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

02-structural/composite/Main.java

@@ -0,0 +1,58 @@
+package composite;
+
+/**
+ * Composite Design Pattern — Runnable Demo
+ *
+ * Builds a file system tree with nested directories and files.
+ * Demonstrates that getSize() and print() work uniformly on
+ * leaves (File) and composites (Directory) without any instanceof checks.
+ *
+ * Run: javac composite/*.java && java composite.Main
+ * Article: https://ankurm.com/composite-design-pattern-java/
+ */
+public class Main {
+
+    public static void main(String[] args) {
+        System.out.println("=== Composite Design Pattern Demo ===\n");
+
+        // Build a file system tree
+        Directory root = new Directory("project");
+
+        Directory src = new Directory("src");
+        Directory main = new Directory("main");
+        main.add(new File("App.java",        4_200))
+            .add(new File("Config.java",     1_800))
+            .add(new File("Application.yml", 3_100));
+
+        Directory test = new Directory("test");
+        test.add(new File("AppTest.java", 2_600))
+            .add(new File("ConfigTest.java", 1_200));
+
+        src.add(main).add(test);
+
+        Directory resources = new Directory("resources");
+        resources.add(new File("application.yml",  1_500))
+                 .add(new File("logback.xml",       900))
+                 .add(new File("banner.txt",        200));
+
+        root.add(src)
+            .add(resources)
+            .add(new File("pom.xml",     8_400))
+            .add(new File("README.md",   2_100));
+
+        // Print entire tree — recursion happens automatically
+        System.out.println("File system tree:");
+        root.print("");
+
+        System.out.printf("%nTotal project size: %,d bytes%n", root.getSize());
+
+        // Client treats File and Directory identically
+        System.out.println("\n-- Treating File and Directory uniformly --");
+        FileSystemItem[] items = { new File("standalone.txt", 500), src };
+        for (FileSystemItem item : items) {
+            System.out.printf("%s -> size: %,d bytes%n", item.getName(), item.getSize());
+        }
+
+        System.out.println("\n=== Demo complete ===");
+    }
+}