比较元素列表并将其添加到另一个列表中

Question

I am trying to compare and add the list of objects inside another list using java8 stream API. Below are the sample json.

studentList:

 [{
  sid:1,
  id:10,
  sname :"xyz"
  sPassOut : "2019"
}, {
  sid:2,
  id:20,
  sname :"abc"
  sPassOut : "2020"
},{
   sid:3,
   id:30,
  sname :"ppp"
  sPassOut : "2021"
},
{
   sid:1,
   id:10,
  sname :"ppp"
  sPassOut : "2021"
},
{
   sid:2,
  id:20,
  sname :"ppp"
  sPassOut : "2021"
}]

myList:

  [{
    id:10,
    status:"active",
    studentList:[]
   },{id:20, 
      status:"active",
       studentList:[]
   },{id:30, 
      status:"active",
       studentList:[]
   },{id:40, 
      status:"active",
       studentList:[]
   }]

I want to add the studentList elements in the myList for each matching id value.

tried the below code, but it is not giving the expected result.

studentList.forEach(studList -> {
    studList.setStudentList(myList.stream().collect(Collectors.toMap(MyList:: getId)));
});

Expected JSON:

[
    {
    id: 10,
    status: "active",
    studentList: [
            {
  sid: 1,
  id: 10,
  sname : "xyz"
  sPassOut : "2019"
            },
            {
   sid: 4,
   id: 10,
  sname : "ppp"
  sPassOut : "2021"
            }
        ]
    },
    {id: 20, 
      status: "active",
       studentList: [
            {
  sid: 2,
  id: 20,
  sname : "abc"
  sPassOut : "2020"
            },
            {
   sid: 5,
  id: 20,
  sname : "ppp"
  sPassOut : "2021"
            }
        ]
    },
    {id: 30, 
      status: "active",
       studentList: [
            {
   sid: 3,
   id: 30,
  sname : "ppp"
  sPassOut : "2021"
            }
        ]
    },
    {id: 40, 
      status: "active",
       studentList: []
    }
]

I wan to add the elements from studentList grouped together with matching id value in the myList as shown in the sample json above.

Answer 1

In order to get the required student list using the stream api, you need to filter the students to get only students with a certain id and then collect them into a list. An example of this is shown below:

studentList.stream().filter(student -> student.id == ID_TO_MATCH).collect(Collectors.toList())

In the code above, ID_TO_MATCH represents the the student id to find for each section of myList . See the following example of getting ID_TO_MATCH :

myList.forEach(section -> {
    int ID_TO_MATCH = section.id;
};

These code snippets can be combined to produce the solution:

myList.forEach(section -> {
    int id = section.id;
    List<Student> studentsFound = studentList.stream().filter(student -> student.id == id).collect(Collectors.toList())
    section.setStudentList(studentsFound);
};