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POObject Relationship Design
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copied!<p>I just happen to ponder of a OO concept which may sound quite trivial but I don't know why I find it quite confusing.</p> <p>Anyway, I am thinking for example, if I have an Animal class and a Location class. And I only allow one animal to be at one location at any time. So it is kind of like a 1-to-1 relationship. At the same time, I wish that the Animal and Location classes do not need some kind of bidirectional reference so that they are kept loosely coupled. If say I have this:</p> <pre><code>class Animal { private Location loc; public Animal(int x, int y) { loc = new Location(x,y); } public static newAnimal(Location[][] map, int x, int y) { if(map[x][y] != null) { return new Animal(x, y); } else return null; } class Location extends Point { public Location(int x, int y) { super(x, y); } } public static void main(String[] args) { //populates a map Location[][] map = new Location[10][10]; for(int x=0; x<10; x++) { for(int y=0; y<10; y++) { map[x][y] = new Location(x, y); } } Animal dog = new Animal(2, 4); //dog is at location 2,4 Animal cat = new Animal(5, 6); //cat is at location 5,6 //But this does not restrict a constraint requirement that there should only be one animal at any one point Animal horse = new Animal(2, 4); //now, horse is at the same location as dog but we only wanted one location to have one animal Animal rabbit = Animal.newAnimal(map, 20, 50); //rabbit is null because it is out of the map size } </code></pre> <p>From this, I foresee 2 problems.</p> <p>First, because my location does not know if an animal is already on it, many animals can all be pointing to a same location on the map array. This would violate the 1-1 multiplicity constraint that I wanted. In my case, I let the Animal to own the Location. this could the be reason why this could happen. If say I let the Location to own the Animal, this could be solved. But in a case if I want to know where my Animal is at, I need to loop through the entire map just to find where one of my Animal's location is? Alternatively, I can keep a bidirectional reference but this would cause the classes to be highly-coupled.</p> <p>The second problem that I feel could be a problem is the design in the Animal class. I have a static newAnimal() method to instantiate new animals. I did it this way because I thought allowing the caller to create new Animal instance directly from the constructor may allow an out of range coordinate input. But I still find the design very awkward.</p> <p>I am using Java codes in my example. And I am thinking the design within the class objects itself and is not involving the database yet.</p> <p>Any suggestions to improve the two issues I raised could be great. Thanks!</p>

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