Java Sealed Classes Cheat Sheet

Syntax for sealed, non-sealed, and final permitted subclasses, plus exhaustive pattern matching with switch, as finalized in modern Java.

1 PageIntermediateFeb 8, 2026

Declaring a Sealed Hierarchy

sealed restricts which classes/interfaces may extend or implement it.

java

public sealed interface Shape    permits Circle, Square, Rectangle {}public final class Circle implements Shape {    public final double radius;    public Circle(double radius) { this.radius = radius; }}public final class Square implements Shape {    public final double side;    public Square(double side) { this.side = side; }}// Permitted subclasses must be final, sealed, or non-sealedpublic non-sealed class Rectangle implements Shape {    public double width, height;}

Same-File / Same-Module Shortcut

permits is optional when all subclasses are in the same file.

java

// If all permitted subtypes live in the same source file, permits can be omittedsealed interface Result<T> {    record Ok<T>(T value) implements Result<T> {}    record Err<T>(String message) implements Result<T> {}}

Exhaustive Pattern Matching

The compiler verifies switch covers every permitted subtype — no default needed.

java

static double area(Shape shape) {    return switch (shape) {        case Circle c -> Math.PI * c.radius * c.radius;        case Square s -> s.side * s.side;        case Rectangle r -> r.width * r.height;        // no default clause required: compiler proves exhaustiveness    };}// With record patterns (deconstruction)static String describe(Result<Integer> result) {    return switch (result) {        case Result.Ok<Integer> ok -> "value: " + ok.value();        case Result.Err<Integer> err -> "error: " + err.message();    };}

Permitted-Subclass Modifiers

Every direct subclass of a sealed type must pick exactly one.

final- no further subclassing allowed
sealed- continues the restriction with its own permits list
non-sealed- reopens the hierarchy, any class may extend it
permits- explicit list of allowed direct subtypes (optional if same file)
record- records are implicitly final, so they satisfy the sealed contract directly

Pro Tip

Combine sealed hierarchies with exhaustive switch expressions to get compile-time enforcement that every case is handled — adding a new permitted subtype breaks the build everywhere a switch forgot to handle it, which is exactly what you want.

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