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Custom listener with interface

I am trying to understand better how to create a custom listener with a simple example but I don't know how to start the interface so that it is not null:
public class MainActivity extends AppCompatActivity implements ListenerButton{
TextView helloToOther;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
helloToOther = findViewById(R.id.helloWorldToOtherActivity);
helloToOther.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
startActivity(new Intent(MainActivity.this, ButtonActivity.class));
}
});
}
#Override
public void onClickButton(View view) {
Toast.makeText(this, "Estoy en la primera activity", Toast.LENGTH_SHORT).show();
}
}
This is the second activity:
public class ButtonActivity extends AppCompatActivity {
Button btnInterface;
ListenerButton listenerButton;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_button);
btnInterface = findViewById(R.id.button_activity__btn__button_interface);
setUpButtonInterface();
}
private void setUpButtonInterface() {
btnInterface.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
listenerButton.onClickButton(v);
}
});
}
}
And there's the interface:
public interface ListenerButton {
void onClickButton(View view);
}
Basically I get a null pointer exception on the second activity because the interface is null, but I don't fall right now as I can start it. Thank you very much.

the reason you are getting a null pointer exception is because you haven't assigned anything to variable listenerButton and therefor it is in fact null!!
you don't need a new interface for that you just need to do this:
public class ButtonActivity extends AppCompatActivity {
Button btnInterface;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_button);
btnInterface = findViewById(R.id.button_activity__btn__button_interface);
setUpButtonInterface();
}
private void setUpButtonInterface() {
btnInterface.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
//do whatever you want to do when button is clicked!
}
});
}
}
if you want to define whatever you want to do when button is clicked you need a class and not an interface:
public class ButtonListener implements View.OnClickListener{
#Override
public void onClick(View v) {
//do whatever you want to do when button is clicked!
}
}
and then in your activity do this:
public class ButtonActivity extends AppCompatActivity {
Button btnInterface;
ListenerButton listenerButton;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_button);
listenerButton = new ButtonListener();
btnInterface = findViewById(R.id.button_activity__btn__button_interface);
setUpButtonInterface();
}
private void setUpButtonInterface() {
btnInterface.setOnClickListener(listenerButton);
}
}

Create a new file:
MyListener.java:
public interface MyListener {
// you can define any parameter as per your requirement
public void callback(View view, String result);
}
In your activity, implement the interface:
MyActivity.java:
public class MyActivity extends Activity implements MyListener {
#override
public void onCreate(){
MyButton m = new MyButton(this);
}
// method is invoked when MyButton is clicked
#override
public void callback(View view, String result) {
// do your stuff here
}
}
In your custom class, invoke the interface when needed:
MyButton.java:
public class MyButton {
MyListener ml;
// constructor
MyButton(MyListener ml) {
//Setting the listener
this.ml = ml;
}
public void MyLogicToIntimateOthers() {
//Invoke the interface
ml.callback(this, "success");
}
}

Updating the status in UI methods from a non-UI thread

I have an Activity that includes a TextView to display the current status of the app. Besides MainActivity there is another class that is used for checking for updates and then updating the app if appropriate.
This is the important part of the code.
public class MainActivity extends AppCompatActivity {
private TextView status;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
status = findViewById(R.id.status);
new Update().check();
}
public void setStatus(String status) {
this.status.setText(status);
}
}
How can I update the status TextView from within the Update class?
public class Update extends Activity {
public void check(final Context c) {
runOnUiThread(new Runnable() {
#Override
public void run() {
TextView status = findViewById(R.id.status);
status.setText("Checking for updates...");
}
});
// final TextView status = getApplicationContext().findViewById(R.id.status);
// status.setText("Checking for updates");
// Check for updates...
}
public class UpdateApp extends AsyncTask<String,Void,Void> {
private Context context;
public void setContext(Context contextf){
context = contextf;
}
#Override
protected Void doInBackground(String... arg0) {
// Update the app
}
}
}
Attempting to run getApplicationContext().findViewById(R.id.status) crashes the app. When I put it in a try block it still crashes the app - how can that be when I catch a Throwable object? ... that doesn't make sense!!!
Using runOnUiThread the app runs but the status is not updated.

Put the UpdateApp AsynTask in new file.
public class MainActivity extends AppCompatActivity {
private TextView status;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
status = findViewById(R.id.status);
new UpdateApp(status).execute();
}
}
UpdateApp.java
public class UpdateApp extends AsyncTask<String,Void,Void> {
private TextView status;
public UpdateApp(TextView status){
this.status = status;
}
#Override
protected void onPostExecute(String result) {
status.setText("Update complete");
}
#Override
protected void onPreExecute() {
status.setText("Checking for updates...");
}
#Override
protected Void doInBackground(String... arg0) {
// Update the app
}
}

You just don't need another class to make it works, just copy your UpdateApp class inside MainActivity to access your status view.
In case you want your UpdateApp class to be in a different file (witch I recommend, to separate classes responsibility) you'd need a callback: a reference to your activity to update the status view from the AsyncTask class. The way you can make it is using Interfaces. Take a look to this example
MainActivity.java
public class MainActivity extends AppCompatActivity implements CallbackInterface {
private TextView status;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
status = findViewById(R.id.status);
new LoaderClass(this).execute();
}
#Override
public void setStatusText(String text) {
status.setText(text);
}
}
CallbackInterface.java
public interface CallbackInterface {
void setStatusText(String text);
}
LoaderClass.java
public class LoaderClass extends AsyncTask<String, Void, Void> {
private CallbackInterface callbackInterface;
public LoaderClass(CallbackInterface callback) {
callbackInterface = callback;
}
#Override
protected Void doInBackground(String... arg0) {
callbackInterface.setStatusText("Loaded text");
return null;
}
}

If you are in any Activity you can use runonUIthread() like -
runOnUiThread(new Runnable() {
#Override
public void run () {
// do your magic here
}
});

How to start an Activity task using Threads?

I want to start the Main2Activity's task(Loading the WebView) when button is pressed but not to show the Main2Activity's Screen until onPageFinished() is called. i had some ideas to get this by Intents however it seems doesn't work.
Here my code :
MainActivity.java :
public class MainActivity extends AppCompatActivity {
Handler handler = new Handler(){
#Override
public void handleMessage(Message msg) {
super.handleMessage(msg);
Intent intent = new Intent(MainActivity.this,Main2Activity.class);
startActivity(intent);
}
};
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final Button button = (Button)findViewById(R.id.Button);
button.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
final Runnable runnable = new Runnable() {
#Override
public void run() {
handler.sendEmptyMessage(0);
}
};
Thread thread = new Thread(runnable);
thread.start();
}
}
);
}
}
Main2Activity.java :
public class Main2Activity extends AppCompatActivity {
Intent intent;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main2);
final WebView webView = (WebView)findViewById(R.id.webview);
webView.setWebViewClient(new WebViewClient() {
#Override
public void onPageFinished(WebView view, String url) {
super.onPageFinished(webView, url);
intent.putExtra("DONE",1);
// Toast.makeText(getApplicationContext(), "Done!",
//Toast.LENGTH_SHORT).show();
}
});
webView.loadUrl("file:///android_asset/2.2.html");
}
}
is this possible using Intents or i have to do this with some other techniques?
How can i Achieve this goal ?
Thanks All.

I think you should directly go to Main2Activity on button click, without calling any thread in MainActivity.
Then in Main2Activity show an image or something until onPageFinished() is called.

Can i declare and use the same button in an Activity and a class?

I want to use the button for different calls in the different classes. However, when I declare and try to call the Button click method in the Activity class, it throws a null exception. In my Class I want to do this:
public class CustomFeedListViewAdapter extends BaseAdapter {
holder.feedUpVoteButton = (Button) view.findViewById(R.id.feedUpVoteButton);
holder.feedUpVoteButton.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
//do stuff
}
});
private class ViewHolder {
Button feedUpVoteButton;
}
And in my main activity I want to do this:
public class Feed extends AppCompatActivity {
Button upVoteButton;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_feed);
upVoteButton = (Button) findViewById(R.id.feedUpVoteButton);
feedUpVoteButton.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
//do stuff
}
});
}
}

You are getting NullPointerException because you are trying to access feedUpVoteButton which is present in the item layout used in CustomFeedListViewAdapter and not in the layout used by Feed activity.
Do you want to have access to the ClickListener whenever you create an adapter of type CustomFeedListViewAdapter? If yes then do this...
Your Activity class
public class Feed extends AppCompatActivity {
CustomFeedListViewAdapter adapter;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_feed);
adapter = new CustomFeedListViewAdapter( new OnClickListener() {
#Override
public void onClick(View v) {
// Handle click here
}
});
}
}
And your custom adapter class
public class CustomFeedListViewAdapter extends BaseAdapter {
View.OnClickListener clickListener;
// Constructor
public CustomFeedListViewAdapter(View.OnClickListener listener) {
clickListener = listener;
}
#Override
void getView(){
holder.feedUpVoteButton = (Button) view.findViewById(R.id.feedUpVoteButton);
holder.feedUpVoteButton.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
//do stuff
}
});
}
}
This way you can set a different listener for each instance of the adapter you create.

How to access arraylist outside the oncreate

Hi i am new to programming. I want access arraylist outside from the onCreate but i get error.Below is code.
public class WelcomeOnFootActivity extends AppCompatActivity {
ImageButton next;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_welcome_on_foot);
Bundle bundle=getIntent().getExtra;
List<UserDate> data= new ArrayList();
data.add((UserData)bundle.get("data"));
next=(ImageButton)findViewById(R.id.next);
next.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
handle();
}
});
}
private void handle(){
String info=data.getUserInfo;// here i get error
}
}

Just declare the list globally at class level. In your case before OnCreate.
List<UserDate> data= new ArrayList();
en#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_welcome_on_foot);
Bundle bundle=getIntent().getExtra;
data.add((UserData)bundle.get("data"));
next=(ImageButton)findViewById(R.id.next);
next.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
handle();
}
});
}
private void handle(){
String info=data.getUserInfo;
}
Hope it will help.

public class WelcomeOnFootActivity extends AppCompatActivity {
ImageButton next;
List<UserDate> data;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_welcome_on_foot);
Bundle bundle=getIntent().getExtra;
data= new ArrayList();
data.add((UserData)bundle.get("data"));
next=(ImageButton)findViewById(R.id.next);
next.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
handle();
}
});
}
private void handle(){
String info=data.getUserInfo;// here i get error
}
}

public class WelcomeOnFootActivity extends AppCompatActivity {
ImageButton next;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_welcome_on_foot);
Bundle bundle=getIntent().getExtra;
final List<UserDate> data= new ArrayList();
data.add((UserData)bundle.get("data"));
next=(ImageButton)findViewById(R.id.next);
next.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
handle(data);
}
});
}
private void handle(List<UserDate> listUD){
String info=listUD.getUserInfo;// here i get error
}
}