In this case class Point is mutable and some user can modify state of object of this class.

class Point {
private int x, y;

public Point(int x, int y) {
    this.x = x;
    this.y = y;
}

public int getX() {
    return x;
}

public void setX(int x) {
    this.x = x;
}

public int getY() {
    return y;
}

public void setY(int y) {
    this.y = y;
}
}

//...

public final class ImmutableCircle {
private final Point center;
private final double radius;

public ImmutableCircle(Point center, double radius) {
    // we create new object here because it shouldn't be changed
    this.center = new Point(center.getX(), center.getY());
    this.radius = radius;
}