Develop Reference

Attempt to write to null Array error when passing data to main activity

I am still a beginner and this is my first post here on the forum, sorry if the format of the question is not the right one.
I have an array of Resources in a class called DatabaseTagWalk that I use to find the id of a button in the MainActivity class.
This is the class where I store the id (in the activity_main.xml file i have created 6 buttons and assigned to them the same names as down here
public class DatabaseTagWalk {
int[] iconWalkId;
public DatabaseTagWalk(){
iconWalkId = new int[6];
iconWalkId[0] = R.id.topsightswalk;
iconWalkId[1] = R.id.literarypariswalk;
iconWalkId[2] = R.id.secretpassageswalk;
iconWalkId[3] = R.id.picnictimewalk;
iconWalkId[4] = R.id.joggitwalk;
iconWalkId[5] = R.id.deluxegardenswalk;
}
}
And this is the main activity class where I look for the id of the button to set a specific text and to specify the parameters used when the button is clicked
public class MainActivity extends AppCompatActivity {
DatabaseWalks listofwalks;
DatabaseTagWalk tagWalk;
Button[] buttons;
int buttonClicked;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
listofwalks = new DatabaseWalks();
tagWalk = new DatabaseTagWalk();
for (int i = 0; i < listofwalks.walkList.length; i++) {
buttons[i] = (Button) findViewById(tagWalk.iconWalkId[i]);
buttons[i].setText(listofwalks.walkList[i].returnWalk());
setOnClick(buttons[i], tagWalk.iconWalkId[i]);
}
}
public void setOnClick(Button button, final int buttonTag ){
button.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
buttonClicked = buttonTag;
Intent startingGame = new Intent("android.intent.action.SECONDACTIVITY");
startingGame.putExtra("buttonTag", buttonClicked);
startActivity(startingGame);
}
});
}
}
When i run the app I get the error:
"Attempt to write to null array" in the MainActivity at the line:
buttons[i] = (Button) findViewById(tagWalk.iconWalkId[i]);

You did not initialize your buttons list. You told that this variable exist, but you did not assign any value.
Change your line:
Button[] buttons;
to (if you know size - e.g. 10):
Button[] buttons = new Button[10];
or if wyou would like base on walkList size:
// After this line
listofwalks = new DatabaseWalks();
// Add new line, because here you know the list size
buttons = new Button[listofwalks.walkList.length];

Parsing JSON to array not working when back button pressed

I'm having some trouble with an issue in my Android Studio application that is to do with pressing the back button. Basically the user inputs into a text view and another method (not included as it works fine but I can post if requested) searches an API for the relevant data and displays it in a recycler view. Then the user makes a selection from the recycler view and the method below parses a JSON file and adds relevant data from it to an array in a separate class "Director". Once this happens the next activity "GraphActivity" starts and draws a number of nodes on a canvas for each director in the array.
The issue I am having is that when I make an input into the text view and then press the back button to close the soft keyboard on my Android device (it covers up the recycler view) and go through the steps I just mentioned, when the GraphActivity activity starts it does not work (no nodes are drawn). However, if I then press the back button to go back to the first activity and then make the same selection from the recycler view again, it works.
From what I can tell, the issue seems to be with the data being added to the "Director" array but I cannot figure out what is causing it, Logcat shows that the first time the GraphActivity activity starts the "Director" array is empty and therefore, no nodes are drawn but when I press back and then make the same selection again the array has the correct data and the nodes are drawn.
I would really appreciate some help with this as I am very new to Android Studio and have been searching around for hours to try and find a solution.
public class MainActivity extends AppCompatActivity implements MainAdapter.OnCompanyClickedListener {
private RecyclerView recyclerView;
private TextView userInput;
private Button search;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
recyclerView = (RecyclerView) findViewById(R.id.rView);
RecyclerView.LayoutManager manager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(manager);
}
//Parse JSON File
public void searchCompanyDirectors(final String companyNumber){
Thread thread = new Thread(new Runnable(){
#Override
public void run(){
HttpClient httpClient = new HttpClient();
try{
final String response = httpClient.run("myURL");
runOnUiThread(new Runnable(){
#Override
public void run(){
try {
JSONObject jsonObject = new JSONObject(response);
JSONArray array = jsonObject.getJSONArray("items");
for (int i = 0; i < array.length();i++){
JSONObject o = array.getJSONObject(i);
String appointment = o.getString("links");
String parsedAppointment = parseApp(appointment);
Director director = new Director(o.getString("name"),parsedAppointment);
Director.directorList.add(director);
}
}catch (JSONException e){
e.printStackTrace();
}
}
});
}
catch (Exception ex){
ex.printStackTrace();
}
}
});
thread.start();
}
#Override
public void onCompanyClicked(int position) {
Toast.makeText(this,Company.companyList.get(position).getTitle(),Toast.LENGTH_LONG).show();
chosenCompanyNumber = Company.companyList.get(position).getCompanyNumber();
chosenCompany = Company.companyList.get(position).getTitle();
searchCompanyDirectors(chosenCompanyNumber);
Intent intent = new Intent(this,GraphActivity.class);
startActivity(intent);
}
}
The GraphActivity class:
public class GraphActivity extends AppCompatActivity {
DrawGraph drawGraph;
#SuppressLint("ClickableViewAccessibility")
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
drawGraph = new DrawGraph(this);
setContentView(drawGraph);
The DrawGraph class:
public class DrawGraph extends View {
private Paint p1;
private Paint p2;
private Paint p3;
private Paint paint;
private Path path;
private Point point1;
private int radius = 300;
double x;
double y;
double angle;
int mWidth = this.getResources().getDisplayMetrics().widthPixels;
int mHeight = this.getResources().getDisplayMetrics().heightPixels;
List<Director> directorList = Director.getDirectorList();
private String chosenCompany = MainActivity.getChosenCompany();
private static List<Float> ordinates = new ArrayList<>();
public DrawGraph(Context context) {
super(context);
p1 = new Paint(Paint.ANTI_ALIAS_FLAG);
p1.setStrokeWidth(10);
p2 = new Paint(Paint.ANTI_ALIAS_FLAG);
p2.setStrokeWidth(10);
p3 = new Paint(Paint.ANTI_ALIAS_FLAG);
p3.setStrokeWidth(5);
path = new Path();
paint = new Paint();
point1 = new Point(mWidth/2, mHeight/2);
}
#Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
// Colors
p1.setStyle(Paint.Style.FILL);
p1.setColor(Color.RED);
p2.setStyle(Paint.Style.FILL);
p2.setColor(Color.GREEN);
p3.setStyle(Paint.Style.STROKE);
p3.setColor(Color.CYAN);
paint.setColor(Color.BLACK);
paint.setTextSize(18f);
paint.setAntiAlias(true);
paint.setTextAlign(Paint.Align.CENTER);
Rect bounds = new Rect();
paint.getTextBounds(chosenCompany, 0, chosenCompany.length(),bounds);
//Draw company and directors
for (int i = 1; i <= directorList.size(); i++)
{
//Divide the company node
angle = i * (360/directorList.size());
x = point1.x + radius * cos(angle);
y = point1.y + radius * sin(angle);
//Draw directors
canvas.drawCircle((float)x,(float)y - (bounds.height() / 2), bounds.width() + 5,p2);
canvas.drawText(directorList.get(i-1).getName(),(float)x,(float)y,paint);
}

Okay, I've found an issue in DrawGraph class noted here;
List<Director> directorList = Director.getDirectorList();
private String chosenCompany = MainActivity.getChosenCompany();
You should not directly access the data objects of one activity from another. The previous activity could be destroyed etc as per Android OS and its lifecycle. You must think of it like out of sight, out of memory; which can cause the current activity to not be able to access the data in the previous one.
You'd need to properly send the parsed data from your JSON file, the variables List directorList and the String chosenCompany from your MainActivity to GraphActivity.
You can send data from one activity to another by adding it to the intent with which you call the next activity. The Intent class has features to help you send a small amount of information via itself; called putExtra() and getExtra(). More about them here.
You will need to add those variable datasets through intent.putExtra and fetch them back in your other activity onCreate by using getExtras.
Activity A >> Intent call to Activity B + Data >> Activity B
Hope this helps.

Java class extends MainActivity but findViewById comes back null?

This class extends my main Activity.
public class Numbers extends MainActivity{
public ArrayList<ImageView> getNumbers () {
ArrayList<ImageView> numbers = new ArrayList<>();
ImageView one = (ImageView) findViewById(R.id.one);
numbers.add(one);
return numbers;
}
And I've done some digging but can figure out why my variable "one" is coming back null.
My MainActivity has a ContentView set.
This is the content of my onCreate in MainActivity
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ImageView start = (ImageView) findViewById(R.id.start);
sceneRoot = (ViewGroup) findViewById(R.id.scene_root);
questionView = findViewById(R.id.questionView);
startView = findViewById(R.id.startView);
gameOverView = findViewById(R.id.gameOver);
animSlide = AnimationUtils.loadAnimation(getApplicationContext(), R.anim.slide);
animSlide.setAnimationListener(this);
animZoom = AnimationUtils.loadAnimation(getApplicationContext(), R.anim.zoom_fade);
animZoom.setAnimationListener(this);
set.addTransition(new Fade())
.addTransition(new Slide(Gravity.RIGHT));
start.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
getQuestion();
TransitionManager.beginDelayedTransition(sceneRoot, set);
startView.setVisibility(View.GONE);
questionView.setVisibility(View.VISIBLE);
}
});
}
public void getQuestion (){
time = (TextView) findViewById(R.id.timeBar);
time.startAnimation(animSlide);
}
I don't call getNumbers() until after start has been clicked and the animation has started.
public void onAnimationStart(Animation animation){
if(animation == animSlide) {
final Questions questions = new Questions();
Numbers n = new Numbers();
for (int i = 0; i < n.getNumbers().size(); i++) {
n.getNumbers().get(i).setVisibility(View.GONE);
n.getNumbersTen().get(i).setVisibility(View.GONE);
}
n.getNumbers().get(0).setVisibility(View.VISIBLE);
EDIT:
If anyone was wondering, I got it to work by extending the class as a Fragment instead of my MainActivity. Then I just used the fragment in my xml.

Because you extended an Activity class doesn't mean setContentView gets called for that class also. It will only do so if properly started and you call super.onCreate(bundle) from your own implementation of onCreate within Numbers
Basically, you should never new any Activity. It has no life-cycle, and therefore no content view, so findViewById just won't work.
Numbers n = new Numbers();
You could not extend anything and have a data-only class around your list of images.
public class Numbers {
private List<ImageView> numbers = new ArrayList<ImageView>();
public Numbers() {}
public void addNumber(ImageView v) { numbers.add(v); }
public List<ImageView> getNumbers() { return numbers; }
}
And from MainActivity you can find and add as you want.
Number n = new Numbers();
n.addNumber((ImageView) findViewById(R.id.one));
However, I don't know if that is useful, really...
Maybe a Fragment would serve a better purpose if you want a "sub-view" of your Activity, but it's hard to tell.

How to access a variable declared in the main activity in another activity

i don't know if this is possible or if they is a better programming practice but i'd like to know how to use a varible declared in the main activity in another activity. Source code snippet example is given like so...
public class MainActivity extends ActionBarActivity {
private int a, b;
private EditText first, second;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
first = (EditText) findViewById(R.id.first_entry);
second = (EditText) findViewById(R.id.second_entry); }
public void doMath(View view) throws Exception {
try {
if (!first.getText().toString().equals("")) {
a = Integer.parseInt(first.getText().toString());
}
else {
a = 0;
}
if (!second.getText().toString().equals("")) {
b = Integer.parseInt(gpa_credits1.getText().toString());
}
else {
b = 0;
}
int sum = a + b; }catch (Exception e) {}
now, i would like to create a new activity to use the variable "sum" to do some math like...
public class first_year_second_semester extends Activity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.newActivity);
int blah = 5;
private TextView soln;
soln = (TextView) findViewById(R.id.the_soln);
soln = (blah / sum) //i.e sum from the previous activity...please i need this
}

You can pass the value from one to other activity using intent or using shared preferences.
Intent intent=new Intent(MainActivity.this,year_second_semester.class);
intent.putExtras("sum",sum);
startActivity(intent);
Then in next activity you can get this value.
Or using shared preferences:
Inside class:
SharedPreferences shared;
Inside onCreate:
shared=getSharedPreferences("app",Context.MODE_PRIVATE);
After computing the sum I.e. after doing sum=a+b;
Editor edit=shared.edit();
edit.putString("sum",sum);
edit.commit();
And in the next activity:
Inside the class:
SharedPreferences shared;
Inside oncreate:
shared=getSharedPreferences("app",Context.MODE_PRIVATE);
Wherver you want:
String yourvalue=shared.getString("sum","default value");
Here your value will be the sum from the main activity.

Create a public static variable in your main activity (where the do math method is) like so:
public static int SUM;
Then in the do math function set that value to the one calculated.
int sum = a+b;
SUM = sum;
Then you can access it via:
MainActivity.SUM

Android ArrayList, stopped unexpectedly

I'm a novice programmer and I'm trying to learn Android coding using Eclipse.
This is my first time using StackOverflow.
Just for tutorial purposes, I want to make a simple Animal Encyclopedia.
So in my Home class, there are some buttons: "Dog", "Cat", "Bird", etc. When I click the button, it will bring me to the same layout but of course with different content.
So I created a class named AnimalData that holds the
ArrayList<Integer> to store R.drawable.xxx and ArrayList<String>
to store the text that I will put below the picture (like "Bulldog"
or "Husky")
Then I created a class named ChangeContent to set all those drawable
and text to the XML
But whenever I click the button, it results in Stopped Unexpectedly Error
Below are the shortened Home class, The "crash-maker" isn't here. I have checked the whole code line per line using Thread.sleep(2000), so if my app crashes before 2 second, the error is before the sleep() code and vice versa.
public class Home extends Activity implements OnClickListener{
Button dog, cat, bird;
AnimalData ad;
ChangeContent cc;
private ArrayList<Integer> drawable;
private ArrayList<String> title;
public Home(){
ad = new AnimalData();
cc = new ChangeContent(ad);
drawable = new ArrayList<Integer>();
title = new ArrayList<String>()
}
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.home);
//set the findViewById for all the buttons
//set onClickListener() to all the buttons
}
public void onClick(View v) {
switch (v.getId()){
case R.id.bDog:
drawable.add(R.drawable.xxx);
drawable.add(R.drawable.yyy);
title.add("Bulldog");
title.add("Husky");
break;
case R.id.Cat:
//same
break;
case R.id.bBird:
//same
break;
}
ad.setDrawable(drawable);
ad.setTitle(title);
Intent i = new Intent("animal.ChangeContent"); //from Manifest
startActivity(i);
}
}
The AnimalData is just a typical getter setter, so I will just skip the code for that
The error is right after ChangeContent started because even when I put the sleep() on the first line of constructor, it doesn't have any effect.
public class ChangeContent extends Activity {
TextView title1, title2;
ImageView pic1, pic2;
private ArrayList<Integer> drawable;
private ArrayList<String> title;
public ChangeContent(AnimalData data){
drawable = new ArrayList<Integer>();
title = new ArrayList<String>();
drawable = data.getDrawable();
title = data.getTitle();
}
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.animal_info);
//findViewById for the TextView and ImageView
//setText() for TextView and setImageResource() for ImageView
}
}
Sorry for the long question, I tried to make it as short as possible
Can you guys help me figure the error out?
Thanks before

You are trying to get arraylist from intent, whereas you are not putting putStringArrayList and putIntegerArrayList methods.
putIntegerArrayListExtra(String name, ArrayList<Integer> value)
putStringArrayListExtra(String name, ArrayList<String> value)
so Change calling activity to following:
Intent i = new Intent(Home.this, ChangeContent.class); //from Manifest
i.putIntegerArrayListExtra("drawables", drawable);
i.putStringArrayListExtra("titles", titles);
startActivity(i);
and get data from intent in onCreate method by following methods:
getIntegerArrayListExtra, and getStringArrayListExtra
you also can do following by making contentChanged method to static, by this you wont need to do much changes in your application code, just do following:
public class ChangeContent extends Activity {
TextView title1, title2;
ImageView pic1, pic2;
private static ArrayList<Integer> drawable;
private static ArrayList<String> title;
public static ChangeContent(AnimalData data){
drawable = new ArrayList<Integer>();
title = new ArrayList<String>();
drawable = data.getDrawable();
title = data.getTitle();
}
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.animal_info);
//findViewById for the TextView and ImageView
//setText() for TextView and setImageResource() for ImageView
}
}

See the answer by RajaReddy P here:
https://stackoverflow.com/a/9937854
In the send class use:
Intent intent = new Intent(PhotoActivity.this,PhotoActivity1.class );
intent.putIntegerArrayListExtra("VALUES", image);
startActivity(cameraIntent);
.. and in the receiver class use:
Intent i = getIntent();
ArrayList<Integer> img = i.getIntegerArrayListExtra("VALUES");

Your approach is wrong -
public Home(){
ad = new AnimalData();
cc = new ChangeContent(ad);
drawable = new ArrayList<Integer>();
title = new ArrayList<String>()
}
ChangeContent is a Activity class so, you don't pass parameter like this.
Passsing class should be serializable and use it -
startActivity(new Intent(this, ChangeContent.class)
.putExtra("key", ad);
And in your ChangeContent class extract the class from Intent
So, change your code design and go ahead.
Best of luck.