在启动HomeActivity之前启动的MainActivity的以下代码（下载另一个活动中需要的json并显示加载进度条）中，我收到一个空对象引用错误（在getProgress（）方法上），但是如果我按下“后退”按钮，HomeActivity随即会启动。
我已经搜索了这种错误，但是似乎可行的唯一解决方案是让我在一段时间延迟后启动HomeActivity的解决方案，更改了这样的setProgress方法：（但似乎栏没有改变，并且我认为这不是解决问题的真正方法）
public void setProgress(ProgressBar bar, int progress) {
int oldProgress = bar.getProgress();
int totalProgress = oldProgress + progress;
bar.setProgress(totalProgress);
if (bar.getProgress() == 100) {
new Handler().postDelayed(new Runnable() {
@Override
public void run() {
final Intent intent = new Intent(MainActivity.this, HomePage.class);
startActivity(intent);
}
}, 4000);
}
}
----------------------------------------------------------------------
package ...
import ...
public class MainActivity extends AppCompatActivity {
private ProgressBar progressBar;
private TextView textView;
private OkHttpClient httpClient;
private String url;
private Request request;
private static boolean loaded = false;
private FileInputStream inputStream;
private FileOutputStream outputStream;
private static final String FILE_NAME = "data.txt";
private AlertDialogConnection noResponse;
private AlertDialogConnection noInternet;
private AlertDialogConnection serverError;
private AlertDialogConnection internalError;
public MainActivity(){
this.inputStream = null;
this.outputStream = null;
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
this.setContentView(R.layout.activity_main);
getWindow().getDecorView().setSystemUiVisibility(View.SYSTEM_UI_FLAG_HIDE_NAVIGATION);
this.httpClient = new OkHttpClient();
this.url = "my url";
this.request = new Request.Builder().url(url).build();
this.progressBar = findViewById(R.id.progressBar);
this.progressBar.getProgressDrawable().setColorFilter(getResources().getColor(R.color.progressBarColor), android.graphics.PorterDuff.Mode.SRC_IN);
this.textView = findViewById(R.id.textView);
this.noResponse = new AlertDialogConnection();
this.noInternet = new AlertDialogConnection();
this.serverError = new AlertDialogConnection();
this.internalError = new AlertDialogConnection();
checkConnectionStatusAndStart();
}
@Override
public void onWindowFocusChanged(boolean hasFocus) {
super.onWindowFocusChanged(hasFocus);
if (hasFocus) {
getWindow().getDecorView().setSystemUiVisibility(
View.SYSTEM_UI_FLAG_LAYOUT_STABLE
| View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
| View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
| View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
| View.SYSTEM_UI_FLAG_FULLSCREEN
| View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY);
}
}
public void checkConnectionStatusAndStart(){
if(checkConnectionStatus())
requestData();
else{
this.noInternet.setTitle("no connection!");
this.noInternet.setText("connection error!");
this.noInternet.show(getSupportFragmentManager(), "no connection!");
}
}
public boolean checkConnectionStatus(){
boolean wifiConnection = false;
boolean mobileConnection = false;
ConnectivityManager connectivityManager = (ConnectivityManager)getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo networkInfo = null;
if (connectivityManager != null)
networkInfo = connectivityManager.getActiveNetworkInfo();
if((networkInfo != null) && networkInfo.isConnected()){
if(networkInfo.getType() == ConnectivityManager.TYPE_WIFI){
/* Wifi On */
wifiConnection = true;
}
if(networkInfo.getType() == ConnectivityManager.TYPE_MOBILE){
/* Mobile data On */
mobileConnection = true;
}
return (wifiConnection || mobileConnection);
}else{
/* You are not connected */
return false;
}
}
public void requestData(){
textView.setText("waiting server...");
httpClient.connectTimeoutMillis(); //Timeout
httpClient.writeTimeoutMillis();
httpClient.newCall(request).enqueue(new Callback() {
@Override
public void onFailure(Call call, IOException e) {
if(!call.isExecuted()){
Toast.makeText(getApplicationContext(), "server error!", Toast.LENGTH_LONG).show();
}
Log.d("HTTP-Error", "HTTP error!");
AlertDialogConnection errorDialog = new AlertDialogConnection();
errorDialog.setTitle("server error!");
errorDialog.setText("server error, try later");
errorDialog.show(getSupportFragmentManager(), "messaging error!");
}
@Override
public void onResponse(@NotNull Call call, Response response) throws IOException {
if(response.isSuccessful()){
/* HTTP-code: 200 */
final String body = response.body().string();
MainActivity.this.runOnUiThread(new Runnable() {
@Override
public void run() {
((TextView)findViewById(R.id.textView)).setText("Setting of suggestion variables!");
setProgress(progressBar, 10);
try {
JSONObject json = new JSONObject(body);
try {
((TextView)findViewById(R.id.textView)).setText("retrieving server infos!");
saveJSON(json, getApplicationContext());
} catch (FileNotFoundException e) {
Log.d("File Not Found!", "FileNotFoundException!");
internalError.setTitle("error while saving data!");
internalError.setText("server error, try later");
internalError.show(getSupportFragmentManager(), "error while saving data!");
e.printStackTrace();
} catch (IOException e) {
Log.d("File Not Found!", "IOException!");
internalError.setTitle("error while saving data!");
internalError.setText("server error, try later");
internalError.show(getSupportFragmentManager(), "error while saving data!");
e.printStackTrace();
}
setProgress(progressBar, 90);
MainActivity.loaded = true;
} catch (JSONException e) {
e.printStackTrace();
Log.d("JSON-Error", "parsing JSON error!");
internalError.setTitle("server error!");
internalError.setText("server error, try later");
internalError.show(getSupportFragmentManager(), "JSON messaging error!");
}
}
});
}else{
/* Http-code: 500 */
Log.d("HTTP-Error", "server error!");
serverError.setTitle("server error!");
serverError.setText("server error");
serverError.show(getSupportFragmentManager(), "server error!");
}
}
});
}
public void setProgress(ProgressBar bar, int progress){
int oldProgress = bar.getProgress();
int totalProgress = oldProgress + progress;
bar.setProgress(totalProgress);
if(bar.getProgress() == 100){
Intent intent = new Intent(this, HomePage.class);
startActivity(intent);
this.onDestroy();
}
}
private void saveJSON(JSONObject json, Context context) throws IOException {
outputStream = null;
try{
outputStream = context.openFileOutput(FILE_NAME, MODE_PRIVATE);
outputStream.write(json.toString().getBytes());
}finally {
if(outputStream != null){
outputStream.close();
}
}
}
public JSONObject loadJSON(Context context) throws IOException, JSONException {
inputStream = context.openFileInput(FILE_NAME);
BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
JSONObject jsonObject = new JSONObject(reader.readLine());
inputStream.close();
return jsonObject;
}
@Override
protected void onStart() {
super.onStart();
}
@Override
protected void onStop() {
super.onStop();
}
@Override
protected void onDestroy() {
this.httpClient = null;
this.noResponse = null;
this.noInternet = null;
this.serverError = null;
this.internalError = null;
this.request = null;
this.textView = null;
this.progressBar = null;
this.url = null;
super.onDestroy();
}
@Override
protected void onResume() {
if(!loaded){
checkConnectionStatusAndStart();
}
super.onResume();
}
@Override
protected void onRestart() {
if(!loaded){
checkConnectionStatusAndStart();
}
super.onRestart();
}
@Override
protected void onPause() {
if(!loaded ){
checkConnectionStatusAndStart();
}
super.onPause();
}
}