java - 如何迭代HashMap>

Question

如何从 HashMap 打印 ArrayList？

Map<String, ArrayList<Car>> cars = new HashMap<String, ArrayList<Car>>;

ArrayList<Car> carList = cars.get("bmw");

    for (int i = 0; i < carList.size(); i++) {
        System.out.println(carList.get(i));
    }

代码导致：

java.lang.NullPointerException

尽管“bmw”键存在并已填充。

Answer 1

Iterator<String> itr = carList.iterator();
    while (itr.hasNext()) {
      String element = itr.next();
      System.out.print(element + " ");
    }

Inoder to do this carList should not be a null value.
To add values to ArrayList you can use

carList.add("Some value");

Answer 2

尝试添加一个

System.out.println(cars.get("bmw"));

检查并查看其中的确切内容（在地图和汽车的 ArrayList 中）。

Answer 3

遍历列表的最简单方法是使用 foreach：

ArrayList<Car> carList = cars.get("bmw");  
for (Car car : carList) {
    System.out.println(car.getYourValueToPrint());
}

Answer 4

遍历 ArrayList 最安全的方法是通过增强每个循环，从而为您省去 a 的痛苦NullPointerException：

ArrayList<Car> carList = cars.get("bmw");
for(Car car: carList){
    car.doAnyOpertation();
    System.out.println(car.getAnyValue());
}

Answer 5

ArrayList<Car> carList = cars.get("bmw");
for(Car car: carList){
    car.doAnyOpertation();
    System.out.println(car.getAnyValue());
}

即使在这种情况下，如果 carList 为空，则
for(Car car: carList)与 throw NPE一致