I am writing an Android app where I need to pass a string array between two classes. The string initializes fine and I can output the contents of the string fine in the one class but as I try to pass it to another class I get a Null Pointer Exception error. The following is the stripped down version of my code:
accelerometer.java:
public class accelerometer extends Service {
public String movement[];
public void onCreate() {
movement = new String[1000000];
}
public void updatearray() {
movement[arraypos]=getCurrentTimeString();
//Toast.makeText(this, movement[arraypos] , Toast.LENGTH_SHORT).show(); //this correctly displays each position in the array every time it updates so I know the array is working correctly in this file
arraypos+=1;
}
public String[] getmovement(){
return movement;
}
}
wakeupalarm.java:
public class wakeupalarm extends Activity {
private TextView herestext_;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.wakeup);
herestext_ = (TextView) findViewById(R.id.TextView01);
accelerometer accelerometercall = new accelerometer();
String movearray[] = accelerometercall.getmovement();
herestext_.setText(movearray[2]);
}
}
I have a feeling I'm missing something very simple but any help would be greatly appreciated!
Thanks,
Scott
You're creating a new accelerometer class, which is completely uninitialized since there is no constructor, then you access its member. Of course it'll be null.
Not sure how your two classes are related, but if the activity is called by the service, then you need to pass the string through the intent (through an extra, for example).
Side note: Class names should always start with a capital letter. Method/variable names should have camel case, i.e. "updateArray". Also, you can format your code here by selecting it and pressing CTRL+K.
Your first problem, I think, is that you are creating an array with a million slots in it. Do you really mean to be doing that? It's going to take a lot of memory---quite possibly more than is available. You should instead look to having a Vector of Strings that you extend as necessary.
Related
I am new to android studio but I am getting better at it as I program more and more. I have a MainActivity.java and the .xml file. And a friend provided me some code that it suppose to work with the input areas. The problem is I do not know how to access that regular java file. So that I can use it the way it is intended. He was using eclipse to build everything while I use android studio. I have the buttons all good to go and areas of input good to go but I just dont know how to implement his code. Any guidance will be greatly appreciated.
See examples to understand what I am trying to do.
"In android studio" a class is created called WaterDetails.java with a .xml file called activity_water_details.xml. There are calculations that were made for the duration that I need to be able to use or access from a java file created in eclipse called DurationCalculations.java. I have tried importing. I have tried opening the folder in explorer and putting the class in the same project. But, nothing seems to work.
Code:
public class WaterDetails extends AppCompatActivity {
Button continueWaterDetailsPart2;
EditText duration;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_water_details);
duration = (EditText)findViewById(R.id.enter_duration);
duration.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
String user = duration.getText().toString();
if(duration.equals(" "))// if user inputs information
//Then get calculations from other java file.
}
});
Sample Code:
Second Java fie. The file I need to access.
package ScubanauiTables;
import java.util.Arrays;
public class DurationCalculations {
private int duration;
//Constructor
DurationCalculations(int duration, int maxDepth, int avgDepth, int temp, int visibility, int pressureStart,
int pressureEnd, String[] diveConditions, String[] diveActivities) {
setDuration(duration);
setMaxDepth(maxDepth);
setAvgDepth(avgDepth);
setTemp(temp);
setVisibility(visibility);
setPressureStart(pressureStart);
setPressureEnd(pressureEnd);
setAirType(21);
setDiveConditions(diveConditions);
setDiveActivities(diveActivities);
setPressureGroup();
public int getDuration() {
int temp = duration;
return temp;
}
private void setDuration(int duration) {
this.duration = duration;
}
I hope this sample code makes sense. Thank you all for your help in advance.
You want to use methods of your DurationCalculation class, and for that, you've to create an instance of that class.
You can instantiate and use your class like this
DurationCalculations durationCalculation = new DurationCalculations(
/*enter your constructor values*/);
Now you can call all public methods of your DurationCalculations class using durationCalculation variable like this
durationCalculation.getDuration();
You cannot call any private methods from outside of the class, like your setDuration() whose scope is set to private. For it be accessed outside of DurationCalculations class. You need to set it to public
Yes I'm really new to android and I'm wokring on a very simple app.
On my mainActivity I'm creating and array, and want to access the array from a different activity.
public class Activity extends {
MyAreas[] myArea;
#Override
protected void onCreate(Bundle savedInstanceState) {
myArea= new MyAreas[2];
myArea[0] = new MyAreas(33, 44, "Location ", "active");
myArea[1] = new MyAreas(32, 434, "Location 2", "active");
Class
public class MyAreas{
public double val;
public double val2;
public String name;
public String status;
public MyAreas(double val, double val2, String name, String status) {
this.val= val;
this.val2= val2;
this.name = name;
this.status = status;
}
I'm trying to access myarea array from my activity2.java, I tried this but didn't work.
private ArrayList<MyAreas> mMyAreasList;
Using Parcelable to pass data between Activity
Here is answer which should help.
In regular Java you can use getters to obtain objects or any variable from a different class. Here is a good article on encapsulation.
In Android, there is a class called Intent that lets you start one activity from another and pass any necessary information to it. Take a look at the developer docs and this other answer which should help you.
For your begginer level, rather than using intents, just set the array object public and static, like that:
public static MyAreas[] myArea;
By that way you can access it from any activity in your app..
Then, go to the activity2.java wherever you want to access it.
MyAreas area = Activity.myArea[0];
The problem with this approach is that you do not have complete control when and in what order the activities are created or destroyed. Activities are sometimes destroyed and automatically restarted in response to some events. So it may happen that the second activity is started before the first activity, and the data is not initialized. For this reason it is not a good idea to use static variables initialized by another activity.
The best approach is to pass the data via the intent. The intents are preserved across activity restarts, so the data will be preserved as well.
Another approach is to have a static field to keep the data, and initialize the data in an Application instance.
I need some pointers on doing the following:
lets say i have 10/20 (number doesn't matter) of activities.
each of these activities has a textview that should work like a counter.
each of these activities has a button to go to the next activity.
this counter starts when the app is launched, and increment itself every second.
So what i did so far is:
have in my main activity a method that instantiate a class that extends Thread.
In that class in the run() method, i increment a variable when a second passes.
Now i'm stuck on what i should do next. Any pointers would be appreciated thanks.
Edit: i need a way to communicate from inside the run method, to whichever activity is now currently on screen, to update its textview.
Just a bit of theory here for standard Object Oriented Programming : stick to the recommended principles like Loose Coupling which makes your project code less tied to each other. You can read more on that later.
Now, using Events, you can setup a system that is synonymous with the natural Publisher/Subscriber design pattern. Like this:
The activity that needs to notify the other activities is called Publisher and the other activities that need to be notified are called Subscribers.
From here:
There are already built and tested libraries to do Events in android. Like my favorite EventBus.
Step 1 Add this line to your app-level build.gradle file:
compile 'org.greenrobot:eventbus:3.0.0'
Then create a simple Plain Old Java Object aka POJO class like this:
public class UpdateTextViewEvent{
private String textToShow;
public UpdateTextViewEvent(String text){
this.textToShow = text;
}
//add your public getters and setters here
}
Step 2 Notify others:
When you want to notify anyone of the changes, you simply called this method:
EventBus.getDefault().post(new UpdateTextViewEvent("Some new Text"));
Step 3 Receive notifications
For those who want to be notified of this event, simply do this:
#Override
public void onStart() {
super.onStart();
EventBus.getDefault().register(this);
}
#Override
public void onStop() {
super.onStop();
EventBus.getDefault().unregister(this);
}
NOTE: to actually handle the event:
#Subscribe
public void onEvent(UpdateTextViewEvent event){
String text = event.getTextToShow();
//now you can show by setting accordingly on the TextView;
}
This is so much easier to do, do decouple your code by eliminating static references in your different activities
I hope this helps! Good luck!
make that Textview in second class as
public static Textview text;
and call it in main activity as
SecondActivity obj=new SecondActivity();
obj.text.settext("");
You can create one another activity e.g. BaseActivity extend with Activity class and your all 10/20 activity extends with created BaseActivity Class.
You can use your textview with protected access specifiers.
What you need to do is inside the counter class, create an a method and passed in a TextView as the parameter. Then create an int variable and set the counter as the instance:
Like this
public static class Counter extends Thread{
private static int x;
#Override
public void run(){
x = counter;
}
public void setCounter(TextView tv){
tv.setText(String.valueOf(x));
}
}
Now call this method setCounter(TextView) in all the activity's onCreate() method you'll like to display the counter, and passed in your the layout TextView as the argument. Like this
...
#Override
public void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState):
....
TextView cTextView = (TextView)findViewById(R.id.texT1);
Counter c = new Counter();
c.setCounter(cTextView);
}
I've got an Android app with custom objects which implement the Parcelable interface. They way I have it set it up is that my program initially creates an ArrayList of a custom class Products from a file in the bundle. I can see and confirm that the arraylist and it's instance variabels are populated appropriately. This class has several instance variables along with one being another ArrayList but with the String class. Remember that fact.
I am trying to pass the ArrayList<Product> into a new activity like so:
try {
Intent i = new Intent(RootActivity.this, ProductsActivity.class); //Intent from this activity to the next
i.putParcelableArrayListExtra("Products", app_products); //Puts my ArrayList<Class A> as an extra
startActivity(i); //Launch the activity
}
catch(Exception e){
Log.d("Activity Error", "Error Here:" + e.getMessage());
}
I am collecting the information back from the intent in my new activity by pulling the ArrayList out by using
app_products = getIntent().getParcelableArrayListExtra("Products");
For my custom class, it looks something like this, along with the implemented Parcelable methods.
public class Product implements Parcelable{
private String name;
private String cost;
private ArrayList<String> similarItems;
public Product{
name = null;
cost = null;
similarItems = new ArrayList<String>();
}
public Product(String name, String cost){
this();
this.name = name;
this.cost = cost;
}
public addSimilarItem(String item){
similarItems.add(item);
}
public static final Parcelable.Creator<Product> CREATOR
= new Parcelable.Creator<Product>()
{
public Product createFromParcel(Parcel in) {
return new Product(in);
}
public Product[] newArray(int size) {
return new Product[size];
}
};
public int describeContents(){
return 0;
}
private Product(Parcel in){
name = in.readString();
cost = in.readString();
similarItems = in.readArrayList(String.class.getClassLoader());
}
public void writeToParcel(Parcel out, int flags){
out.writeString(name);
out.writeString(cost);
out.writeList(similarItems);
}
}
So this works well WITHOUT my String arraylist being added in the class
Comment out out.writeList(similarItems); and also similarItems = in.readArrayList(String.class.getClassLoader());
but once you add them back in into the class, the app crashes but it doesn't even throw a message for debugging. I've wrapped everything around try-catch statements and android doesn't even report the app crashed with the normal dialog on the springboard. I am truly at a loss.
It is worth mentioning that I've used some log statements to understand where the program is crashing despite the fact that android wont throw an exception. I can see that all of the items in my ArrayList undergoes the writeToParcelMethod and completes writing. The Product(Parcel in) method is never called. Lastly, I can also see the class I am launching the new activity from enters the Pause State and my new Activity is never created.
Let me know if I can provide any additional information.
Fairly certain your problem is the use of writeList(). writeList() seems to indicate that it follows the same contract as writeValue() for the items contained in the list. readList() however, seems to indicate that the values must be Parcelable (which String is not).
Either way, typically these calls have to be very specifically linked to their inverse (e.g. writeString() must be read in with readString(), not readValue()) so you should instead use the provided methods for reading/writing String Lists:
// Takes in a List<String> which may be null
out.writeStringList(similarItems);
// Returns either null or a new ArrayList<String> with the contents
similarItems = in.createStringArrayList();
These seemed to be due to some malformed XML which my app uses as a resource. Not sure why this was this issue but after many hours of hunting it down, I was able to remove the bad XML and will revisit this issue at a later date towards when I need to release the app.
Right now, I'm just gonna worry about continuing to develop it. I'll try to remember to check back here if I find anything interesting about my XML.
i am on the creation of an app in android. its a calculator app. the main activity is where the user could input the equation, and the second activity is where the user can add/edit/delete variables. so i made a new class in another file named Global.java. then i extended it to application, imported everything i need, made s private string, made some public functions, edited the manifest, and initialized it right on my main activity. everything works fine while im only using a string to be passed by the functions but when i started adding what i need, an ArrayList, and made some functions so i could access the list then run it, the app closes. i think its because the arraylist is not allowed to be passed to different classes? am i right or am i just missing something?
please dont downvote my post if i didn't post something needed. i am using aide so there is no log output. code:
Global.java
...
import android.app.*;
import java.util.*;
public class Global extends Application
{
private String s;
public static ArrayList<String> sList;
public String getS() {
return s;
}
public void setS(String ss) {
s=ss;
}
public void add() {
sList.add(s);
}
}
MainActivity.java
...
String s;
...
global=(Global)getApplicationContext();
...
global.setS("jian"); //this one works
global.sList.add("jian"); // this one dont
...
Are you sure you initialized sList, like this:
sList = new ArrayList<String>();
If you didn't, you might want to change its declaration to include this initialization.
public static ArrayList<String> sList = new ArrayList<String>();
Just do
global.add("jian");
since you have an add function to take care of the addition of item to arraylist.
Also, try with this:
public void add(String ss) {
sList.add(ss);
}
You are not instantiating your arraylist.
public static ArrayList<String> sList = new Arraylist<String>();
Also you should read beginner tutorials on Java and android, using a public extension of application like this is a bad idea and you can get log outputs from different apps if Aide doesn't provide that, search play store