arrayList i only show the last element - arraylist

Question

I have the following problem. I'm trying to show all the elements of a display arrayList but I can only see the last item repeated as many times as there are number of elements in the ArrayList.
import java.util.ArrayList;
import java.util.Iterator;
public class PurebaArrayList {
public static void main(String[] args) {
ArrayList<PuntoDouble> puntos = new ArrayList<>();
PuntoDouble p = new PuntoDouble();
for(int cont = 0; cont< 100; cont++){
p.setX(cont);
p.setY(cont);
puntos.add(p);
}
System.out.println(puntos.toString());
}
}
public class PuntoDouble{
private double x;
private double y;
public PuntoDouble(double x, double y) {
this.x = x;
this.y = y;
}
public PuntoDouble(){
}
public double getX() {
return x;
}
public void setX(double x) {
this.x = x;
}
public double getY() {
return y;
}
public void setY(double y) {
this.y = y;
}
#Override
public String toString() {
return "PuntoDouble{" + "x=" + x + ", y=" + y + '}';
}
}
Thanks

Answer 1

You need to create a new PuntoDouble for every entry you add to the list. At the moment you add just a single instance of a PuntoDouble which you modify.
Therefore change the code to:
for(int cont = 0; cont< 100; cont++){
PuntoDouble p = new PuntoDouble();
p.setX(cont);
p.setY(cont);
puntos.add(p);
}

Answer 2

You are, literally, adding the same PuntoDouble as each of the 100 elements of the array. If you want them to be distinct from each other, you need to make a new one for each.

Answer 3

For debugging,slightly change 100 to 3 and add for each iteration to print out the elements in puntos.
// adds to ArrayList
for(int cont = 0; cont< 10; cont++){
p.setX(cont);
p.setY(cont);
System.out.println("[add]" + p);
puntos.add(p);
}
// iterates ArrayList puntos' elements
for(PuntoDouble pd : puntos){
System.out.println("[contain]" + pd);
}
debug output lines:
[add]PuntoDouble{x=0.0, y=0.0}
[add]PuntoDouble{x=1.0, y=1.0}
[add]PuntoDouble{x=2.0, y=2.0}
[add]PuntoDouble{x=3.0, y= 3.0}
[contain]PuntoDouble{x=9.0, y=9.0}
[contain]PuntoDouble{x=9.0, y=9.0}
[contain]PuntoDouble{x=9.0, y=9.0}
What you did is created an PuntoDouble object p and initilized it by invoking default constructor. In the for loop, the values of p modifies, but the reference of p never changed, therefore you kept adding the same object into ArrayList 100 times, all of those added objects refer to the same address in memory(new an object once before the for loop) which the value x is 99.0 and y is 99.0 (the last added values)
What you should do is creating a new object each iterate.
for(int cont = 0; cont< 100; cont++){
puntos.add(new PuntoDouble(cont, cont));
}