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[英]How do I get the values(ArrayList<Book> Books variable) generated in my getBooks() into my onCreate method?
[英]My Arraylist is null in oncreate
我正在用android studio編程,我想解析一個json並將詳細信息放在MainActivity中的函數中的對象中,我在該函數中做得很好,一切正常! 但是,當我想在onCreate中使用它時,它為null。 我該怎么辦? 主要活動:
public class MainActivity extends ActionBarActivity {
ArrayList<Details> details= new ArrayList<>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
new myserver2().execute("");
Adapter adptor=new Adapter(this,details);
ListView l= (ListView) findViewById(R.id.listview);
l.setAdapter(adptor);
}
private class myserver2 extends AsyncTask<String,String,String> {
String s;
@Override
protected String doInBackground(String... params) {
HttpClient client = new DefaultHttpClient();
HttpGet hp = new HttpGet("http://jsonplaceholder.typicode.com/posts");
try
{
HttpResponse r = client.execute(hp);
HttpEntity ent = r.getEntity();
String s = EntityUtils.toString(ent);
JSONArray array = new JSONArray(s);
JSONObject obj = new JSONObject();
for(int i=0;i<array.length();i++){
Details d = new Details();
obj=array.getJSONObject(i);
d.setId(Integer.parseInt(obj.getString("id")));
d.setUserid(Integer.parseInt(obj.getString("userId")));
d.setBody(obj.getString("body"));
d.setTitle(obj.getString("title"));
Log.i("**(*", obj.getString("title"));
details.add(d);
}
Log.i("**(*", details.get(0).getTitle());
} catch (
IOException e
)
{
e.printStackTrace();
} catch (
JSONException e
)
{
e.printStackTrace();
}
return null;
}
}
細節:
public class Details {
int id=0,userid=0;
String body,title;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public int getUserid() {
return userid;
}
public void setUserid(int userid) {
this.userid = userid;
}
public String getBody() {
return body;
}
public void setBody(String body) {
this.body = body;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
}
我在myserver中的for之后的日志給出了真實的標題,但是如果我在onCreate中擁有此日志,它將引發異常:java.lang.IndexOutOfBoundsException這是因為我的Arraylis在onCreate中為null,但是我為什么以及該怎么辦,我不知道!
您的onCreates嘗試在asyc任務未完成時訪問arraylist。 為了確保僅在完成asyc任務后才設置適配器,請在onPostExecute()
設置適配器
@Override
protected void onPostExecute(String result)
{
super.onPostExecute(result);
if(details.size > 0){
Adapter adptor=new Adapter(this,details);
l.setAdapter(adptor);
}else{
l.setAdapter(null);
}
}
希望有幫助!
您可以在AsyncTask
覆蓋onPostExecute()
方法,並且可以在此處正常設置適配器。 將此代碼添加到myserver2
類中
@Override
protected void onPostExecute(String message) {
super.onPostExecute(message);
Adapter adptor=new Adapter(MainActivity.this,details);
ListView l= (ListView) findViewById(R.id.listview);
l.setAdapter(adptor);
}
並從onCreate()
刪除這三行。
您可以在asynctask
Onpost()
方法中執行此操作,因為網絡操作需要花費一些時間才能獲取日期,但是
Adapter adptor=new Adapter(this,details);
l.setAdapter(adptor);
由於在主UI線程中調用了AsyncTask的onPostExecute-Method,因此您可以在onPostExecute-Method中初始化列表視圖和適配器。
onPostExecute(Result),在后台計算完成后在UI線程上調用。 后台計算的結果作為參數傳遞到此步驟。 http://developer.android.com/reference/android/os/AsyncTask.html
@Override
protected void onPostExecute(String message) {
super.onPostExecute(message);
Adapter adptor=new Adapter(MainActivity.this,details);
ListView l= (ListView) findViewById(R.id.listview);
l.setAdapter(adptor);
}
在您的oncreate方法中執行此操作
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Adapter adptor=new Adapter(this,details);
ListView l= (ListView) findViewById(R.id.listview);
l.setAdapter(adptor);
new myserver2().execute("");
}
並在您的AsyncTask中添加此方法
@Override
protected void onPostExecute(String message) {
super.onPostExecute(message);
adpter.notifydatasetchanged()
}
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