private-interface-methods.yaml

---
id: 13
slug: "private-interface-methods"
title: "Private interface methods"
category: "language"
difficulty: "intermediate"
jdkVersion: "9"
oldLabel: "Java 8"
modernLabel: "Java 9+"
oldApproach: "Duplicated Logic"
modernApproach: "Private Methods"
oldCode: |-
  interface Logger {
      default void logInfo(String msg) {
          System.out.println(
              "[INFO] " + timestamp() + msg);
      }
      default void logWarn(String msg) {
          System.out.println(
              "[WARN] " + timestamp() + msg);
      }
  }
modernCode: |-
  interface Logger {
      private String format(String lvl, String msg) {
          return "[" + lvl + "] " + timestamp() + msg;
      }
      default void logInfo(String msg) {
          IO.println(format("INFO", msg));
      }
      default void logWarn(String msg) {
          IO.println(format("WARN", msg));
      }
  }
summary: "Extract shared logic in interfaces using private methods."
explanation: "Java 9 allows private methods in interfaces, enabling you to share code\
  \ between default methods without exposing implementation details to implementing\
  \ classes."
whyModernWins:
- icon: "🧩"
  title: "Code reuse"
  desc: "Share logic between default methods without duplication."
- icon: "🔐"
  title: "Encapsulation"
  desc: "Implementation details stay hidden from implementing classes."
- icon: "🧹"
  title: "DRY interfaces"
  desc: "No more copy-paste between default methods."
support:
  state: "available"
  description: "Widely available since JDK 9 (Sept 2017)"
prev: "language/diamond-operator"
next: "language/pattern-matching-switch"
related:
- "language/compact-source-files"
- "language/flexible-constructor-bodies"
- "language/type-inference-with-var"
docs:
- title: "Private Interface Methods"
  href: "https://openjdk.org/jeps/213"