Sorting the List of Type Dto with List <String> contacting Ids in java-CodePudding

I have a List of objects like this:

List<Student> student= new ArrayList<Student>();

Student class looks like:

public class Student {
    private String Id;
    private String name;
}

I have another List<String> stuIds = new ArrayList<String>();

I want to sort student list based on stuIds list.

Tried this, but not getting the right order:

student.forEach(Id -> {
            
    student.sort(Comparator.comparing(items->stuIds.indexOf(student.getId())));     
});

AND

Collections.sort(student, 
    Comparator.comparing(item -> stuIds.indexOf(item)));
});

Is the sorting not happening because it is List<Dto> and List<String>? Could someone help here?

CodePudding user response：

stuIds.indexOf(item) will always return -1 since item is a Student and stuIds contains Strings.

Try:

Collections.sort(student, 
    Comparator.comparing(item -> stuIds.indexOf(item.getID())));
});

CodePudding user response：

There are two approaches to solving this problem:

Approach 1: Using a HashMap for fast sorting

This will take extra space but will be faster than normal sorting for your scenario:

List<Student> student = new ArrayList<Student>();
List<String> stuIds = new ArrayList<String>();
HashMap<String, Student> stringStudentHashMap = new HashMap<>();
for (Student student1 : student) {
    stringStudentHashMap.put(student1.getId(), student1);
}
ArrayList<Student> sortedStudent = new ArrayList<>();
for (String stuId : stuIds) {
    sortedStudent.add(stringStudentHashMap.get(stuId));
}

Approach 2: Using java Comparator:

You are not able to get the required result because you are trying to get the index of the Student object (instead of studentId) in the studentId List. Try changing the code with below code:

Collections.sort(student,Comparator.comparing(t -> stuIds.indexOf(t.getId())));