返回ArrayList和在另一种方法中problm使用它 [英] Return ArrayList and use it in another method problm

问题描述

我有一个返回对象的ArrayList的方法。我返回数组列表创建的，它充满了许多对象，我检查他们在那里通过同样的方法打印。当我使用的ArrayList作为其他方法的参数，它给了我一个错误，就像如果数组列表是空的。

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.

可以说物体的位置（Y，X），用方法getFila和getColumna。
和失败的方法（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;
    }

我回到里面许多对象的数组列表，但我不知道什么是与该发生的事情。

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

摘要：

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.

谁能帮我在这个过程中？我需要帮助，很快很快。

Can anyone help me in the process? I need that help very very soon.

推荐答案

如果您在（看不见的）方法，它提供了创建ArrayList中，要退呢？或者，你修改参数？

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

在换句话说，你在做这样的：

In other words, are you doing this:

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

或你在做这样的：

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

这样做的第二个版本将无法正常工作，因为当你在方法内部创建一个对象，它可以逃脱的方法的边界的唯一方法是通过该方法返回。否则，超出范围时，该方法结束。

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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