处理/ Java ArrayList和LinkedList异常

Question

I am trying to implement a pathfind system into one of my games, So i got following problem.

I got here a nice ArrayList :

ArrayList<PVector> path = new ArrayList<>();

Now its empty, later in the Process it fills with PVector entrys :

{5.0,6.0,0},{5.0,7.0,0},{5.0,8.0,0},{5.0,9.0,0}

Thats nice isnt it ? But i cant work with it, because i need only the 5.0 out of the {5.0,6.0,0} ....

i tried it with path.get(0) ... there i only get {5.0,6.0,0} ... so i found something new this here :

path.get(0)[0]; that didnt worked either... because the expression type needs to be an array but its resolved to an object

So how do i get an single entry out of an index ? :/ How do i get 5.0 out of {5.0,6.0,0} ?

Answer 1

So you have a ArrayList of PVector s, right? This means, when you get from the ArrayList , you get an PVector back. I do not know PVector, but there is (hopefully) a method in PVector to get the first int ( x() or something).

Answer 2

For questions like these, the reference is your best friend.

But remember that path.get(0) returns a PVector . You can then use the PVector API to get at its position. Something like this:

ArrayList<PVector> path = new ArrayList<PVector>();
//add PVectors to path
PVector p = path.get(0);
float x = p.x;

Notice that I'm using <PVector> generics so that the ArrayList knows what types of objects it holds. The p variable isn't necessary; I'm just using it to show that path.get() returns a PVector . You could do it in one line as well:

ArrayList<PVector> path = new ArrayList<PVector>();
//add PVectors to path
float x = path.get(0).x;

Answer 3

When declaring variables, always parametrize the generic types with the most specific type you will store in it:

// Declaration:
List<PVector> path = new ArrayList<PVector>();

// Storing:
path.add(new PVector(...));
path.add(new PVector(...));
...

// Reading:
PVector pVector=path.get(n);
pVector.get(...)

In this way, when reading items from your list, you will get exactly the same type of object you stored.