如何从另一个类访问 ArrayList? [英] How can I access to an ArrayList from another class?

问题描述

我有一个Car的ArrayList:

ArrayList<Car> cars = new ArrayList<Car>();

我用 AsyncTask 填充我的 MainActivity.java 的 onCreate 方法:

that I fill on my onCreate method of my MainActivity.java with an AsyncTask:

new fillCars().execute();

我想要的是在其他名为 ModifyCars.java 的类中使用这个 ArrayList cars.我的问题是在 AsyncTask 之后它在方法 onCreate 上执行，在所有类 MainActivity.java ArrayList cars 它已填充，我可以使用其中的数据，但是当我将它传递给 ModifyCars.java 时，它的大小等于 0.

What I want it's to use this ArrayList cars in other class, named ModifyCars.java. My problem it's that after the AsyncTask it's executed on the method onCreate, in all the class MainActivity.java the ArrayList cars it's fill and I can use the data inside of it but when I pass it to ModifyCars.java the size it's equals to 0.

我用来传递这个 ArrayList 的方法是创建一个 MainActivity.java 类的空构造函数并创建一个 get 方法.在 ModifyCars.java 类中，我创建了一个 MainActivity 对象并尝试使用我之前创建的 get 方法.这是我用于此目的的代码:

The method that I use to pass this ArrayList it's to create an empty constructor of the class MainActivity.java and create a get method. In the class ModifyCars.java I create an object of MainActivity and try to use the get method that I created before. Here the code that I used for this purpose:

在MainActivity.java

public MainActivity(){

}

//Here the method onCreate 

public ArrayList<Car> getCars(){
    return cars;
}

在ModifyCars.java

MainActivity MA = new MainActivity();
Log.d("prove",Integer.toString(MA.getCars().size())); //Here the size it's 0

如何在我的 ModifyCars.java 类中使用我的 cars ArrayList(当然，已填充)?我做错了什么?

How can I use my cars ArrayList (of course, filled) in my ModifyCars.java class? What am I doing wrong?

我在互联网上搜索，但没有任何对我有用的东西.任何帮助将不胜感激.

I search on the Internet but nothing had been useful for me. Any help would really appreciated.

提前致谢！

推荐答案

您正在创建 MainActivity 的一个新实例，这就是为什么您在那里得到一个空数组列表的原因.一个简单的解决方案是使您的 arryalist 保持静态并按如下方式访问它，

You are creating a new instance of the MainActivity that's why you are getting an empty array list there. A simple solution to that would be making your arryalist static and access it as below,

在主活动中:

static ArrayList<Car> cars = new ArrayList<Car>();

并在您的 ModifyCars.java 中访问它，

and in your ModifyCars.java access it as,

    ArrayList<Car> myCars = MainActivity.cars

但是执行此操作的标准方法是，将数组列表发送到 ModifyCars 活动，同时通过如下意图调用它，

But a standard way to do this would be, sending the array list to the ModifyCars activity while calling it through intent as below,

            Intent i = new Intent(this, ModifyCars.class);
            i.putExtra("CAR_ARRAY_LIST", this.cars);
            startActivity(i);

并且在 ModifyCars 活动中，您应该将其从意图中提取为，

and in ModifyCars activity you should extract it from intent as,

Intent i = getIntent();
ArrayList<Car> myCars = (ArrayList<Car>) i.getSerializableExtra("CAR_ARRAY_LIST");

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