j21-switch-exhaustiveness - Blog of Vincent VAUBAN

🧑‍🍳 Exhaustiveness of switch Expressions
— à la française 🇫🇷

In Java, switch expressions must be exhaustive. That means they must cover all possible values of the expression being switched on. If not, the compiler won't be happy. Normally, you’ll need a default branch to handle all remaining cases.
However, there’s a tasty exception 🍽️: if you're switching on an enum and you've listed all possible enum constants, then Java considers it exhaustive — no need for default, it adds one implicitly.
Example with French Dishes: Enum Edition
Let’s say we have a lovely enum representing some famous French dishes:

enum PlatFrancais { RATATOUILLE, BOUILLABAISSE, COQ_AU_VIN }

If we switch on that enum and cover all known values, there’s no need for default:

String accompagnement = switch (plat) { case RATATOUILLE -> "Pain de campagne"; case BOUILLABAISSE -> "Rouille et croûtons"; case COQ_AU_VIN -> "Pâtes fraîches"; };

This is fine because the compiler knows all the PlatFrancais options and we covered them all.

🧂 The Yield Rule: No Missing Ingredients!
Every case in a switch must return a value using yield, unless it's a simple arrow -> form.
Let’s see what happens when we forget the yield, like a chef who forgets the salt:

String sauce = switch (plat) { case RATATOUILLE -> { System.out.println("Légumes mijotés"); // ❌ ERROR! No yield in the block } default -> "Sauce inconnue"; };

🔴 This won’t compile. If you use a block ({ }) instead of the simple arrow (->), you must use yield.
Same issue if you group cases but forget to yield for all of them:

String vin = switch (plat) { case COQ_AU_VIN, BOUILLABAISSE -> { System.out.println("Accord mets et vins"); // ❌ ERROR! Missing yield } default -> "Eau pétillante"; };

🚫 Illegal Jumps: No Escaping the Kitchen!
Inside a switch expression, you can’t jump out with break, continue, or return. Java requires the switch to resolve to one single value, not an escape route.
Here’s a forbidden move:

platLoop: for (PlatFrancais plat : menu) { String dessert = switch (plat) { case RATATOUILLE -> "Tarte Tatin"; case COQ_AU_VIN -> "Île flottante"; default -> { continue platLoop; // ❌ ERROR! Illegal jump through a switch expression } }; }

🍽️ TL;DR: A switch expression is like a recipe. You must:
🟣 Provide an ingredient (value) for each case.
🟣 Use yield in blocks.
🟣 Avoid jumping out mid-recipe!

🧑‍🍳 Exhaustiveness of switch Expressions

In Java, switch expressions must be exhaustive. That means they must cover all possible values of the expression being switched on. If not, the compiler won't be happy. Normally, you’ll need a default branch to handle all remaining cases.

However, there’s a tasty exception 🍽️: if you're switching on an enum and you've listed all possible enum constants, then Java considers it exhaustive — no need for default, it adds one implicitly.

Example with French Dishes: Enum Edition

Let’s say we have a lovely enum representing some famous French dishes:

If we switch on that enum and cover all known values, there’s no need for default:

This is fine because the compiler knows all the PlatFrancais options and we covered them all.

🧂 The Yield Rule: No Missing Ingredients!

Every case in a switch must return a value using yield, unless it's a simple arrow -> form.

Let’s see what happens when we forget the yield, like a chef who forgets the salt:

🔴 This won’t compile. If you use a block ({ }) instead of the simple arrow (->), you must use yield.

Same issue if you group cases but forget to yield for all of them:

🚫 Illegal Jumps: No Escaping the Kitchen!

Inside a switch expression, you can’t jump out with break, continue, or return. Java requires the switch to resolve to one single value, not an escape route.

Here’s a forbidden move:

🍽️ TL;DR: A switch expression is like a recipe. You must:

🟣 Provide an ingredient (value) for each case.

🟣 Use yield in blocks.

🟣 Avoid jumping out mid-recipe!