Good day. I'm having some issues with my android project specifically listview. I tried searching for other information here in this site, and implemented some of the answers. However, it is still not working.
The error specifically is
NullPointerException at line 76 at MainActivity
Here is the code of my MainActivity
import java.util.ArrayList;
import android.os.Bundle;
import android.app.Activity;
import android.content.Intent;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
public class MainActivity extends Activity {
final ArrayList<String> studentName = new ArrayList<String>();
ArrayAdapter<String> aa;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ListView myList = (ListView) findViewById(R.id.listName);
aa = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, studentName);
myList.setAdapter(aa);
//droid.R.id.list;
//add
Button bAdd = (Button) findViewById(R.id.addstudent);
bAdd.setOnClickListener(new View.OnClickListener(){
#Override
public void onClick(View v) {
startActivity(new Intent("android.intent.action.ADDSTUDENTS"));
}
});
//edit
Button bEdit = (Button) findViewById(R.id.editstudent);
bEdit.setOnClickListener(new View.OnClickListener(){
#Override
public void onClick(View x) {
startActivity(new Intent("android.intent.action.EDITSTUDENTS"));
}
});
//edit
Button bDelete = (Button) findViewById(R.id.deletestudent);
bDelete.setOnClickListener(new View.OnClickListener(){
#Override
public void onClick(View x) {
startActivity(new Intent("android.intent.action.DELETESTUDENTS"));
}
});
}
public ArrayList<String> getArray(){
return studentName;
}
public void notifyArray(){
aa.notifyDataSetChanged();
}
#Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.activity_main, menu);
return true;
}
}
and line 76 by the way is
aa.notifyDataSetChanged();
Here is my code for the AddStudents class
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
public class AddStudents extends Activity{
#Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.add_student);
Button bAddStudents = (Button) findViewById(R.id.add);
final EditText et = (EditText) findViewById(R.id.student_name);
bAddStudents.setOnClickListener(new View.OnClickListener(){
#Override
public void onClick(View v) {
MainActivity as = new MainActivity();
as.getArray().add(et.getText().toString());
as.notifyArray();
finish();
}
});
Button bBack = (Button) findViewById(R.id.backadd);
bBack.setOnClickListener(new View.OnClickListener(){
#Override
public void onClick(View v) {
finish();
}
});
}
}
and the xml part with the list view is
<ListView
android:id="#+id/listName"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="1" >
</ListView>
I hope you can help me cause I want to also learn what my mistakes are. I can add other information if you want.
In your AddStudents class, you're calling notifyArray() right after you instantiated MainActivity. MainActivity.onCreate() will not be called just by instantiating it.
Instantiating your MainActivity there is probably not what you want anyway (because that object will be disposed directly after the onClick handler is done).
What you want instead is to access the existing instance of MainActivity. For that, add a reference to the current instance to a static member of your MainActivity class, e.g.
public class MainActivity extends Activity {
public static MainActivity activity;
#Override
protected void onCreate(Bundle savedInstanceState) {
activity = this;
}
}
Then in your AddStudent class access it via
MainActivity.activity.notifyArray()
This is not the most beautiful way to solve your issue, but it works as long as you can be sure to only have one MainActivity instance. (If not, you could make the array itself static; or create a Singleton wrapper class for it.)
notifyArray() is being called before onCreate.
Try calling getArray().add(et.getText().toString()); and notifyArray(); inside onResume() of MainActivity and NOT from AddStudentActivity( not recommended!)
So onResume() you would ideally want to add a new student to the list, so in your case, you can retrieve the student name using a common sharable object like a hashtable or somethiing similar, make it a singleton, and use it from anywhere in the applciation
The common class may go something like:
class CommonHashtable{
private static Hashtable<String, Object> commonHashtable = null;
public static getInstance(){
if(commonHashtable == null)
commonHashtable = new Hashtable<String, Object>();
return commonHashtable;
}
on getInstance(), it returns a commonHashtable which can be used to store values temporarily!
so, add this on addbutton click event
Hashtable hash = CommonHashtable.getInstance();
hash.put("NEW_STUDENT_NAME", et.getText().toString());
and add this in you onResume() of MainActivity
Hashtable hash = CommonHashtable.getInstance();
Object studentName = (String) hash.get("NEW_STUDENT_NAME");
if(studentName != null){
notifyArray();
}
Related
We are working on creating a basic app in my online class and I keep running into this issue where the Instructions have us type a method with an #Override for an onClick. I was able to get one of the methods to work with an autofill help from A.S. itself but this next one won't fix itself out. I have run into other issues over this class as well and I know the Instructions are a few years old. Possibly out of date now?
I'm pretty new to Android Java and coding overall. It seems that the app is trying to use the onClick method incorrectly. I will post the whole class I'm working with because I'm not sure where the problem is exactly.
package com.test.firstandroidapp;
import androidx.appcompat.app.AppCompatActivity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.SeekBar;
import android.widget.Spinner;
import android.widget.Switch;
import android.widget.TextView;
public class InformationPageActivity extends AppCompatActivity {
Button btnSave;
Spinner spnSeason;
SeekBar skbTemp;
TextView lblSeekValue;
Switch swchAllergy;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_information_page);
spnSeason = (Spinner) findViewById(R.id.spnSeason);
skbTemp = (SeekBar) findViewById(R.id.skbTemp);
swchAllergy = (Switch) findViewById(R.id.swchAllergy);
lblSeekValue = (TextView) findViewById(R.id.lblSeekValue);
btnSave = (Button) findViewById(R.id.btnSave);
btnSave.setOnClickListener((View.OnClickListener) this);
skbTemp.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
#Override
public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
String display = String.valueOf(progress);
lblSeekValue.setText(display);
}
#Override
public void onStartTrackingTouch(SeekBar seekBar) {
}
#Override
public void onStopTrackingTouch(SeekBar seekBar) {
}
});
}
#Override
public void onClick(View v) {
String season, allergies;
int temperature;
season = spnSeason.getSelectedItem().toString();
temperature=skbTemp.getProgress();
allergies = (String) (swchAllergy.isChecked() ? swchAllergy.getTextOn() : swchAllergy.getTextOff()); //if the switch is on then the user has allergies
Intent intent = new Intent(InformationPageActivity.this, InformationResultsActivity.class); //pass information to the results activity
intent.putExtra("season", season);
intent.putExtra("temperature", temperature);
intent.putExtra("allergies", allergies);
this.startActivity(intent);
}
}
And here is the Main Menu Class parts that go to the view i'm trying to have opened, The onclicklistener is inside the onCreate method and the goInfo is it's own method
Button btnInfo = (Button) findViewById(R.id.btnInfo);
btnInfo.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
goInfo();
}
}
);
}
private void goInfo() {
Intent intent = new Intent(MainMenuActivity.this, InformationPageActivity.class);
this.startActivity(intent);
}
I was able to find the issue. I needed to add an implements statement to the class for the onclicklistener. I'm answering my own question in case someone in the future has the same error. Here is what I did.
My problematic class was
public class InformationPageActivity extends AppCompatActivity {
This was causing the error "java.lang.RuntimeException: Unable to start activity ComponentInfo{com.test.firstandroidapp/com.test.firstandroidapp.InformationPageActivity}: java.lang.ClassCastException: com.test.firstandroidapp.InformationPageActivity cannot be cast to android.view.View$OnClickListener"
I solved the issue by fixing the class adding the implements
public class InformationPageActivity extends AppCompatActivity implements View.OnClickListener {
I created a ListDialog extending a DialogFragment class and I have a problem with understanding of this code in the DijalogX class
((MainActivity)getActivity()).setTextField(selectedItem);
I understand that with this code above I put selected String variable to the setTextField method as an argument and after that this variable is showed in TextView on MainActivity class.
My questions:
Why I need a cast from getActivity() to the MainActivity and how I get access from DijalogX(fragment) to the method setTextField in MainActivity? Please explain a little about this process.
I also tried instead of ((MainActivity)getActivity()).setTextField(selectedItem)
use an Interface and everything works nice and I got the same resoult but I am wondering what is better solution here Interface or ((MainActivity)getActivity()).setTextField(selectedItem)?
MainActivity
package com.example.dezox.dijaloglist;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity{
private Button btnStartDialog;
private TextView tvSelectedOption;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initWidgets();
setupListener();
}
private void initWidgets() {
btnStartDialog = findViewById(R.id.btnDialog);
tvSelectedOption = findViewById(R.id.tvselectedOption);
}
private void setupListener() {
btnStartDialog.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
DijalogX dijalogX = new DijalogX();
dijalogX.show(getSupportFragmentManager(), "dx");
tvSelectedOption.setText("");
}
});
}
public void setTextField(String odabrano){
tvSelectedOption.setText(odabrano);
}
public String getTextField(){
return tvSelectedOption.getText().toString();
}
}
DijalogX
package com.example.dezox.dijaloglist;
import android.app.Dialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.DialogFragment;
import android.support.v7.app.AlertDialog;
public class DijalogX extends DialogFragment {
private String[] languageList;
#Override
public void onCreate(#Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
initListResource();
}
private void initListResource() {
languageList = getResources().getStringArray(R.array.language_list);
}
#NonNull
#Override
public Dialog onCreateDialog(Bundle savedInstanceState) {
AlertDialog.Builder builder = new AlertDialog.Builder(getActivity(),
android.R.style.Theme_Material_Dialog_Alert)
.setTitle("Select Language: ")
.setItems(languageList, new DialogInterface.OnClickListener() {
#Override
public void onClick(DialogInterface dialog, int which) {
String selectedItem = languageList[which];
//THIS PART OF THE CODE I DONT UNDERSTAND:
((MainActivity)getActivity()).setTextField(selectedItem);
}
});
return builder.create();
}
}
You have declared a method in MainActivity called setTextField. If you called
Activity a = getActivity();
you would not be able to call your custom method (it is on your derived class, not the base Activity class).
a.setTextField(selectedIte); // WON'T WORK - NO SUCH METHOD
If instead you call
MainActivity ma = (MainActivity)getActivity();
it is now cast as your derived class and you can then call
ma.setTextField(selectedItem);
Doing it in two lines like this is the same as calling the one-liner in your code
((MainActivity)getActivity()).setTextField(selectedItem);
As far as casting vs. an interface, an interface is a bit more flexible of an approach. If you tried to use this fragment in a different activity (not MainActivity) the casting approach would fail. If you are only ever going to use the fragment in this Activity then either would work.
I try to develop a simple Android App with one Button which generates new TextViews on each click.
import android.app.Activity;
import android.os.Bundle;
import android.text.Layout;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
public class CreateTV extends Activity {
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
Button mCreate = (Button)findViewById(R.id.btnCreate);
mCreate.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
// TODO Auto-generated method stub
((Button) v).setText("Clicked");
TextView mTV1 = new TextView(this);
}
});
}
}
My code is wrong because of:
TextView mTV1 = new TextView(this);
I could find some similar examples, which generate objects programmatically in onCreate(). But I want to generate and modify new objects in onClick().
Would anybody please help?
Change
TextView mTV1 = new TextView(this);
to
TextView mTV1 = new TextView(CreateTV.this);
Views can only be instantiated with a context as parameter
As you can see in the documentation a TextView needs the context to be created. TextView(Context context)
Since you are trying to create a TextView inside a ClickListener you can not use this as a reference to a Context-extending object.
As McAdam331 pointed out, use new TextView(getActivity), this works because Activity extends Context.
In addition to change TextView mTV1 = new TextView(this); to TextView mTV1 = new TextView(CreateTV.this);, you must add the TextView within a view like the following:
import android.app.Activity;
import android.os.Bundle;
import android.text.Layout;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
public class CreateTV extends Activity {
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
Button mCreate = (Button)findViewById(R.id.btnCreate);
mCreate.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
// TODO Auto-generated method stub
((Button) v).setText("Clicked");
TextView mTV1 = new TextView(CreateTV.this);
addContentView(mTV1);
}
});
}
}
I would prefer adding a Context, setting it to final and then call the Textview using the Context.
Example:
public class CreateTV extends Activity {
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
Button mCreate = (Button)findViewById(R.id.btnCreate);
final Context mContext = this;
mCreate.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
// TODO Auto-generated method stub
((Button) v).setText("Clicked");
TextView mTV1 = new TextView(mContext);
addContentView(mTV1);
}
});
}
}
If you want to use the Context outside the onCreate method (and within Listeners) you can define a Context.
private Context context;
public void onCreate(....) {
this.context = this;
}
private void aMethod() {
context....
}
Theres another way doing such cool stuff. Create a Class and extends it by Application.
public class MainApplication extends Application {
public static Context getContext() { return this; }
}
Then add the MainApplication to your Manifest.
<application
android:name=".MainApplication"
>
and access it from everywhere with MainApplication.getContext();
I want to implement 2 activities , one is a listview with image and text which works well now. The other one is a onClicklistener when I click any ListItem it will start another activity to show more information, with 1 previous button and next button to see the Prev information or next. However there is something wrong with the button. It can only see the previous one and next one once. Which means you can't click the next button twice. Any suggestions will be appreciated .
Here is my main activity code
package com.example.manchesterunited;
import com.example.manchesterunited.R;
import android.os.Bundle;
import android.app.AlertDialog;
import android.app.ListActivity;
import android.content.Intent;
import android.view.View;
import android.widget.ListView;
import android.widget.Toast;
public class MainActivity extends ListActivity {
static PlayerData data = new PlayerData();
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
CustomAdapter adapter = new CustomAdapter(this);
setListAdapter(adapter);
}
public void onListItemClick(ListView l,View v, int pos, long id){
int playerId = (int)id;
Toast.makeText(getApplicationContext(),"Selected "+data.getName(pos),Toast.LENGTH_SHORT).show();
Intent intent = new Intent(this, InfoActivity.class);
intent.putExtra("playerId", playerId);
startActivity(intent);
}
}
And here is the second activity
package com.example.manchesterunited;
import android.os.Bundle;
import android.app.Activity;
import android.content.Intent;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;
public class InfoActivity extends Activity {
TextView dobText;
TextView pobText;
TextView internationalText;
Button prevButton;
Button nextButton;
PlayerData data = new PlayerData();
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_info);
dobText = (TextView)findViewById(R.id.textView1);
pobText = (TextView)findViewById(R.id.textView2);
internationalText = (TextView)findViewById(R.id.textView3);
prevButton = (Button)findViewById(R.id.prev);
nextButton = (Button)findViewById(R.id.next);
Intent intent = getIntent();
final int playerId = intent.getExtras().getInt("playerId");
dobText.setText("Birthdate:"+String.valueOf(data.getPlayer(playerId).getDob()));
pobText.setText("Birthplace:"+String.valueOf(data.getPlayer(playerId).getPob()));
internationalText.setText("International:"+data.getPlayer(playerId).getInternational());
prevButton.setOnClickListener(new OnClickListener(){
#Override
public void onClick(View v) {
// TODO Auto-generated method stub
int newId = playerId-1;
dobText.setText("Birthdate:"+String.valueOf(data.getPlayer(newId).getDob()));
pobText.setText("Birthplace:"+String.valueOf(data.getPlayer(newId).getPob()));
internationalText.setText("International:"+data.getPlayer(newId).getInternational());
}
});
nextButton.setOnClickListener(new OnClickListener(){
#Override
public void onClick(View v) {
// TODO Auto-generated method stub
int newId = playerId+1;
dobText.setText("Birthdate:"+String.valueOf(data.getPlayer(newId).getDob()));
pobText.setText("Birthplace:"+String.valueOf(data.getPlayer(newId).getPob()));
internationalText.setText("International:"+data.getPlayer(newId).getInternational());
}
});}
#Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.activity_info, menu);
return true;
}
}
You're not updating the playerID.
You're just getting a new ID.
you should put
int newId = playerId+1;
playerID = newId; //insert this line
so that when you click again,
playerID gets updated.
You are always adding just 1 to player ID which will now point to next item. But as soon as you re-click the button it will have the same player ID that was passed through intent. So again it will add in the same value. Make a new variable which will store the current player ID that is visible and keep on incrementing it then you will be able to browse next and vice versa in the previous case.
You can't increment player ID as it has been declared as final, so you can either make a global variable or put a variable in intent and increment it on button clicks.
nextButton.setOnClickListener(new OnClickListener(){
#Override
public void onClick(View v)
{
// TODO Auto-generated method stub
//make newId global then it will work. And keep on updating it when the button is clicked accordingly.
newId = playerId+1;
dobText.setText("Birthdate:"+String.valueOf(data.getPlayer(newId).getDob()));
pobText.setText("Birthplace:"+String.valueOf(data.getPlayer(newId).getPob()));
internationalText.setText("International:"+data.getPlayer(newId).getInternational());
}
});}
Try this on InfoActivity,
private int playerId;
...
...onCreate(...){
...
playerId = intent.getExtras().getInt("playerId");
prevButton.setOnClickListener(new OnClickListener(){
#Override
public void onClick(View v) {
...
playerId--;
dobText.setText("Birthdate:"+String.valueOf(data.getPlayer(playerId).getDob()));
pobText.setText("Birthplace:"+String.valueOf(data.getPlayer(playerId).getPob()));
internationalText.setText("International:"+data.getPlayer(playerId).getInternational());
}
}
nextButton.setOnClickListener(new OnClickListener(){
#Override
public void onClick(View v) {
...
playerId++;
dobText.setText("Birthdate:"+String.valueOf(data.getPlayer(playerId).getDob()));
pobText.setText("Birthplace:"+String.valueOf(data.getPlayer(playerId).getPob()));
internationalText.setText("International:"+data.getPlayer(playerId).getInternational());
}
}
}
I have some problems with the "setOnClickListener(onSave)” and “View.OnClickListener”
Below is my code:
package apt.tutorial;
import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
public class LunchList extends Activity {
Restaurant r = new Restaurant ();
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
//save button
//be notified when the button is clicked
Button save =(Button)findViewById(R.id.save);
save.setOnClickListener(onSave){
}
private View.OnClickListener onSave=new View.OnClickListener();
public void onClick(View v) {
EditText name=(EditText)findViewById(R.id.name);
EditText address=(EditText)findViewById(R.id.addr);
r.setName(name.getText().toString());
r.setAdress(address.getText().toString());
}
#Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.activity_main, menu);
return true;
}
}
for save.setOnClickListener(onSave) I get the error save.setOnClickListener(onSave) and for View.OnClickListener I get the error Cannot instantiate the type View.OnClickListener.
I looked on this website and googled for this problem put I didn’t find a solution. I hope you guys can help me out.
Greets.
Remove semicolon after
private View.OnClickListener onSave=new View.OnClickListener();
and write onClick(View v) inside anonomous constructor of onClickListener
or else change your code with below code.
private View.OnClickListener onSave=new View.OnClickListener(
public void onClick(View v) {
EditText name=(EditText)findViewById(R.id.name);
EditText address=(EditText)findViewById(R.id.addr);
r.setName(name.getText().toString());
r.setAdress(address.getText().toString());
});
Button save =(Button)findViewById(R.id.save);
save.setOnClickListener(onSave);
try the following code:
start.setOnClickListener(new OnClickListener() {
public void onClick(View arg0) {
mainactivity();
}
});