Question

It came to my notice that a TreeSet doesn’t keep the mutable objects in sorted order if object attribute values are changed later on. For example,

public class Wrap { 
    static TreeSet<Student> ts = new TreeSet<Student>(new Comparator<Student>(){
        @Override
        public int compare(Student o1, Student o2) {            
            return o1.age - o2.age;
        }       
    }); 
    public static void main(String []args){
        Student s = new Student(10);
        ts.add(s); 
        ts.add(new Student(50));
        ts.add(new Student(30));
        ts.add(new Student(15));
        System.out.println(ts);
        s.age = 24;      //Here I change the age of a student in the TreeSet
        System.out.println(ts);     
    }
}
class Student{
    int age;
    Student(int age){
        this.age = age;
    }   
    @Override
    public String toString() {
        return "Student [age=" + age + "]";
    }   
}

The output is :

[Student [age=10], Student [age=15], Student [age=30], Student [age=50]]
[Student [age=24], Student [age=15], Student [age=30], Student [age=50]]

After I change the age of a particular student, and then print the TreeSet, the Set seems no longer in sorted order. Why does this happen? and how to keep it sorted always?

Answer 1

Why does this happen?

Because the set cannot monitor all its objects for changes… How would it be able to do that?!

Same problem arises for HashSets. You can’t change values affecting an objects hash-code when a HashSet holds the object.

and how to keep it sorted always?

You typically remove the element from the set, modify it, and then reinsert it. In other words, change

s.age = 24;      //Here I change the age of a student in the TreeSet

to

ts.remove(s);
s.age = 24;      //Here I change the age of a student in the TreeSet
ts.add(s);

You can also use for example a list, and call Collections.sort on the list each time you’ve modified an object.