如何在不复制数据的情况下自动刷新（添加/更新）listview项目？

i made a simple code to retrieve some data from mysql database and put them into android listview everything works fine but the old items stays on the list view and it adds the new items(data is duplicated), below is php code :

$sql = "SELECT * FROM persons";

$res = mysqli_query($con,$sql);

$result = array();

while($row = mysqli_fetch_array($res)){
array_push($result,
array('id'=>$row[0],
'name'=>$row[1],
'address'=>$row[2]
 ));
}

echo json_encode(array("result"=>$result));

mysqli_close($con);

and here the android code :

    public class FetchData extends Activity{
    String myJSON;

    private static final String TAG_RESULTS="result";
    private static final String TAG_ID = "id";
    private static final String TAG_NAME = "name";
    private static final String TAG_ADD ="address";

    JSONArray peoples = null;

    ArrayList<HashMap<String, String>> personList;

    ListView list;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.test);
        list = (ListView) findViewById(R.id.listView);
        personList = new ArrayList<HashMap<String,String>>();
        getData();
        final Handler handler = new Handler();
        handler.postDelayed( new Runnable() {

            @Override
            public void run() {


                refreshData();
                handler.postDelayed( this, 2 * 100 );
            }
        }, 2 * 100 );

    }
    public void refreshData(){

    }
    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://192.168.1.4/get-data.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 + "
");
                    }
                    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;

                showdata();
            }
        }
        GetDataJSON g = new GetDataJSON();
        g.execute();
    }
    protected void showdata(){
        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 id = c.getString(TAG_ID);
                String name = c.getString(TAG_NAME);
                String address = c.getString(TAG_ADD);

                HashMap<String,String> persons = new HashMap<String,String>();

                persons.put(TAG_ID,id);
                persons.put(TAG_NAME,name);
                persons.put(TAG_ADD,address);

                personList.add(persons);
            }

            ListAdapter adapter = new SimpleAdapter(
                    FetchData.this, personList, R.layout.list_item,
                    new String[]{TAG_ID,TAG_NAME,TAG_ADD},
                    new int[]{R.id.id, R.id.name, R.id.address}
            );

            list.setAdapter(adapter);

        } catch (JSONException e) {
            e.printStackTrace();
        }

    }


    }

i would like to update automatically specific listview item if mysql database data changed for that specific item and add new items without duplicating

Try to modify your showdata() like this:

    protected void showdata(){
        try {
            JSONObject jsonObj = new JSONObject(myJSON);
            peoples = jsonObj.getJSONArray(TAG_RESULTS);

            personList.clear();
            for(int i=0;i<peoples.length();i++){
                JSONObject c = peoples.getJSONObject(i);
                String id = c.getString(TAG_ID);
                String name = c.getString(TAG_NAME);
                String address = c.getString(TAG_ADD);

                HashMap<String,String> persons = new HashMap<String,String>();

                persons.put(TAG_ID,id);
                persons.put(TAG_NAME,name);
                persons.put(TAG_ADD,address);

                personList.add(persons);
            }

            ListAdapter adapter = new SimpleAdapter(
                    FetchData.this, personList, R.layout.list_item,
                    new String[]{TAG_ID,TAG_NAME,TAG_ADD},
                    new int[]{R.id.id, R.id.name, R.id.address}
            );

            list.setAdapter(adapter);

        } catch (JSONException e) {
            e.printStackTrace();
        }

    }

personList.clear(); - for cleaning old result.

the best way to solve the problem to send last id recovered from php file so you will not see any duplicated data , will load more items by default send id zero and each time you make a request send last id stored

$id = abs(intval($_GET['id']));
$sql = "SELECT * FROM persons where id > ".$id;
$res = mysqli_query($con,$sql);
$result = array();

while($row = mysqli_fetch_array($res)){
array_push($result,
array('id'=>$row[0],
'name'=>$row[1],
'address'=>$row[2]
 ));
}

echo json_encode(array("result"=>$result));

mysqli_close($con);

java class

public class FetchData extends Activity{
String myJSON;

private static final String TAG_RESULTS="result";
private static final String TAG_ID = "id";
private static final String TAG_NAME = "name";
private static final String TAG_ADD ="address";

JSONArray peoples = null;

ArrayList<HashMap<String, String>> personList;

ListView list;
private static int lastid = 0;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.test);
    list = (ListView) findViewById(R.id.listView);
    personList = new ArrayList<HashMap<String,String>>();
    getData();
    final Handler handler = new Handler();
    handler.postDelayed( new Runnable() {

        @Override
        public void run() {


            refreshData();
            handler.postDelayed( this, 2 * 100 );
        }
    }, 2 * 100 );

}
public void refreshData(){

}
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://192.168.1.4/get-data.php?id="+lastid);

            // 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 + "
");
                }
                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;

            showdata();
        }
    }
    GetDataJSON g = new GetDataJSON();
    g.execute();
}
protected void showdata(){
    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 id = c.getString(TAG_ID);
            String name = c.getString(TAG_NAME);
            String address = c.getString(TAG_ADD);

            HashMap<String,String> persons = new HashMap<String,String>();

            persons.put(TAG_ID,id);
            persons.put(TAG_NAME,name);
            persons.put(TAG_ADD,address);

            personList.add(persons);
            lastid = Integer.parseInt(id);
        }

        ListAdapter adapter = new SimpleAdapter(
                FetchData.this, personList, R.layout.list_item,
                new String[]{TAG_ID,TAG_NAME,TAG_ADD},
                new int[]{R.id.id, R.id.name, R.id.address}
        );

        list.setAdapter(adapter);

    } catch (JSONException e) {
        e.printStackTrace();
    }

}


}