返回 ArrayList 并在另一个方法中使用它 [英] Return ArrayList and use it in another method

问题描述

我有一个方法可以返回对象的 ArrayList.我返回创建的数组列表，其中填充了许多对象，我通过以相同的方法打印它们来检查它们是否存在.当我在另一种方法中使用该 Arraylist 作为参数时，它会给我一个错误，就像 arraylist 为空一样.

I have a method that returns an ArrayList of Objects. I return the arraylist created, and its filled with many objects, that I check they are there by printing them in the same method. When I use that Arraylist as parameter in another method, it gives me an error, like if the arraylist were empty.

让我们用 getFila 和 getColumna 方法说对象的 Position(y,x).以及失败的方法(实际上什么都不做):

Lets say the object its Position(y,x), with methods getFila and getColumna. And the method that fails (actually does nothing):

public static ArrayList<Position> getTrayectory (ArrayList<Position> aListPos){
        for (int ii = 0; ii < aListPos.size();ii++ ) {
            System.out.println("(" +  aListPos.get(ii).getFila() + "," + aListPos.get(ii).getColumna()+")");
        }
        return aListPos;
    }

我返回了包含许多对象的 Arraylist，但我不知道发生了什么.

I return the Arraylist with many objects inside, but I dont know what is happening with that.

总结:

1- I have a method that returns an ArryList
2- I create a new Arraylist with method in 1
3- I use new method with Arraylist in 2 as parameter.

(4-) Nothing happens, because new arraylist in 2 look empty.

有人可以帮助我吗?

推荐答案

如果您在提供它的(看不见的)方法中创建 ArrayList，您会返回它吗?或者你在修改一个参数?

If you are creating the ArrayList in the (unseen) method that provides it, are you returning it? Or are you modifying a parameter?

换句话说，你在这样做:

In other words, are you doing this:

ArrayList<Position> myArrayList = generateArrayList(); // or whatever you call it

或者你正在这样做:

ArrayList<Position> myArrayList;
generateArrayList(myArrayList);
ArrayList<Position> result = getTrayjectory(myArrayList);

第二个版本不起作用，因为当您在方法中创建对象时，它可以逃脱方法边界的唯一方法是由该方法返回.否则，它会在方法结束时超出范围.

The second version of this won't work, because when you create an object inside a method, the only way that it can escape the method's bounds is to be returned by that method. Otherwise, it goes out of scope when the method ends.

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