Question

我使用HashMap将密钥作为字符串，值为ArrayList：

HashMap<String, ArrayList<Student>> students;

如何取回所有 Student个对象，即所有ArrayList个对象的所有元素？

Answer 1

如果您想要Student的所有Stream，则values()可以Map Stream flatMap，List<Students> studentList = students.values() // Collection<ArrayList<Students>> .stream() // Stream<ArrayList<Students>> .flatMap(List::stream) // Stream<Students> .collect(Collectors.toList()); values() ：

Map

如果您无法使用Java 8，则必须迭代List的{{1}}并将所有这些内容添加到List<Students> studentList = new ArrayList<>(); for (ArrayList<Students> list : students.values()) studentList.addAll(list);：< / p>

class SiteController extends Controller {

public function behaviors() {
    return [
        'access' => [
            'class' => AccessControl::className(),
            'only' => ['logout', 'signup'],
            'rules' => [
                [
                    'actions' => ['signup'],
                    'allow' => true,
                    'roles' => ['?'],
                ],
                [
                    'actions' => ['logout', 'index'],
                    'allow' => true,
                    'roles' => ['@'],
                ],
            ],
        ],
        'verbs' => [
            'class' => VerbFilter::className(),
            'actions' => [
                'logout',
            ],
        ],
    ];
}

public function actions() {
    return [
        'error' => [
            'class' => 'yii\web\ErrorAction',
        ],
        'captcha' => [
            'class' => 'yii\captcha\CaptchaAction',
            //'fixedVerifyCode' => YII_ENV_TEST ? 'testme' : null,
        ],
    ];
}

Answer 2

ArrayList<Students> studentList = students.get("stringKey");

如果你想获得所有学生名单。

声明一个单独的数组列表并使用addAll()方法。

for (String key : studentList .keySet()) {
       finalList.addAll(studentList.get(key));
}

Answer 3

您可以使用values()功能将学生地图中的所有ArrayLists映射到单个Collection对象。然后我们可以通过以下方法将集合转换为ArrayList。

Collection<ArrayList<Students>> studentCollection = students.values();
List<Students> studentList = new ArrayList<Students>(studentCollection);

从Hashmp的所有值中获取所有嵌套项

3 个答案: