import for bringing in namespaces/packages
Console.WriteLine vs System.out.println for printing
static void Main() vs public static void main(String[]) — Java requires public and lowercase main

Common Pitfall

Don't assume Java works exactly like C#. While the concepts may be similar, the syntax and behavior can differ significantly.

Key Takeaways

Java entry point is 'public static void main(String[] args)' — lowercase, public, args required
Use import instead of using; java.lang is always available
System.out.println replaces Console.WriteLine
Both platforms are managed runtimes with GC and JIT

Rule of Thumb

The best way to learn is by doing. Try rewriting some of your C# code in Java to practice these concepts.