Doubt about Trees Implementations

[Mateus Brum]

Hi guys, this is my first post !

What is rules to manipulatee a TreeMap and TreeSet?
I think some element is equals to other one if it pass to comparators implementations(Comparable and Comparator) rules. Am I correct?

package mateus.estudos;
 
import java.util.Set;
import java.util.TreeSet;
 
public class Main {
 
public static void main(String[] args) {
Cat c1 = new Cat("A", 2);
Cat c2 = new Cat("A", 6);
 
System.out.println("Equals : "+c1.equals(c2));
 
Set gat = new TreeSet();
gat.add(c1);
gat.add(c2);

System.out.println(gat.size());

for (Object d : gat) {
System.out.println(d);
}
 
}
}
 
class Cat implements Comparable<Cat> {
String nome;
int idade;
 
public Cat(String nome, int idade) {
this.nome = nome;
this.idade = idade;
}
 
public int compareTo(Cat o) {
return this.nome.compareTo(o.nome);
}

@Override
public String toString() {
return this.nome+" "+this.idade;
}
 
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + idade;
result = prime * result + ((nome == null) ? 0 : nome.hashCode());
return result;
}
 
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
final Cat other = (Cat) obj;
if (idade != other.idade)
return false;
if (nome == null) {
if (other.nome != null)
return false;
} else if (!nome.equals(other.nome))
return false;
return true;
}
}

In case below, the TreeSet don�t add c2 object, because de method compareTo return zero

But if I modify to most especific compareTo method like:

...
public int compareTo(Cat o) {
return this.nome.compareTo(o.nome)+ Integer.valueOf(this.idade).compareTo(Integer.valueOf(o.idade));
}
...

It�s work, it�s non-unique (by the view of compareTo) that it not return 0.

I mean, Am I correct o to say than implementations of Tree don�t care about equals to add, remove e verify elements.

Thanks...

[Deepak Jain]

This is a very good question.

The natural ordering for a class C is said to be consistent with equals if and only if (e1.compareTo((Object)e2) == 0) has the same boolean value as e1.equals((Object)e2) for every e1 and e2 of class C.
In your first c1.equals(c2) = false but c1.compareTo(c2) = true. Hence your implementation of Comparable is inconsistent because compareTo() and equals() are not consistent.

It is strongly recommended (though not required) that natural orderings be consistent with equals. This is so because sorted sets (and sorted maps) without explicit comparators behave "strangely" when they are used with elements (or keys) whose natural ordering is inconsistent with equals. In particular, such a sorted set (or sorted map) violates the general contract for set (or map), which is defined in terms of the equals method.

For example, if one adds two keys a and b such that (!a.equals((Object)b) && a.compareTo((Object)b) == 0) to a sorted set that does not use an explicit comparator, the second add operation returns false (and the size of the sorted set does not increase) because a and b are equivalent from the sorted set's perspective.

Virtually all Java core classes that implement comparable have natural orderings that are consistent with equals.

In your first code you are aware that c1 and c2 are two different objects and when added to set must be added but because compareTo() and equals() are not consistentlt implemented, the second Cat object is not added to Set.
Hence the second implementation is correct.

[Mateus Brum]

Thank very much for explanations.
I am studing K&B book for exam but this subjection is not treated!