Pattern Matching (instanceof & switch)

Pattern matching lets instanceof and switch test a value's type and bind it to a ready-to-use, correctly typed variable in a single step — no separate cast required.

Learn Pattern Matching (instanceof & switch) in our free Java course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a…

Part of the free Java course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

You should know inheritance and casting and the new switch expressions . instanceof patterns need Java 16+ ; switch patterns need Java 21+ .

💡 Analogy: The old way was like a mailroom worker who first checks "is this a parcel?" and then, in a separate step, picks up the parcel to weigh it ( if (o instanceof Box) {" Box b = (Box) o; ... "} ). Pattern matching is the worker who recognises the parcel and already has it in hand: if (o instanceof Box b) — one motion. A pattern switch is the sorting table where each chute is labelled by type , and whatever lands there arrives already identified.

The win is fewer casts, fewer chances to cast to the wrong type, and code that reads as "when it's a String named s , do this".

Write the type test and a variable name together: o instanceof String s . If o is a String , the binding variable s is in scope and already typed — call s.length() with no cast. Compare the before/after:

A pattern switch dispatches on the type of its selector. Each case Type name -> ... matches when the value is that type and binds name . This collapses a tower of if/else instanceof checks into one tidy switch. Add an explicit case null so a null selector is handled instead of throwing.

A guard adds a boolean condition to a pattern using when : case String s when s.isEmpty() -> ... . The arm matches only if the type matches and the guard is true. Because arms are tried top to bottom, list the most specific guarded patterns first and the plain pattern last as a catch-all for that type.

Predict the result before opening each answer.

Answer: No. Outside the if , the pattern isn't provably true, so s is out of scope: error: cannot find symbol s . Use s only where the match holds.

Answer: This won't compile. The plain case Integer i comes first and matches every Integer , so the guarded arm below is unreachable — a compile error. Swap their order.

Answer: If obj is null , it throws NullPointerException — a pattern switch only handles null if you add case null . Add case null -> "none"; to be safe.

🎯 Your Turn — instanceof with a binding

Combine a pattern with && to only match long strings, and handle integers too.

🧩 Mini-Challenge — A grading pattern switch

Use guarded patterns and a case null label — and get the order right.

You can now bind a typed variable straight from instanceof , dispatch on type with a pattern switch , refine matches with when guards, handle null with case null , and order cases so the first match is the right one.

Next up: Sealed Classes & Interfaces — restrict who may implement a type so a pattern switch can be exhaustive.

Practice quiz

• Browse all free courses
• Cheat sheets & quizzes
• Coding dictionary