Java 如何解决此空指针异常?
MainActivity.javaJava 如何解决此空指针异常?,java,android,nullpointerexception,Java,Android,Nullpointerexception,MainActivity.java package com.example.yashpachisia.fetchjson; import android.os.AsyncTask; import android.support.v7.app.ActionBarActivity; import android.os.Bundle; import android.view.Menu; import android.view.MenuItem; import android.widget.ListA
package com.example.yashpachisia.fetchjson;
import android.os.AsyncTask;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.TextView;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.params.BasicHttpParams;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.HashMap;
public class MainActivity extends ActionBarActivity {
String myJSON;
private static final String TAG_RESULTS="result";
private static final String TAG_NAME = "name";
private static final String TAG_AGE = "age";
private static final String TAG_SEX ="sex";
JSONArray peoples = null;
ArrayList<HashMap<String, String>> personList;
ListView list;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
list = (ListView) findViewById(R.id.listView);
personList = new ArrayList<HashMap<String,String>>();
getData();
}
protected void showList(){
try {
JSONObject jsonObj = new JSONObject(myJSON);
peoples = jsonObj.getJSONArray(TAG_RESULTS);
for(int i=0;i<peoples.length();i++){
JSONObject c = peoples.getJSONObject(i);
String name = c.getString(TAG_NAME);
String age = c.getString(TAG_AGE);
String sex = c.getString(TAG_SEX);
HashMap<String,String> persons = new HashMap<String,String>();
persons.put(TAG_NAME,name);
persons.put(TAG_AGE,age);
persons.put(TAG_SEX,sex);
personList.add(persons);
}
ListAdapter adapter = new SimpleAdapter(
MainActivity.this, personList, R.layout.list_item,
new String[]{TAG_NAME,TAG_AGE,TAG_SEX},
new int[]{R.id.name, R.id.age, R.id.sex}
);
list.setAdapter(adapter);
} catch (JSONException e) {
e.printStackTrace();
}
}
public void getData(){
class GetDataJSON extends AsyncTask<String, Void, String>{
@Override
protected String doInBackground(String... params) {
DefaultHttpClient httpclient = new DefaultHttpClient(new BasicHttpParams());
HttpPost httppost = new HttpPost("http://localhost/test/form.php");
// Depends on your web service
httppost.setHeader("Content-type", "application/json");
InputStream inputStream = null;
String result = null;
try {
HttpResponse response = httpclient.execute(httppost);
HttpEntity entity = response.getEntity();
inputStream = entity.getContent();
// json is UTF-8 by default
BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"), 8);
StringBuilder sb = new StringBuilder();
String line = null;
while ((line = reader.readLine()) != null)
{
sb.append(line + "\n");
}
result = sb.toString();
} catch (Exception e) {
// Oops
}
finally {
try{if(inputStream != null)inputStream.close();}catch(Exception squish){}
}
return result;
}
@Override
protected void onPostExecute(String result){
myJSON=result;
showList();
}
}
GetDataJSON g = new GetDataJSON();
g.execute();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
/* @Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_user) {
return true;
}
return super.onOptionsItemSelected(item);
}*/
}
package com.example.yashpachisia.fetchjson;
导入android.os.AsyncTask;
导入android.support.v7.app.ActionBarActivity;
导入android.os.Bundle;
导入android.view.Menu;
导入android.view.MenuItem;
导入android.widget.ListAdapter;
导入android.widget.ListView;
导入android.widget.simpledapter;
导入android.widget.TextView;
导入org.apache.http.HttpEntity;
导入org.apache.http.HttpResponse;
导入org.apache.http.client.methods.HttpPost;
导入org.apache.http.impl.client.DefaultHttpClient;
导入org.apache.http.params.BasicHttpParams;
导入org.json.JSONArray;
导入org.json.JSONException;
导入org.json.JSONObject;
导入java.io.BufferedReader;
导入java.io.InputStream;
导入java.io.InputStreamReader;
导入java.util.ArrayList;
导入java.util.HashMap;
公共类MainActivity扩展了ActionBarActivity{
字符串myJSON;
私有静态最终字符串标记_RESULTS=“result”;
私有静态最终字符串标记_NAME=“NAME”;
私有静态最终字符串标记_AGE=“AGE”;
私有静态最终字符串标记_SEX=“SEX”;
JSONArray-peoples=null;
ArrayList个人主义者;
列表视图列表;
@凌驾
创建时受保护的void(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
list=(ListView)findViewById(R.id.ListView);
personList=新的ArrayList();
getData();
}
受保护的无效显示列表(){
试一试{
JSONObject jsonObj=新的JSONObject(myJSON);
peoples=jsonObj.getJSONArray(标记结果);
对于(int i=0;i只有在检查您在onPostExecute()
中得到的结果不为null或空后,才能调用此方法showList()
if(result != null && result.length >0 ){
myJSON=result;
showList()
}
由于myJson可能为空,因此您正在尝试在其上运行Json
code,因此会得到numpointererror
更新
如果您得到的是空值,这意味着您没有从web服务中获得数据,或者您调用它是错误的
我发现你的代码中有一些错误,所以我给你写了一个新的。我的代码使用commons-io-2.4.jar
库,但你可以编写自己的代码来查看结果
String chartString = "your url which you want to call";
URL chartURL = new URL(chartString);
HttpURLConnection chartHttpURLConnection = (HttpURLConnection) chartURL.openConnection();
chartHttpURLConnection.connect();
String result = IOUtils.toString(chartHttpURLConnection.getInputStream());
System.out.println(""+result);
当您试图通过emulator调用本地服务时,IP地址将为10.0.2.2
所以你要调用的URL是
URL url = new URL(http://10.0.2.2/test/form.php);
请看一看这个其中的行抛出NullPointException我不知道是什么导致了这段代码中的异常。while((line=reader.readLine())!=null){sb.append(line+“\n”);}result=sb.toString();}catch(异常e){//Oops}最后{try{if(inputStream!=null)inputStream.close();}catch(Exception squish){}返回结果;您能为我们显示JSON响应吗?或者告诉witch line是ShowList()方法中的56行吗。但看起来您试图访问JSON中不存在的内容。请在“myJSON=result”行上设置断点,并检查结果是否包含有效的JSON。如果是,请在“JSONObject JSONObject=new JSONObject(myJSON);”行上设置断点再次检查JSON是否良好。然后你可能会知道更多。谢谢你的回复。我设置了条件,我的JSON似乎返回空值。你能告诉我为什么吗?这是我页面输出的屏幕截图。@Taurus,问题在于你得到的数据,JSON数据应该包含在[]中表示JSON数组或数据应在{}表示JSON对象,您必须确保删除在php中响应的表单file@Taurus,代码中的另一个问题是您正在进行的API调用。我已更新了答案,以便您可以查看。如果您可以从web浏览器中获取API的结果,则表示这是一个get请求,而不是帖子
String chartString = "your url which you want to call";
URL chartURL = new URL(chartString);
HttpURLConnection chartHttpURLConnection = (HttpURLConnection) chartURL.openConnection();
chartHttpURLConnection.connect();
String result = IOUtils.toString(chartHttpURLConnection.getInputStream());
System.out.println(""+result);
URL url = new URL(http://10.0.2.2/test/form.php);