Modern Java Features - C# to Java | codemirr

Introduction

In this lesson, you'll learn about modern java features 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 java records, pattern matching, sealed classes vs c# equivalents.

In Java:

Java has its own approach to java records, pattern matching, sealed classes vs c# equivalents, 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

// Java record — same as C# record
public record Point(double x, double y) {}
var p = new Point(1, 2);
// No 'with' — use copy constructor or builder pattern

// Sealed interface + records
public sealed interface Shape
    permits Circle, Rect {}

public record Circle(double radius) implements Shape {}
public record Rect(double w, double h)   implements Shape {}

// Pattern matching switch (Java 21)
String describe(Shape s) {
    return switch (s) {
        case Circle c when c.radius() > 10 -> "large circle r=" + c.radius();
        case Circle c                      -> "circle r=" + c.radius();
        case Rect r                        -> "rect " + r.w() + "x" + r.h();
    };
}

// instanceof pattern binding (Java 16+)
if (s instanceof Circle c) {
    System.out.println("radius: " + c.radius());
}

// var — local type inference (Java 10+)
var list = new ArrayList<String>();

// Text blocks (Java 15+)
var json = """
    {"name": "Alice"}
    """;

Comparing to C#

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

C# (What you know)

// C# record — immutable with value equality
public record Point(double X, double Y);
var p = new Point(1, 2);
var p2 = p with { X = 5 }; // non-destructive mutation

// switch expression
string Describe(Shape s) => s switch {
    Circle { Radius: > 10 } c => $"large circle r={c.Radius}",
    Circle c                  => $"circle r={c.Radius}",
    _                         => "other"
};

// Sealed hierarchy
public abstract record Shape;
public record Circle(double Radius) : Shape;
public record Rect(double W, double H) : Shape;

// Primary constructors (C# 12)
public class Config(string Host, int Port)
{
    public string Host { get; } = Host;
    public int Port { get; } = Port;
}

// Required properties (C# 11)
public class UserDto
{
    public required string Name { get; init; }
    public required string Email { get; init; }
}

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

Java records use record (lower-case); C# uses record keyword too — similar syntax

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

Java has no 'with' expression — must create a new record manually

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

Java pattern matching uses 'when' guard (C# uses 'when' too)

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

Java sealed uses 'permits' to list allowed subclasses; C# uses no special syntax

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

Java switch expression uses -> (like C#); each arm doesn't need break

Step-by-Step Breakdown

1. Records

Both C# and Java have records for immutable data. Java getters use the field name directly (c.radius()), not C#'s property syntax (c.Radius).

public record Point(double X, double Y);
var p = new Point(1, 2);
p.X; // C# property

Java

record Point(double x, double y) {}
var p = new Point(1, 2);
p.x(); // Java accessor — note parentheses!

2. Sealed Hierarchy

Java uses 'sealed interface permits ...' to list all allowed implementors. C# uses 'sealed' on the subclasses but doesn't enumerate them.

public abstract record Shape;
public record Circle(double Radius) : Shape;

Java

sealed interface Shape permits Circle, Rect {}
record Circle(double radius) implements Shape {}
record Rect(double w, double h)   implements Shape {}

3. Pattern Matching Switch

Java 21 switch expressions support type patterns and guards with 'when'. Sealed types make the switch exhaustive.

s switch {
    Circle { Radius: > 10 } c => $"large {c.Radius}",
    _ => "other"
}

Java

switch (s) {
    case Circle c when c.radius() > 10 -> "large " + c.radius();
    case Circle c                      -> "circle " + c.radius();
    case Rect r                        -> "rect";
}

4. var and Text Blocks

Java 10+ var for local type inference. Java 15+ text blocks use triple-quote strings — identical purpose to C# verbatim/raw strings.

var list = new List<string>();
var json = """
    {"name": "Alice"}
    """;

Java

var list = new ArrayList<String>(); // Java 10+
var json = """
    {"name": "Alice"}
    """; // Java 15+ text block

Common Mistakes

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

Java records use record (lower-case); C# uses record keyword too — similar syntax
Java has no 'with' expression — must create a new record manually
Java pattern matching uses 'when' guard (C# uses 'when' too)

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 records similar to C# records; Java getters use method syntax: p.x() not p.X
sealed interface permits A, B, C makes switch exhaustive — Java's version of discriminated unions
Java pattern switch uses 'when' for guards (C# uses 'when' too)
var (Java 10+) for type inference; text blocks (Java 15+) for multi-line strings

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 java records, pattern matching, sealed classes vs c# equivalents.

In Java:

Java has its own approach to java records, pattern matching, sealed classes vs c# equivalents, 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

// Java record — same as C# record
public record Point(double x, double y) {}
var p = new Point(1, 2);
// No 'with' — use copy constructor or builder pattern

// Sealed interface + records
public sealed interface Shape
    permits Circle, Rect {}

public record Circle(double radius) implements Shape {}
public record Rect(double w, double h)   implements Shape {}

// Pattern matching switch (Java 21)
String describe(Shape s) {
    return switch (s) {
        case Circle c when c.radius() > 10 -> "large circle r=" + c.radius();
        case Circle c                      -> "circle r=" + c.radius();
        case Rect r                        -> "rect " + r.w() + "x" + r.h();
    };
}

// instanceof pattern binding (Java 16+)
if (s instanceof Circle c) {
    System.out.println("radius: " + c.radius());
}

// var — local type inference (Java 10+)
var list = new ArrayList<String>();

// Text blocks (Java 15+)
var json = """
    {"name": "Alice"}
    """;

Comparing to C#

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

C# (What you know)

// C# record — immutable with value equality
public record Point(double X, double Y);
var p = new Point(1, 2);
var p2 = p with { X = 5 }; // non-destructive mutation

// switch expression
string Describe(Shape s) => s switch {
    Circle { Radius: > 10 } c => $"large circle r={c.Radius}",
    Circle c                  => $"circle r={c.Radius}",
    _                         => "other"
};

// Sealed hierarchy
public abstract record Shape;
public record Circle(double Radius) : Shape;
public record Rect(double W, double H) : Shape;

// Primary constructors (C# 12)
public class Config(string Host, int Port)
{
    public string Host { get; } = Host;
    public int Port { get; } = Port;
}

// Required properties (C# 11)
public class UserDto
{
    public required string Name { get; init; }
    public required string Email { get; init; }
}

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

Java records use record (lower-case); C# uses record keyword too — similar syntax

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

Java has no 'with' expression — must create a new record manually

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

Java pattern matching uses 'when' guard (C# uses 'when' too)

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

Java sealed uses 'permits' to list allowed subclasses; C# uses no special syntax

Mirror Card

From C#:

You may be used to different syntax or behavior.

In Java:

Java switch expression uses -> (like C#); each arm doesn't need break

Step-by-Step Breakdown

1. Records

Both C# and Java have records for immutable data. Java getters use the field name directly (c.radius()), not C#'s property syntax (c.Radius).

public record Point(double X, double Y);
var p = new Point(1, 2);
p.X; // C# property

Java

record Point(double x, double y) {}
var p = new Point(1, 2);
p.x(); // Java accessor — note parentheses!

2. Sealed Hierarchy

Java uses 'sealed interface permits ...' to list all allowed implementors. C# uses 'sealed' on the subclasses but doesn't enumerate them.

public abstract record Shape;
public record Circle(double Radius) : Shape;

Java

sealed interface Shape permits Circle, Rect {}
record Circle(double radius) implements Shape {}
record Rect(double w, double h)   implements Shape {}

3. Pattern Matching Switch

Java 21 switch expressions support type patterns and guards with 'when'. Sealed types make the switch exhaustive.

s switch {
    Circle { Radius: > 10 } c => $"large {c.Radius}",
    _ => "other"
}

Java

switch (s) {
    case Circle c when c.radius() > 10 -> "large " + c.radius();
    case Circle c                      -> "circle " + c.radius();
    case Rect r                        -> "rect";
}

4. var and Text Blocks

Java 10+ var for local type inference. Java 15+ text blocks use triple-quote strings — identical purpose to C# verbatim/raw strings.

var list = new List<string>();
var json = """
    {"name": "Alice"}
    """;

Java

var list = new ArrayList<String>(); // Java 10+
var json = """
    {"name": "Alice"}
    """; // Java 15+ text block

Common Mistakes

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

Java records use record (lower-case); C# uses record keyword too — similar syntax
Java has no 'with' expression — must create a new record manually
Java pattern matching uses 'when' guard (C# uses 'when' too)

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 records similar to C# records; Java getters use method syntax: p.x() not p.X
sealed interface permits A, B, C makes switch exhaustive — Java's version of discriminated unions
Java pattern switch uses 'when' for guards (C# uses 'when' too)
var (Java 10+) for type inference; text blocks (Java 15+) for multi-line strings

Rule of Thumb

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