树集的行为不同，但对于自定义对象，哈希集不会

Question

In below created code have Employee class with attributes Id and Name,Created 3 objects of Employee class e1,e2,e3 with respective name attribute a,b,a. when I add these object in hash set the size shows 3 but on same while in Tree Set it show size 2 even i am not overriding the hash code and equals method.

    Emplyoee class.............................


        package collection.core.concept;

        public class Employee implements Comparable<Employee>{

            private int id;
            private String name;

            public int getId() {
                return id;
            }
            public void setId(int id) {
                this.id = id;
            }
            public String getName() {
                return name;
            }
            public void setName(String name) {
                this.name = name;
            }

           @Override
           public int compareTo(Employee o) {
            if(this.name.equal(o.getName()){
               return 0;
            }else{
              return 1;
              }
           }
            @Override
            public String toString(){
            return name;

            }
    }

    main class...............................................
    public class CustomTreeSetSorting {
        public static void main(String[] args) {

            Employee e1 = new Employee();
            e1.setId(1);
            e1.setName("alok");
            Employee e2 = new Employee();
            e2.setId(2);
            e2.setName("vaibhav");
            Employee e3 = new Employee();
            e3.setId(3);
            e3.setName("alok");


            Set<Employee> Employee1 = new TreeSet<Employee>();

            Employee1.add(e1);
            Employee1.add(e2);
            Employee1.add(e3);
            System.out.println(Employee1.size());

        }
    }

Answer 1

In general sets you cannot have repetitions. Hence the size is two.

Since

 @Override
    public boolean equals(Object obj) {
        Employee emp = (Employee) obj;  
           if (name.equals(emp. getName())) {  
               return true;  
           }
           return false;  
}

It checks only name even though the id is different it treats them as same objects.

Change it to

    @Override
    public boolean equals(Object obj) {
       if(obj instanceof this) {
           Employee emp = (Employee) obj;  
           if (name.equals(emp. getName()) && id == emp.getId()) {  
               return true;  
           }
           return false;  
       }
       return false;
    }

Also the code is wrong for

@Override
public int compareTo(Employee o) {
    if(this.name==o.name){
        return 0;
    }else{
        return 1;
    }
}

Change it to

@Override
public int compareTo(Employee o) {
    return name.compareTo(o.getName());
}

You cannot compare strings with == use .equals method.

Answer 2

Whenever you are add something to TreeSet then compareTo method of class Employee is getting invoked and in compareTo method you are stating that if 2 names are equal then both objects are the same. Thats why TreeSet is adding only 2 objects alok and vaibhav . Moreover, currently you are comparing string wrongly , in order to compare Strings use compareTo on String objects that compare objects based on natural ordering ie lexicographically

Answer 3

How could we get different results according to the set that is used? It's quite simple : the HashSet uses both equals and hashCode to check object equality. You've got 3 different ids, that's why you get 3 objects in your hashset.

On the other hand, the TreeSet will use your compareTo method (or a provided Comparator) to compare items and check equality. If you compareTo method uses names only to give its result, and knowing you have only 2 unique names, then the TreeSet will only have 2 elements (probably the last one will be discarded)

Answer 4

Personally if I was going to base my comparison on a single string for insertion into the tree I would use what is already given to us:

public int compareTo(Employee o) {
    return this.name.compareTo(o.getName());
}

Not sure why this isn't the first choice? Am I missing something?

Given that, you probably need more than just the String, otherwise why have id's? I can't fathom two employee objects being the same if they have different id's (ie if they have more than one ID, I would create an array of id's as names are not unique.)

Answer 5

Both HashSet and Treeset doesn't allow duplicates.

The behavior is explained by understanding How do you define a Duplicate!

---

HashSet

In case of HashSet , the hashcodes for two objects are different even for same names, hence they are not considered duplicates.Unless you override hashCode and equals all the objects are considered distinct/unique

---

TreeSet:

You are providing the definition of "Duplicate" by overriding compareTo . ie If the names are same then the Objects are same

To allow duplicates in your TreeSet , Change your compareTo to

@Override
  public int compareTo(Employee o) {
    if (this.name.equals(o.getName())) {
      return -1; //return 0 says that two objects are same 
    } else {
      return 1;  
    }
  }