如何在不同的服务类中使用泛型类T？

public class Box<T> {
  private T item;

  public void setItem(T item) {
    this.item = item;
  }

  public T getItem() {
    return item;
  }
}

Box<String> stringBox = new Box<>();
stringBox.setItem("Hello");
String str = stringBox.getItem();

Box<Integer> intBox = new Box<>();
intBox.setItem(10);
int num = intBox.getItem();

public class Printer {
  public <T> void printItem(T item) {
    System.out.println(item.toString());
  }
}

Printer printer = new Printer();
printer.printItem("Hello");
printer.printItem(10);