pattern-matching-instanceof.yaml

---
id: 4
slug: "pattern-matching-instanceof"
title: "Pattern matching for instanceof"
category: "language"
difficulty: "beginner"
jdkVersion: "16"
oldLabel: "Java 8"
modernLabel: "Java 16+"
oldApproach: "instanceof + Cast"
modernApproach: "Pattern Variable"
oldCode: |-
  if (obj instanceof String) {
      String s = (String) obj;
      System.out.println(s.length());
  }
modernCode: |-
  if (obj instanceof String s) {
      IO.println(s.length());
  }
summary: "Combine type check and cast in one step with pattern matching."
explanation: "Pattern matching for instanceof eliminates the redundant cast after\
  \ a type check. The variable is automatically scoped to where the pattern matches,\
  \ making code safer and shorter."
whyModernWins:
- icon: "🔄"
  title: "No redundant cast"
  desc: "Type check and variable binding happen in a single expression."
- icon: "📏"
  title: "Fewer lines"
  desc: "One line instead of two — the cast line disappears entirely."
- icon: "🛡️"
  title: "Scope safety"
  desc: "The pattern variable is only in scope where the type is guaranteed."
support:
  state: "available"
  description: "Widely available since JDK 16 (March 2021)"
prev: "language/switch-expressions"
next: "language/records-for-data-classes"
related:
- "language/compact-source-files"
- "language/type-inference-with-var"
- "language/text-blocks-for-multiline-strings"
docs:
- title: "Pattern Matching for instanceof (JEP 394)"
  href: "https://openjdk.org/jeps/394"