Question

Vector <Double> x = new Vector<Double>();
Vector <Integer> y = new Vector <Integer>();  
System.out.print(x.equals(y));

This prints:

true

Why? Isn t equals() -by default- supposed to compare if two references point to the same object?

Answer 1

equals is implemented in AbstractList. It goes through elements in the list and returns false if any are not equal. Because your lists have no elements, true is returned.

public boolean equals(Object o) {
if (o == this)
    return true;
if (!(o instanceof List))
    return false;

ListIterator<E> e1 = listIterator();
ListIterator e2 = ((List) o).listIterator();
while(e1.hasNext() && e2.hasNext()) {
    E o1 = e1.next();
    Object o2 = e2.next();
    if (!(o1==null ? o2==null : o1.equals(o2)))
    return false;
}
return !(e1.hasNext() || e2.hasNext());
}

As Tom mentioned in the comments, reading the contract of the List Interface you will see that it defines the behavior.

Returns true if and only if the specified object is also a list, both lists have the same size, and all corresponding pairs of elements in the two lists are equal.

Answer 2

NO, equals() is not supposed to compare if two references point to the same object.
It compares if the objects referenced are equal, mostly that means having the same content.

It only compares for the same object if the method is not implemented for the given Object. In that case the method from Object is used with just use the == operator.

== is the one which compares if it is the same object (instance).

Answer 3

Vector has its own equals method. It compares its content. And both your Vector are empty, so they are equal.

Please note: Because of type erasure at runtime there are only two instances of Vector.

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