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      <span>Default interface methods</span>
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        <span class="badge language">Language</span>
        <span class="badge intermediate">intermediate</span>
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      <h1>Default interface methods</h1>
      <p>Add method implementations directly in interfaces, enabling multiple inheritance of behavior.</p>
    </div>

    <section class="compare-section">
      <div class="section-label">Code Comparison</div>
      <div class="compare-container">
        <div class="compare-panel old-panel">
          <div class="compare-panel-header">
            <span class="compare-tag old">✕ Java 7</span>
            <button class="copy-btn" data-code="old">Copy</button>
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          <div class="compare-code">
            <pre class="code-text">// Need abstract class to share behavior
public abstract class AbstractLogger {
    public void log(String msg) {
        System.out.println(
            timestamp() + &quot;: &quot; + msg);
    }
    abstract String timestamp();
}

// Single inheritance only
public class FileLogger
    extends AbstractLogger { ... }</pre>
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            <span class="compare-tag modern">✓ Java 8+</span>
            <button class="copy-btn" data-code="modern">Copy</button>
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          <div class="compare-code">
            <pre class="code-text">public interface Logger {
    default void log(String msg) {
        System.out.println(
            timestamp() + &quot;: &quot; + msg);
    }
    String timestamp();
}

// Multiple interfaces allowed
public class FileLogger
    implements Logger, Closeable { ... }</pre>
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      <div class="section-label">Why the modern way wins</div>
      <div class="why-grid">
        <div class="why-card">
          <div class="why-icon">🔀</div>
          <h3>Multiple inheritance</h3>
          <p>Classes can implement many interfaces with default methods, unlike single abstract class inheritance.</p>
        </div>
        <div class="why-card">
          <div class="why-icon">📦</div>
          <h3>API evolution</h3>
          <p>Add new methods to interfaces without breaking existing implementations.</p>
        </div>
        <div class="why-card">
          <div class="why-icon">🧩</div>
          <h3>Composable behavior</h3>
          <p>Mix and match capabilities from multiple interfaces freely.</p>
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        <div class="info-label">Old Approach</div>
        <div class="info-value red">Abstract classes for shared behavior</div>
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        <div class="info-label">Modern Approach</div>
        <div class="info-value green">Default methods on interfaces</div>
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        <div class="info-label">Since JDK</div>
        <div class="info-value accent">8</div>
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        <div class="info-label">Difficulty</div>
        <div class="info-value blue">intermediate</div>
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      <div class="section-label">JDK Support</div>
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        <div class="bs-feature-name">Default interface methods</div>
        <span class="baseline-badge widely">Available</span>
        <p class="bs-desc">Available since JDK 8 (March 2014).</p>
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    <section class="explanation">
      <h2>How it works</h2>
      <p>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.</p>
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              <div class="mini-label">Java 8</div>
              <pre class="code-text">interface Logger {
    default void logInfo(String msg) {
        System.out.println(
            &quot;[INFO] &quot; + timestamp() + msg);
    }
    default void logWarn(String msg) {
        System.out.println(
            &quot;[WARN] &quot; + timestamp() + msg);
    }
}</pre>
            </div>
            <div class="card-code-layer modern-layer">
              <div class="mini-label">Java 9+</div>
              <pre class="code-text">interface Logger {
    private String format(String lvl, String msg) {
        return &quot;[&quot; + lvl + &quot;] &quot; + timestamp() + msg;
    }
    default void logInfo(String msg) {
        System.out.println(format(&quot;INFO&quot;, msg));
    }
    default void logWarn(String msg) {
        System.out.println(format(&quot;WARN&quot;, msg));
    }
}</pre>
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              <pre class="code-text">if (obj instanceof String) {
    String s = (String) obj;
    System.out.println(s.length());
}</pre>
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              <div class="mini-label">Java 16+</div>
              <pre class="code-text">if (obj instanceof String s) {
    System.out.println(s.length());
}</pre>
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              <pre class="code-text">// Anyone can extend Shape
public abstract class Shape { }
public class Circle extends Shape { }
public class Rect extends Shape { }
// unknown subclasses possible</pre>
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    permits Circle, Rect {}
public record Circle(double r)
    implements Shape {}
public record Rect(double w, double h)
    implements Shape {}</pre>
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