نمط Command: تغليف الإجراءات

public interface Command {
    void execute();
    void undo();
}

public class InsertTextCommand implements Command {
    private final TextEditor editor;
    private final String text;
    private final int position;
    public InsertTextCommand(TextEditor editor, String text, int pos) {
        this.editor = editor; this.text = text; this.position = pos;
    }
    public void execute() { editor.insert(position, text); }
    public void undo()    { editor.delete(position, text.length()); }
}

public class CommandHistory {
    private final Deque<Command> history = new ArrayDeque<>();
    public void execute(Command cmd) {
        cmd.execute();
        history.push(cmd);
    }
    public void undo() {
        if (!history.isEmpty()) history.pop().undo();
    }
}

private final Deque<Command> undoStack = new ArrayDeque<>();
private final Deque<Command> redoStack = new ArrayDeque<>();
public void redo() {
    if (!redoStack.isEmpty()) {
        Command cmd = redoStack.pop();
        cmd.execute();
        undoStack.push(cmd);
    }
}

public class MacroCommand implements Command {
    private final List<Command> commands;
    public MacroCommand(List<Command> commands) { this.commands = commands; }
    public void execute() { commands.forEach(Command::execute); }
    public void undo()    { for (int i = commands.size()-1; i>=0; i--) commands.get(i).undo(); }
}

BlockingQueue<Command> queue = new LinkedBlockingQueue<>();
// Producer:
queue.put(new SendEmailCommand(recipient, subject, body));
// Consumer thread:
Command cmd = queue.take();
cmd.execute();

ExecutorService exec = Executors.newFixedThreadPool(4);
exec.submit(() -> sendEmail(recipient, subject));
exec.submit(() -> generateReport(params));

void execute(Command cmd) {
    cmd.execute();
    auditLog.append(cmd.toString()); // log every action
    history.push(cmd);
}

Button saveBtn = new Button("Save");
saveBtn.setOnAction(() -> new SaveDocumentCommand(doc).execute());