ConcurrentModificationException同时使用嵌套的for-each移除元素

Question

EmployeeID,FirstName,LastName,Dept
1, John, Smith, Maintenance 
1, John, Smith, Engineering 
1, John, Smith, Transport 

从CSV文件中，我读取每一行并创建一个employee对象，然后将其存储到employee的ArrayList中。

在对员工的排序之前：

employeeID='1', firstName='John', lastName='Smith', department='Maintenance'}
employeeID='1', firstName='John', lastName='Smith', department='Engineering'}
employeeID='1', firstName='John', lastName='Smith', department='Transport'}

一旦文件处理完成，我就开始对员工的。

employeeID='1', firstName='John', lastName='Smith', department='Maintenance, Engineering, Transport'}

Exception in thread "main" java.util.ConcurrentModificationException
    at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1042)
    at java.base/java.util.ArrayList$Itr.next(ArrayList.java:996)
    at com.gillott.csvtomap.CsvToMap.employeeSorter(CsvToMap.java:73)
    at com.gillott.csvtomap.CsvToMap.main(CsvToMap.java:34)

for (Employee emp1: employees) {

    for (Employee emp2: employees){

        if(emp1.getEmployeeID().equals(emp2.getEmployeeID())){

            emp1.setdepartment(emp1.getdepartment() + ", " + emp2.getdepartment());
            System.out.println("Duplicate found for Employee ID: " + emp1.getEmployeeID());
            employees.remove(emp2); //CME Exception Throw here.
        }
    }
}

循环迭代器示例：

for (Iterator<Employee> employeeIterator1 = employees.iterator(); employeeIterator1.hasNext();) {
    Employee emp1 = employeeIterator1.next();

    for (Iterator<Employee> employeeIterator2 = employees.iterator(); employeeIterator2.hasNext();){
        Employee emp2 = employeeIterator2.next();

        if(emp1.getEmployeeID().equals(emp2.getEmployeeID())){

            emp1.setdepartment(emp1.getdepartment() + ", " + emp2.getdepartment());
            System.out.println("Duplicate found for Employee ID: " + emp1.getEmployeeID());
            employeeIterator1.remove();
        }
    }
}

我的迭代器的实现可能是不正确的，这是第一次直接使用迭代器。

Answer 1

按id对雇员进行分组，并使用下游收集器合并实例：

Map<Integer, Employee> map =
    employees.stream()
        .collect(groupingBy(
            Employee::getId,
            reducing((a, b) -> a.setdepartment(a.getdepartment() + "," + b.getdepartment()));

然后在employees中保留实例，这些实例是此映射中的值：

employees.retainAll(map.values());

如果您愿意，您可以在一个语句中完成所有这些。