Question

I want add HashMap of student to ArrayList of students at each iteration but the output in the student HashMap is different with student_list, specifically the value of "id" i is not changing instead it always print the upper bound of the loop termination value. Below is the code. I need to know where i got it wrong.

I suppose to get unique id for each student on the final list as it appears on the individual student details but the result is otherwise. even if I print inside the loop, the final list is the same with below result.

HashMap<String, Object> student = new HashMap<String, Object>();
ArrayList<HashMap<String, Object>> students_list = new 
ArrayList<HashMap<String, Object>>();
for (int i=0; i<3; i++) {
    student.put("Name", "James");
    student.put("id", i);
    student.put("Level", "Two");
    System.out.println("The student details are  "+student);
    students_list.add(student);
}   
System.out.println("List of students list is "+students_list.toString());

Output:

The student details are  {Level=Two, id=0, Name=James}
The student details are  {Level=Two, id=1, Name=James}
The student details are  {Level=Two, id=2, Name=James}
List of students is      [{Level=Two, id=2, Name=James}, {Level=Two, 
id=2, Name=James}, {Level=Two, id=2, Name=James}]

Answer 1

您仅构造一个$LATEST；向下移动第一行，使其位于for循环内：

student

Answer 2

您将在每次迭代中更改相同的对象“学生”，并将其放入具有新键的哈希图中。指向该对象的链接将存储在HashMap存储桶中，但它们仍将指向同一对象。

在每次迭代中重新创建对象

public static void main(String[] args) {
    List<Map<String, Object>> students = IntStream.range(0, 3).mapToObj(index -> {
        Map<String, Object> student = new HashMap<>();
        student.put("Name", "James");
        student.put("id", index);
        student.put("Level", "Two");
        return student;
    }).collect(Collectors.toList());
    System.out.println(students);
}

Add HashMap to ArrayList in For-loop Iteration

2 个答案: