A DESCRIPTION OF THE PROBLEM :
The current documentation of HashMap does not explicitly mention that treeification (conversion of a bucket to a Red-Black Tree) relies on the compareTo method if keys implement Comparable. This can lead to unexpected behavior if the compareTo method is inconsistent with equals, even though such inconsistency violates the Comparable contract. The documentation should:

Clearly state that during treeification, compareTo is used for ordering.
Emphasize that compareTo must be consistent with equals for predictable behavior.
By documenting this behavior, developers will better understand the interplay between Comparable and HashMap, avoiding misuse and debugging challenges.


In the attached example, we can see that because of incorrect implementation of compareTo, the entries go missing in the map when we search for them after treeification.

ACTUAL -
/*
Adding: Employee{id=1, name='Employee1'}. Size=1. Bucket:java.util.HashMap$Node
Adding: Employee{id=2, name='Employee2'}. Size=2. Bucket:java.util.HashMap$Node
Adding: Employee{id=3, name='Employee3'}. Size=3. Bucket:java.util.HashMap$Node
Adding: Employee{id=4, name='Employee4'}. Size=4. Bucket:java.util.HashMap$Node
Adding: Employee{id=5, name='Employee5'}. Size=5. Bucket:java.util.HashMap$Node
Adding: Employee{id=6, name='Employee6'}. Size=6. Bucket:java.util.HashMap$Node
Adding: Employee{id=7, name='Employee7'}. Size=7. Bucket:java.util.HashMap$Node
Adding: Employee{id=8, name='Employee8'}. Size=8. Bucket:java.util.HashMap$Node
Adding: Employee{id=9, name='Employee9'}. Size=9. Bucket:java.util.HashMap$TreeNode
Adding: Employee{id=10, name='Employee10'}. Size=10. Bucket:java.util.HashMap$TreeNode


################################
Print all the HashMap entries
Employee{id=4, name='Employee4'} -> Address4
Employee{id=1, name='Employee1'} -> Address1
Employee{id=2, name='Employee2'} -> Address2
Employee{id=3, name='Employee3'} -> Address3
Employee{id=5, name='Employee5'} -> Address5
Employee{id=6, name='Employee6'} -> Address6
Employee{id=7, name='Employee7'} -> Address7
Employee{id=8, name='Employee8'} -> Address8
Employee{id=9, name='Employee9'} -> Address9
Employee{id=10, name='Employee10'} -> Address10


################################
Search all the HashMap entries
Employee{id=1, name='Employee1'}-null
Employee{id=2, name='Employee2'}-null
Employee{id=3, name='Employee3'}-null
Employee{id=4, name='Employee4'}-Address4
Employee{id=5, name='Employee5'}-null
Employee{id=6, name='Employee6'}-Address6
Employee{id=7, name='Employee7'}-null
Employee{id=8, name='Employee8'}-Address8
Employee{id=9, name='Employee9'}-Address9
Employee{id=10, name='Employee10'}-Address10


################################
Remove all the HashMap entries
Removing: Employee{id=1, name='Employee1'}. Size=10. Bucket:java.util.HashMap$TreeNode
Removing: Employee{id=2, name='Employee2'}. Size=10. Bucket:java.util.HashMap$TreeNode
Removing: Employee{id=3, name='Employee3'}. Size=10. Bucket:java.util.HashMap$TreeNode
Removing: Employee{id=4, name='Employee4'}. Size=9. Bucket:java.util.HashMap$TreeNode
Removing: Employee{id=5, name='Employee5'}. Size=8. Bucket:java.util.HashMap$TreeNode
Removing: Employee{id=6, name='Employee6'}. Size=7. Bucket:java.util.HashMap$TreeNode
Removing: Employee{id=7, name='Employee7'}. Size=6. Bucket:java.util.HashMap$TreeNode
Removing: Employee{id=8, name='Employee8'}. Size=5. Bucket:java.util.HashMap$TreeNode
Removing: Employee{id=9, name='Employee9'}. Size=4. Bucket:java.util.HashMap$TreeNode
Removing: Employee{id=10, name='Employee10'}. Size=3. Bucket:java.util.HashMap$Node


################################
Print all the HashMap entries. HashMap should be empty now.
Employee{id=2, name='Employee2'} -> Address2
Employee{id=1, name='Employee1'} -> Address1
Employee{id=3, name='Employee3'} -> Address3


################################
Search all the HashMap entries. No etries should be found because we removed everything.
Employee{id=1, name='Employee1'}-Address1
Employee{id=2, name='Employee2'}-Address2
Employee{id=3, name='Employee3'}-Address3
Employee{id=4, name='Employee4'}-null
Employee{id=5, name='Employee5'}-null
Employee{id=6, name='Employee6'}-null
Employee{id=7, name='Employee7'}-null
Employee{id=8, name='Employee8'}-null
Employee{id=9, name='Employee9'}-null
Employee{id=10, name='Employee10'}-null

Process finished with exit code 0

 */

---------- BEGIN SOURCE ----------
import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

class Employee implements Comparable<Employee> {
    private final int id;
    private final String name;

    public Employee(int id, String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public int hashCode() {
        // Force hash collision for all Employee objects
        return 42;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        Employee employee = (Employee) obj;
        return id == employee.id && name.equals(employee.name);
    }

    @Override
    public String toString() {
        return "Employee{id=" + id + ", name='" + name + "'}";
    }

    @Override
    public int compareTo(Employee o) {
        return 1; // This line brings the unpredictable behaviour.
    }
}

public class HashMapTreeifyExample {
    public static void main(String[] args) throws Exception {
        // Set initial capacity to at least 64 to prevent resizing
        Map<Employee, String> map = new HashMap<>(64);
        ArrayList<Employee> list = new ArrayList<>();
        // Add 10 Employee-Address pairs to the same bucket
        for (int i = 1; i <= 10; i++) {
            Employee employee = new Employee(i, "Employee" + i);
            String address = "Address" + i;
            map.put(employee, address);
            System.out.println("Adding: " + employee + ". Size=" + map.size() + ". "+ printTreeOrLinkedList(map));
            list.add(employee);
        }

        System.out.println("\n\n################################");
        System.out.println("Print all the HashMap entries");
        map.forEach((key, value) -> {
            System.out.println(key + " -> " + value);
        });

        System.out.println("\n\n################################");
        System.out.println("Search all the HashMap entries");
        for (Employee e : list) {
            System.out.println(e + "-" + map.get(e));
        }

        System.out.println("\n\n################################");
        System.out.println("Remove all the HashMap entries");
        for(Employee employee :list){
            map.remove(employee);
            System.out.println("Removing: " + employee + ". Size=" + map.size() + ". "+ printTreeOrLinkedList(map));
        }

        System.out.println("\n\n################################");
        System.out.println("Print all the HashMap entries. HashMap should be empty now.");
        map.forEach((key, value) -> {
            System.out.println(key + " -> " + value);
        });

        System.out.println("\n\n################################");
        System.out.println("Search all the HashMap entries. No etries should be found because we removed everything.");
        for (Employee e : list) {
            System.out.println(e + "-" + map.get(e));
        }


    }

    static <K,V> String printTreeOrLinkedList(Map<K, V> map) throws Exception{
        String value = "Bucket:";
        Field tableField = HashMap.class.getDeclaredField("table");
        tableField.setAccessible(true);

        // Get the table array from the HashMap
        Object[] table = (Object[]) tableField.get(map);

        // Iterate over the table array
        for (Object bucket : table) {
            if (bucket != null) {
               value += bucket.getClass().getName();
            }
        }
        return value;
    }
}