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

03-behavioral/command/Main.java

@@ -0,0 +1,40 @@
+package command;
+
+/**
+ * Command Design Pattern — Runnable Demo
+ * Run: javac command/*.java -d out/command && java -cp out/command command.Main
+ * Article: https://ankurm.com/command-design-pattern-java/
+ */
+public class Main {
+    public static void main(String[] args) {
+        System.out.println("=== Command Design Pattern Demo ===\n");
+
+        TextEditor editor = new TextEditor();
+        CommandHistory history = new CommandHistory();
+
+        System.out.println("Initial: " + editor);
+
+        history.execute(new InsertCommand(editor, "Hello", 0));
+        System.out.println("After:   " + editor);
+
+        history.execute(new InsertCommand(editor, " World", 5));
+        System.out.println("After:   " + editor);
+
+        history.execute(new DeleteCommand(editor, 5, 6));
+        System.out.println("After:   " + editor);
+
+        System.out.println("\n-- Undo sequence --");
+        history.undo();
+        System.out.println("After undo: " + editor);
+
+        history.undo();
+        System.out.println("After undo: " + editor);
+
+        history.undo();
+        System.out.println("After undo: " + editor);
+
+        history.undo();  // nothing left
+
+        System.out.println("\n=== Demo complete ===");
+    }
+}