Hashmap 在 hashmap 和 arraylist 里面

Question

Declaration:-

private static HashMap<String, HashMap<String, ArrayList>> parentMap = new HashMap<>(); private static HashMap<String, ArrayList> childMap = new HashMap<>();

How do I want to store data in hashmap?

"India":
        "EmployeeName":[A,B,C]
        
"China":
        "EmployeeName":[D,E,F]
    

Methods used:-

public static ArrayList<String> getMap(String parentkey, String childKey) {
    return parentMap.get(parentkey).get(childKey);
}


public static ArrayList<String> setMap(String parentkey, String childKey, String value) {
    childMap.computeIfAbsent(childKey, k -> new ArrayList<>()).add(value);
    parentMap.put(parentkey, childMap);
    return getMap(parentkey, childKey);
}

setMap("India", "EmployeeName", "A")
setMap("India", "EmployeeName", "B")
setMap("India", "EmployeeName", "C")
setMap("China", "EmployeeName", "D")
setMap("China", "EmployeeName", "E")
setMap("China", "EmployeeName", "F")

How data get stored and printed in hashmap while fetchng from getMap method:

System.out.println("India" + getMap("India").get("EmployeeName"));
System.out.println("China" + getMap("China").get("EmployeeName"));

"India" [A,B,C,D,E,F]   
"China" [A,B,C,D,E,F]
        

Whilst i know keeping the childKey name unique would do thejob for me but I wish to keep the same childKey name under each parentkey name and store the respecive value in arraylist.

Any solution to my problem is welcome.

Answer 1

The problem is that you keep reusing the same childMap , regardless of which parentKey is being used. You need to look up the respective child map when adding values.

That means that childMap should be a local variable, nothing more. Delete your private static HashMap<String, ArrayList> childMap .

Try this:

public static ArrayList<String> setMap(String parentkey, String childKey, String value) {
  HashMap<String, ArrayList> childMap = parentMap.computeIfAbsent(parentkey, k->new HashMap<>());
  childMap.computeIfAbsent(childKey, k -> new ArrayList<>()).add(value);
  return getMap(parentkey, childKey);
}

Proof that this works

Answer 2

Suggestion, don't have generic types and dont have static params

private HashMap<String, HashMap<String, ArrayList<String>>> parentMap = new HashMap<>(); 
private HashMap<String, ArrayList<String>> childMap = new HashMap<>();

Try to replace this method

public ArrayList<String> setMap(String parentkey, String childKey, String value) {
    childMap.putIfAbsent(childKey, new ArrayList<>()); // inserts a key only if the key is not already present
    childMap.get(childKey).add(value); // puts the value in the existing key and 
    
    if (!parentMap.containsKey(parentkey)) { // puts in the parent map only if not present. 
        parentMap.put(parentkey, childMap);
    }
}

Since the childmap is referenced already, No need to put again.

Answer 3

If I was you I will do it in more "OOP way" so that you can benefit from static typing. Something like:

import java.util.List;
class Employee{
    String name;
    String getName(){
        return name;
    }
}


public class CompanyBranch{
    String national;
    List<Employee> employees;

    List<String> getEmployeeAllName(){
        return employees.stream().map(Employee::getName).toList();
    }
}