Records and Sealed Classes

Modern Java is moving away from the “everything is a complex object” philosophy toward Data-Oriented Programming. Two of the most important tools for this are Records and Sealed Classes.

1. Records (Data Carriers)

Before Java 14, creating a simple data carrier (POJO) required dozens of lines of boilerplate (Getters, equals, hashCode, toString).

A Record is an immutable data carrier that generates all of this for you in one line.

public record User(String name, String email, int age) {}

Key Features of Records:

Immutable: All fields are final by default.
Concise: No more boilerplate.
Canonical Constructor: Automatically handles assignment.
Validation: You can add validation logic in a “Compact Constructor.”

public record User(String name, String email) {
    public User {
        if (name.isBlank()) throw new IllegalArgumentException("Name cannot be blank");
    }
}

2. Sealed Classes (Restricted Hierarchies)

Traditionally, a class could be extended by anything unless it was final. Sealed Classes give you a middle ground: you define exactly which classes are allowed to extend your class.

public sealed class Shape permits Circle, Square, Triangle {}

public final class Circle extends Shape { ... }
public final class Square extends Shape { ... }
public final class Triangle extends Shape { ... }

Why use Sealed Classes?

Security: Prevents unauthorized subclasses.
Exhaustiveness: When using switch expressions (covered in the next chapter), the compiler knows exactly all possible types, so you don’t need a default case.

3. Interactive: Boilerplate Shredder

See how much code you save using Records.

Legacy Class (Java 8)

              public class User {

                private final String name;

                public User(String n) {this.name=n;}

                public String getName() {return name;}

                @Override public boolean equals(Object o)...

                @Override public int hashCode()...

                @Override public String toString()...

              }

➡️

Modern Record (Java 17+)

              record User(String name) {}
          

4. Best Practices

DTOs: Use Records for Data Transfer Objects (fetching data from DB or API).
Domain Logic: Use Sealed Classes for State Machines or Command patterns.
Immutability: Remember that while a Record is final, its fields (like a List) may still be mutable elements themselves. Always use List.of() or Collections.unmodifiableList().