I found the definition of the class Point may be wrong. The type of x is double, and the method equals use "==" to judge whether a.x equals to b.x. I thought it will always be false.
In the context, I thought may type int be more fit for the issue! As following:

@Override
public boolean equals(Object obj) {
if (obj instanceof Point) {
Point other = (Point) obj;
return x == other.x && y == other.y;
} else {
return false;
}
}