15 — Records & Sealed Classes

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▶️ java pkg1core/core16RecordsDemo.java · java pkg1core/core17SealedClassesDemo.java

Records (Java 16+) — data carriers

1record Point(int x, int y) {}

Compiler auto-generates:

• Constructor
• x() and y() accessors (not getX())
• equals, hashCode, toString

1Point p = new Point(3, 4);2System.out.println(p.x());    // 3

Compact constructor — validation

1record User(String email) {2    User {3        if (!email.contains("@")) throw new IllegalArgumentException();4    }5}

💡 Use records for DTOs, value objects, pattern matching — not JPA entities without care.

Sealed classes (Java 17+) — controlled hierarchy

1sealed interface Expr permits Constant, Add {2    double eval();3}4record Constant(double value) implements Expr {5    public double eval() { return value; }6}7record Add(Expr left, Expr right) implements Expr {8    public double eval() { return left.eval() + right.eval(); }9}

Only listed classes can extend/implement. Enables exhaustive switch:

1double result = switch (expr) {2    case Constant c -> c.value();3    case Add a      -> a.left().eval() + a.right().eval();4};

Why sealed + records together?

Modern Java algebraic data types — model closed sets of variants (expressions, AST nodes, payment types) with compile-time safety.

Milestone: You completed OOP (chapters 10–15). 🎉

Next → 16 Exceptions Related → 01 Core Java