不同类java之间的全局变量

Question

i am on the creation of an app in android. its a calculator app. the main activity is where the user could input the equation, and the second activity is where the user can add/edit/delete variables. so i made a new class in another file named Global.java. then i extended it to application, imported everything i need, made s private string, made some public functions, edited the manifest, and initialized it right on my main activity. everything works fine while im only using a string to be passed by the functions but when i started adding what i need, an ArrayList, and made some functions so i could access the list then run it, the app closes. i think its because the arraylist is not allowed to be passed to different classes? am i right or am i just missing something?

please dont downvote my post if i didn't post something needed. i am using aide so there is no log output. code: Global.java

...
import android.app.*;
import java.util.*;

public class Global extends Application
{
    private String s;
    public static ArrayList<String> sList;

    public String getS() {
        return s;
    }
    public void setS(String ss) {
        s=ss;
    }
    public void add() {
        sList.add(s);
    }
}

MainActivity.java

...
    String s;
... 
        global=(Global)getApplicationContext();
...
        global.setS("jian"); //this one works
        global.sList.add("jian"); // this one dont
...

Answer 1

Are you sure you initialized sList, like this:

sList = new ArrayList<String>();

If you didn't, you might want to change its declaration to include this initialization.

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

Answer 2

Just do

global.add("jian"); 

since you have an add function to take care of the addition of item to arraylist.

Also, try with this:

  public void add(String ss) {
    sList.add(ss);
}

Answer 3

You are not instantiating your arraylist.

    public static ArrayList<String> sList = new Arraylist<String>();

Also you should read beginner tutorials on Java and android, using a public extension of application like this is a bad idea and you can get log outputs from different apps if Aide doesn't provide that, search play store