default-interface-methods.yaml

---
id: 86
slug: "default-interface-methods"
title: "Default interface methods"
category: "language"
difficulty: "intermediate"
jdkVersion: "8"
oldLabel: "Java 7"
modernLabel: "Java 8+"
oldApproach: "Abstract classes for shared behavior"
modernApproach: "Default methods on interfaces"
oldCode: |-
  // Need abstract class to share behavior
  public abstract class AbstractLogger {
      public void log(String msg) {
          System.out.println(
              timestamp() + ": " + msg);
      }
      abstract String timestamp();
  }

  // Single inheritance only
  public class FileLogger
      extends AbstractLogger { ... }
modernCode: |-
  public interface Logger {
      default void log(String msg) {
          IO.println(
              timestamp() + ": " + msg);
      }
      String timestamp();
  }

  // Multiple interfaces allowed
  public class FileLogger
      implements Logger, Closeable { ... }
summary: "Add method implementations directly in interfaces, enabling multiple inheritance\
  \ of behavior."
explanation: "Before Java 8, sharing behavior across unrelated classes required abstract\
  \ classes, which limited you to single inheritance. Default methods let interfaces\
  \ provide method implementations, so classes can inherit behavior from multiple\
  \ interfaces. This was essential for evolving the Collections API (e.g., List.forEach,\
  \ Map.getOrDefault) without breaking existing implementations."
whyModernWins:
- icon: "🔀"
  title: "Multiple inheritance"
  desc: "Classes can implement many interfaces with default methods, unlike single\
    \ abstract class inheritance."
- icon: "📦"
  title: "API evolution"
  desc: "Add new methods to interfaces without breaking existing implementations."
- icon: "🧩"
  title: "Composable behavior"
  desc: "Mix and match capabilities from multiple interfaces freely."
support:
  state: "available"
  description: "Available since JDK 8 (March 2014)."
prev: "tooling/junit6-with-jspecify"
next: "language/markdown-javadoc-comments"
related:
- "language/private-interface-methods"
- "language/pattern-matching-instanceof"
- "language/sealed-classes"
docs:
- title: "Evolving Interfaces (dev.java)"
  href: "https://dev.java/learn/interfaces/examples/"