Add all 23 GoF design pattern implementations

03-behavioral/command/CommandHistory.java

@@ -0,0 +1,28 @@
+package command;
+
+import java.util.ArrayDeque;
+import java.util.Deque;
+
+/**
+ * Invoker — holds command history and triggers execute/undo.
+ * It doesn't know what the commands do; it just calls execute() and undo().
+ */
+public class CommandHistory {
+    private final Deque<Command> history = new ArrayDeque<>();
+
+    public void execute(Command cmd) {
+        cmd.execute();
+        history.push(cmd);
+        System.out.println("  Executed: " + cmd.getDescription());
+    }
+
+    public void undo() {
+        if (history.isEmpty()) {
+            System.out.println("  Nothing to undo.");
+            return;
+        }
+        Command cmd = history.pop();
+        cmd.undo();
+        System.out.println("  Undone: " + cmd.getDescription());
+    }
+}