Java: How to make a copy of an array of object?

Question

Right now, I have an array of Point objects and I want to make a COPY of that array.

I have tried following ways:

1) Point[] temp = mypointarray;

2) Point[] temp = (Point[]) mypointarray.clone();

3)

Point[] temp = new Point[mypointarray.length];
System.arraycopy(mypointarray, 0, temp, 0, mypointarray.length);

But all of those ways turn out to be that only a reference of mypointarray is created for temp, not a copy.

For example, when I changed the x coordinate of mypointarray[0] to 1 (the original value is 0), the x coordinate of temp[0] is changed to 1 too (I swear I didn't touch temp).

So is there any ways to make an copy of Point array?

Thanks

Answer 1

You need to make a deep copy. There's no built-in utility for this, but it's easy enough. If Point has a copy constructor, you can do it like this:

Point[] temp = new Point[mypointarray.length];
for (int i = temp.length - 1; i >= 0; --i) {
    Point p = mypointarray[i];
    if (p != null) {
        temp[i] = new Point(p);
    }
}

This allows for null array elements.

With Java 8, you can do this more compactly with streams:

Point[] temp = Arrays.stream(mypointarray)
                     .map(point -> point == null ? null : new Point(point))
                     .toArray(Point[]::new);

And if you're guaranteed that no element of mypointarray is null, it can be even more compact because you can eliminate the null test and use Point::new instead of writing your own lambda for map():

Point[] temp = Arrays.stream(mypointarray).map(Point::new).toArray(Point[]::new);