Add all 23 GoF design pattern implementations (2026-06-13)

02-structural/facade/Main.java

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+package facade;
+
+/**
+ * Facade Design Pattern — Runnable Demo
+ *
+ * Demonstrates reducing a complex video conversion subsystem
+ * to a single method call via a Facade.
+ *
+ * Run: javac facade/*.java && java facade.Main
+ * Article: https://ankurm.com/facade-design-pattern-java/
+ */
+public class Main {
+
+    public static void main(String[] args) {
+        System.out.println("=== Facade Design Pattern Demo ===\n");
+
+        VideoConversionFacade converter = new VideoConversionFacade();
+
+        System.out.println("-- Client: just one method call --");
+        String result1 = converter.convertVideo("holiday.ogg", "mp4");
+        System.out.println("Output: " + result1);
+
+        System.out.println();
+        String result2 = converter.convertVideo("presentation.mp4", "ogg");
+        System.out.println("Output: " + result2);
+
+        System.out.println("\nClient code: 1 line. Subsystem: 6 classes. Facade hides the complexity.");
+        System.out.println("\n=== Demo complete ===");
+    }
+}

02-structural/facade/Subsystems.java

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+package facade;
+
+/**
+ * Complex subsystem classes — these are what the Facade hides.
+ * Each class has its own complex API; clients shouldn't need to know all of them.
+ */
+
+class VideoFile {
+    private final String filename;
+    private final String codecType;
+
+    VideoFile(String filename) {
+        this(filename, filename.endsWith(".mp4") ? "mpeg4" : "ogg");
+    }
+
+    VideoFile(String filename, String codec) {
+        this.filename  = filename;
+        this.codecType = codec;
+        System.out.println("  VideoFile: " + filename + " [codec: " + codecType + "]");
+    }
+
+    public String getFilename()  { return filename; }
+    public String getCodecType() { return codecType; }
+}
+
+interface Codec { String getName(); }
+
+class MPEG4CompressionCodec implements Codec {
+    @Override public String getName() { return "mpeg4"; }
+}
+
+class OggCompressionCodec implements Codec {
+    @Override public String getName() { return "ogg"; }
+}
+
+class CodecFactory {
+    static Codec extract(VideoFile file) {
+        System.out.println("  CodecFactory: extracting codec from " + file.getFilename());
+        if ("mpeg4".equals(file.getCodecType())) return new MPEG4CompressionCodec();
+        return new OggCompressionCodec();
+    }
+}
+
+class BitrateReader {
+    static VideoFile read(VideoFile file, Codec codec) {
+        System.out.println("  BitrateReader: reading " + file.getFilename()
+                + " with codec " + codec.getName());
+        return new VideoFile(file.getFilename());
+    }
+
+    static VideoFile convert(VideoFile buffer, Codec codec) {
+        System.out.println("  BitrateReader: converting to " + codec.getName());
+        return new VideoFile(buffer.getFilename());
+    }
+}
+
+class AudioMixer {
+    static VideoFile fix(VideoFile result) {
+        System.out.println("  AudioMixer: fixing audio tracks");
+        return new VideoFile(result.getFilename());
+    }
+}

02-structural/facade/VideoConversionFacade.java

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+package facade;
+
+/**
+ * Facade — the single, simple entry point to a complex video subsystem.
+ *
+ * Without this class, clients need to know about CodecFactory,
+ * BitrateReader, AudioMixer, and VideoFile — 4 classes, dozens of methods.
+ * The facade reduces that to ONE method call.
+ *
+ * The subsystem classes still exist and can be used directly
+ * by advanced users who need fine-grained control.
+ */
+public class VideoConversionFacade {
+
+    public String convertVideo(String inputFile, String targetFormat) {
+        System.out.println("VideoConversionFacade: starting conversion of " + inputFile);
+
+        // Step 1: open the file and detect codec
+        VideoFile file = new VideoFile(inputFile);
+        Codec sourceCodec = CodecFactory.extract(file);
+
+        // Step 2: prepare destination codec
+        Codec destCodec;
+        if ("mp4".equals(targetFormat)) {
+            destCodec = new MPEG4CompressionCodec();
+        } else {
+            destCodec = new OggCompressionCodec();
+        }
+
+        // Step 3: read, mix audio, encode
+        VideoFile buffer = BitrateReader.read(file, sourceCodec);
+        VideoFile intermediateResult = BitrateReader.convert(buffer, destCodec);
+        VideoFile result = AudioMixer.fix(intermediateResult);
+
+        String outputFilename = inputFile.replaceAll("\\.[^.]+$", "." + targetFormat);
+        System.out.println("VideoConversionFacade: conversion complete -> " + outputFilename);
+        return outputFilename;
+    }
+}