Java HashMap 为给定键提供错误值答案

【问题标题】：Java HashMap gives wrong values for the given keyJava HashMap 为给定键提供错误值
【发布时间】：2019-11-28 09:55:12
【问题描述】：

我正在使用Map<Integer, HashMap<String, List<Employee>>> 类型的地图。填充地图后，如果我立即使用 String 键检索一些值，则返回的 List<Employee> 是正确的。但是经过一些迭代后，值正在发生变化。

第一次填充地图后，我没有在任何地方使用map.put，只使用了map.get()。

private Map<Integer, HashMap<String, List<Employee>>> createDataMap(List<Employee> employeeList) {

    for(Employee e:employeeList){       
        Map<Integer, HashMap<String, List<Employee>>> dataMap = new HashMap<>();
        int level = Integer.parseInt(e.getLevel());
        HashMap<String, List<Employee>> detailsMap = dataMap.get(level);
        if (Objects.isNull(detailsMap)) {
            detailsMap = new HashMap<>();
            dataMap.put(level, detailsMap);
        }
        String deptName = key.getDepartment();
        List<Employee> keyList = detailsMap.get(deptName);
        if (Objects.isNull(keyList)) {
            keyList = new ArrayList<>();
            detailsMap.put(dataMapKey, keyList);
        }
        keyList.add(key);
    }
    return dataMap;
}

【问题讨论】：

提供您的代码？
为了简化您的代码，您可以使用Map 类的getOrDefault，而不是在获取地图后测试其值是否为空
您的代码没有意义。 dataMap 由每个员工在循环中 构造，然后在在循环之外 返回。那甚至不会编译。一般来说，请查看处理条件创建逻辑的detailsMap = map.computeIfAbsent(level, HashMap::New)

标签： java hashmap hash-collision

【解决方案1】：

dataMap 只需要在循环外实例化一次。
当您使用 key 时，它应该是 e，您应该确保您发布的任何代码都能编译（至少！）
detailsMap.put(dataMapKey, keyList); 不正确，应该是detailsMap.put(e.getDepartment(), keyList);

我认为您的代码应该是这样的。它现在可以编译并正确创建您的地图。我冒昧地重命名了一些变量以使其更具可读性。

private static Map<Integer, HashMap<String, List<Employee>>> createDataMap2(List<Employee> employeeList) {
    Map<Integer, HashMap<String, List<Employee>>> dataMap = new HashMap<>();

    for(Employee e:employeeList){

        // Get the departments for the employee's level
        int level = Integer.parseInt(e.getLevel());
        HashMap<String, List<Employee>> departmentsEmployeesMapForEmployeeLevel = dataMap.get(level);
        if (Objects.isNull(departmentsEmployeesMapForEmployeeLevel)) {
            departmentsEmployeesMapForEmployeeLevel = new HashMap<>();
            dataMap.put(level, departmentsEmployeesMapForEmployeeLevel);
        }

        // Get the employee list for the employee's department
        String deptName = e.getDepartment();
        List<Employee> employeesListForEmployeeLevelAndDepartment = departmentsEmployeesMapForEmployeeLevel.get(deptName);
        if (Objects.isNull(employeesListForEmployeeLevelAndDepartment)) {
            employeesListForEmployeeLevelAndDepartment = new ArrayList<>();
            departmentsEmployeesMapForEmployeeLevel.put(deptName, employeesListForEmployeeLevelAndDepartment);
        }

        // add the employee to its department list
        employeesListForEmployeeLevelAndDepartment.add(e);
    }

    return dataMap;
}

同样如 cmets 中所述，这可以使用 Map.getOrDefault 进一步简化。

private static Map<Integer, HashMap<String, List<Employee>>> createDataMap2(List<Employee> employeeList) {
    Map<Integer, HashMap<String, List<Employee>>> dataMap = new HashMap<>();

    for(Employee employee: employeeList){
        int level = Integer.parseInt(employee.getLevel());
        String deptName = employee.getDepartment();

        // get the list of department employees for level
        HashMap<String, List<Employee>> departmentsEmployeesMapForEmployeeLevel = 
                dataMap.getOrDefault(level, new HashMap<>());
        // get the list of employees for level and department
        List<Employee> employeesListForEmployeeLevelAndDepartment = 
                departmentsEmployeesMapForEmployeeLevel.getOrDefault(deptName, new ArrayList<>());

        // add employee to its department for its level
        employeesListForEmployeeLevelAndDepartment.add(employee);
        // set new employee list to the employee's department
        departmentsEmployeesMapForEmployeeLevel.put(deptName, employeesListForEmployeeLevelAndDepartment);
        // set the updated department to the employee's level
        dataMap.put(level, departmentsEmployeesMapForEmployeeLevel);
    }

    return dataMap;
}

【讨论】：