Keeping track of ArrayList objects

Programming Questions

hampusber..

Keeping track of ArrayList objects

in Programming Questions • 3 years ago

If I make a number of objects in an ArrayList I can keep track of the objects by using their order within the list. But if objects is deleted other objects created later then the deleted objects will get a new "number" in the list and I can no longer track these objects.

My way to solve this problem is by giving all objects a variable called "id" and every time I want to reach an object in the list I need to check all objects if their id match with the one I'm looking for.

My question is:

Is there a more simple way of keeping track of your objects in an ArrayList?

Example (if I want to find the object which initially was number 6):

stuff = new ArrayList();
class Thing{
int id;
Thing(int a)
id = a;
}
for(int i = 0; i < 10; i++) stuff.add(new Thing(i));
Thing = thingThatIWantToUse;
for(int i = 0; i < stuff.size(); i++){
Thing tempThing = (Thing) stuff.get(i);
if(tempThing.id == 5) thingThatImLookingFor = tempThing;
i = stuff.size();
}

Very simple but a bit annoying to do write all time. A more convenient way would be greatly appreciated.

Thanks,

Hampus

Replies(1)

PhiLho

Re: Keeping track of ArrayList objects

3 years ago

If you use Processing 1.2+, you can use a nicer syntax:

ArrayList<Thing> stuff = new ArrayList<Thing>();
class Thing {
int id;
Thing(int a)
{ id = a; }
}
for (int i = 0; i < 10; i++) stuff.add(new Thing(i));
Thing thingThatImLookingFor = null;
for (Thing t : stuff) {
if (t.id == 5) thingThatImLookingFor = t;
break;
}

Or you can exploit the fact you created a class to put that method inside, so you no longer have to write it:

ArrayList<Thing> stuff = new ArrayList<Thing>();
static class Thing { // static -> not an inner class
static int rank;
int id;
String thingy;
Thing(int a) { id = a; thingy = "Thing #" + ++rank; }
static Thing get(int a, ArrayList<Thing> list) {
for (Thing t : list) {
if (t.id == a) return t;
}
return null;
}
// Overrides
public String toString() { return id + " - " + thingy; }
public boolean equals(Object o) {
if (!(o instanceof Thing)) return false;
return ((Thing) o).id == id;
}
public int hashCode() { return id; }
}
for (int i = 11; i >= 0; i--) stuff.add(new Thing(i));
Thing thingThatImLookingFor = Thing.get(5, stuff);
println(thingThatImLookingFor);

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