将数组对象推送到 Array 或 ArrayList 并从 Android 中的另一个类访问它

Question

I have an object of arrays and I am pushing it to an ArrayList like this:

static ArrayList<String> jsonResponse = new ArrayList<>();

//inside the method
strResp = response.body().string(); //the response I get from the server
jsonResponse.clear();  //clear the array first
jsonResponse.add(strResp);

Now from the other class, I am accessing it like this but I can't get it to print because it says NULL:

public class SecondClass extends ReactContextBaseJavaModule {

private FirstClass firstclass;

  public void getLocks(Callback callback) {
  System.out.println("Print object: " + firstclass.jsonResponse);
  }
}

First question, is ArrayList a proper way of achieving what I want in this case?

Second question, why am I not able to print it in the other class?

I do not have any experience in Android/Java so I know this might sound like a dumb question to many of you but please try to understand!

Answer 1

You are creating a FirstClass variable, but you are not initiating it. FirstClass is an object which contains a jsonResponse list. If an object is not initiated then the list itself will always be null. You need to call the constructor of FirstClass like:

firstclass = FirstClass(<add parameters here if any>);

Hope this helps :)

Answer 2

you create private list of strings in the class and then try to access it from another class. In addition your object have not initialized

1) In the code every time before you insert a string, you clear the list. so if you don't use more than one string just use String.

2) Init your object and add 'public' before the static and it should work. ie:

firstclass = FirstClass();

and

public static ArrayList<String> jsonResponse = new ArrayList<>();

Answer 3

In Java to access static elements, you don't need instance of the class(FirstClass). You simply do

FirstClass.jsonResponse();

The reason why you are getting null is be because you are trying to print the List object, which is not a String but a list of Strings. So do a the following:

if(!jsonResponse.isEmpty()){
    //print the first String in the List
    jsonResponse.get(0);
}

To answer your other question, LinkedList has more functionality, but List is faster. Check this out for more explanation .