全局变量未从侦听器中初始化

Question

我正在尝试从侦听器中初始化全局 ArrayList（以在代码的不同位置处理来自 Volley 库的 JSONArray 对象）。但是，当我尝试在 Oncreate 方法中访问它时，它似乎未初始化。

我对不同的静态和非静态变量进行了同样的尝试 i.g。 int[]、String 等，但问题似乎仍然存在。

注意：问题 class 用于应用程序开发目的：

public class Question {

private String question;
private boolean answer;



public Question(String question, boolean answer){
    this.question = question;
    this.answer = answer;
}

public String getQuestion() {
    return question;
}

public boolean isAnswer() {
    return answer;
}

}

public class MainActivity extends AppCompatActivity implements View.OnClickListener {




private String url = "https://raw.githubusercontent.com/curiousily/simple-quiz/master/script/statements-data.json";
private static  ArrayList<Question> questionArrayList = new ArrayList<>();
int  [] intArr = new int[10];
String str = "Hello";



@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);



    // calling the method where the variables are re-initialized
    getQuestions();

    // printing results after re-initialization
    System.out.println("Mainactivity======================");
    System.out.println("intArray: "+ intArr[0]);
    System.out.println("STR: "+ str);
    System.out.println(questionArrayList.get(0).getQuestion());





}



public void getQuestions(){

    JsonArrayRequest arrayRequest = new JsonArrayRequest(Request.Method.GET, url, null,
            new Response.Listener<JSONArray>() {

               // Response Listener
                @Override
                public void onResponse(JSONArray response) {


                    // Test variables:
                    MainActivity.this.intArr[0] = 110;
                    str = "HI";



                    for(int i=0; i< response.length(); i++){
                        try {
                            // initializing ArrayList<Question>
                            questionArrayList.add(new Question(response.getJSONArray(i).getString(0), response.getJSONArray(i).getBoolean(1) ));
                        } catch (JSONException e) {
                            e.printStackTrace();
                        }
                    }




                }
            }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {

        }
    });

   RequestQueue queue = Volley.newRequestQueue(this);
   queue.add(arrayRequest);


}

}

输出：

I/System.out: Mainactivity======================

I/System.out: intArray: 0

STR：你好

D/AndroidRuntime: 正在关闭虚拟机 E/AndroidRuntime：致命异常：main 进程：myapp.com.triviaapp，PID：29167 java.lang.RuntimeException：无法启动 activity ComponentInfo{myapp.com.triviaapp/myapp.com.triviaapp.MainActivity}：java.lang.IndexOutOfBoundsException：索引：0，大小：0

Answer 1

侦听器正在修改字符串，但是侦听器会在稍后的某个时间点执行，在 onCreate 完成后。如果您在侦听器中添加第二个日志，您将看到它随后被调用。这是因为侦听器在收到来自网络的响应之前不会被执行，这总是比 onCreate 完成执行所花费的时间更长

Answer 2

System.out.println(questionArrayList.get(0).getQuestion()); <- 这里你崩溃了，因为数组列表是空的。

得到结果后可以打印信息：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    // calling the method where the variables are re-initialized
    getQuestions();
}

@Override
public void onResponse(JSONArray response) {

    // Test variables:
    MainActivity.this.intArr[0] = 110;
    str = "HI";

    for(int i=0; i< response.length(); i++){
        try {
            // initializing ArrayList<Question>
            questionArrayList.add(new Question(response.getJSONArray(i).getString(0), response.getJSONArray(i).getBoolean(1) ));
        } catch (JSONException e) { 
            e.printStackTrace();
        }
    }

    System.out.println("Mainactivity======================");
    System.out.println("intArray: "+ intArr[0]);
    System.out.println("STR: "+ str);
    if (!questionArrayList.isEmpty()) { // and check if we have at least one element
        System.out.println(questionArrayList.get(0).getQuestion());
    }
}

全局变量未从侦听器中初始化

Global variable are not being initialized from within a listener

android

global

initialization

android-volley