非同期クラスがAndroidで実行される前にUIスレッドが実行されます

Androidフォームのフィールドの検証を行っています。サーバーでユーザー名が使用可能かどうかをサーバーに確認していますが、非同期チェックが完了する前にメインスレッドが次のページに移動します。

コード：

btnnext1.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {

        isallValid=true;
        //check second time for username validation(first time was in onfocus changed)

//      if(txtusername.getText().toString().trim().equals("achuthan")){
//          txtusername.setError("Username exsists!");
//          isUsernameValid=false;
//        }
//      
//        else
//        {
//          isUsernameValid=true;
//        }

        try {
            Void async_result=new validateusername().execute().get();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ExecutionException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

                if(txtfullname.getText().toString().trim().length()==0)
            {
            txtfullname.requestFocus();
            txtfullname.setError("Field required");
            isallValid=false;

            }
        if(txtdob.getText().toString().trim().length()==0)
        {
            txtdob.requestFocus();
            txtdob.setError("Field required");
            isallValid=false;
        }
        if(txtoccupation.getText().toString().trim().length()==0)
        {
            txtoccupation.requestFocus();
            txtoccupation.setError("Field required");
            isallValid=false;
        }
         if(txtusername.getText().toString().trim().length()<6){
            txtusername.requestFocus();
            txtusername.setError("Minimum length of 6 characters");
            isallValid=false;
        }
        if(txtpassword.getText().toString().trim().length()==0)
        {
            txtpassword.requestFocus();
            txtpassword.setError("Field required");
            isallValid=false;
        }
        if(txtconfirmpassword.getText().toString().trim().length()==0)
        {
            txtconfirmpassword.requestFocus();
            txtconfirmpassword.setError("Field required");
            isallValid=false;
        }
        else if(!txtpassword.getText().toString().trim().equals(txtconfirmpassword.getText().toString().trim()))
        {
            //txtconfirmpassword.requestFocus();
            txtconfirmpassword.setError("Passwords not equal");
            txtpassword.setError("Passwords not equal");
            isallValid=false;
        }


         if(isallValid&&isUsernameValid)
        {
            //Toast.makeText(getActivity(),"VALID FORM!!",Toast.LENGTH_LONG).show();
            ((SignUpPage)getActivity()).getValues().fullname=txtfullname.getText().toString().trim();
            ((SignUpPage)getActivity()).getValues().dob=txtdob.getText().toString().trim();


            int id=radiogender.getCheckedRadioButtonId();
            RadioButton rb=(RadioButton) view.findViewById(id);
            String gender=rb.getText().toString();
            ((SignUpPage)getActivity()).getValues().gender=gender;


            int id1=radiomarital.getCheckedRadioButtonId();
            RadioButton rb1=(RadioButton) view.findViewById(id1);
            String marital_status=rb1.getText().toString();
            ((SignUpPage)getActivity()).getValues().marital_status=marital_status;



            ((SignUpPage)getActivity()).getValues().occupation=txtoccupation.getText().toString().trim();
            ((SignUpPage)getActivity()).getValues().username=txtusername.getText().toString().trim();
            ((SignUpPage)getActivity()).getValues().password=txtpassword.getText().toString().trim();



            ((SignUpPage)getActivity()).selectFragment(1);


        }
                    //if all valid , store values and go to next fragment
                    //((SignUpPage)getActivity()).selectFragment(1);
            }
        });



        return view;
    }

非同期クラス：

public class validateusername extends AsyncTask<String,Void,Void>
        {

            @Override
            protected Void doInBackground(String... arg0) {
                HttpClient httpclient = new DefaultHttpClient();
                HttpPost httppost = new HttpPost(URL);
                List<NameValuePair> pairs = new ArrayList<NameValuePair>();
                pairs.add(new BasicNameValuePair("username",txtusername.getText().toString().trim()));
                try {
                    httppost.setEntity(new UrlEncodedFormEntity(pairs));
                    response = httpclient.execute(httppost);
                    result=responsetostring.getResponseBody(response);
                } catch (ClientProtocolException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }




                return null;
            }
            @Override
            protected void onPostExecute(Void result1) {
                try {
                    jsonobj=new JSONObject(result);
                    job2=jsonobj.getJSONObject("server_message");
                } catch (JSONException e1) {
                    // TODO Auto-generated catch block
                    e1.printStackTrace();
                }
                 try {
                    finalresult=job2.getString("username_availability_message");
                } catch (JSONException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                if(finalresult.equals("exist")){
                    txtusername.setError("Username exsists!");
                    isUsernameValid=false;
                }
                else if(finalresult.equals("available"))
                {

                    isUsernameValid=true;
                }
            }

        }

getメソッドを使用して、メインスレッドが非同期クラスが終了するまで待機するようにしましたが、機能しませんでした。助けてください!!