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

02-structural/composite/Directory.java

@@ -0,0 +1,53 @@
+package composite;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Composite — a directory that can hold both Files (leaves)
+ * and other Directories (composites).
+ *
+ * getSize() is recursive: it asks each child for its size and sums them.
+ * The caller doesn't care whether a child is a File or Directory —
+ * both implement FileSystemItem and answer getSize().
+ *
+ * This is the power of Composite: uniform treatment of simple and complex.
+ */
+public class Directory implements FileSystemItem {
+
+    private final String name;
+    private final List<FileSystemItem> children = new ArrayList<>();
+
+    public Directory(String name) {
+        this.name = name;
+    }
+
+    public Directory add(FileSystemItem item) {
+        children.add(item);
+        return this;  // fluent API for easy nesting
+    }
+
+    public void remove(FileSystemItem item) {
+        children.remove(item);
+    }
+
+    @Override
+    public String getName() { return name; }
+
+    @Override
+    public long getSize() {
+        // Recursion: each child knows its own size.
+        // Files return their bytes; directories sum their children.
+        return children.stream()
+                       .mapToLong(FileSystemItem::getSize)
+                       .sum();
+    }
+
+    @Override
+    public void print(String indent) {
+        System.out.printf("%s[DIR]  %s/ (%,d bytes total)%n", indent, name, getSize());
+        for (FileSystemItem child : children) {
+            child.print(indent + "    ");  // recurse with deeper indent
+        }
+    }
+}