This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 1 year ago.
I have the following set-up
public class MainActivity extends AppCompatActivity implements LocationListener {
GeoPoint pointTarget;
#Override
public void onCreate(#Nullable Bundle savedInstanceState) {
pointTarget.setLatitude(51.4245);
pointTarget.setLongitude(3.1455);
}
}
This throws Attempt to invoke virtual method 'void org.osmdroid.util.GeoPoint.setLatitude(double)' on a null object reference at pointTarget.setLatitude(51.4245);
The two arguments are doubles, so that can't be the problem. pointTarget has also been instantiated. Can anyone please tell me what is wrong?
Thank you for referring to "What is a NullPointerException, and how do I fix it?". I have now instantiated as private GeoPoint pointTarget = new GeoPoint(0.0,0.0);, which solved the problem. Perhaps I should switch to Kotlin, which apparently has no NullPointerExceptions.
Related
when I create an interface object implementing the abstract methods (through anonymous classes). What type of object am I creating?
with an example the question will be better understood
public class Class1{
public static void main(String...args){
Interface1 object1= new Interface1(){
#Override
public method1(){
System.out.println("im the method");
}
}
System.out.println(Object1.getClass());
}
this returns the class Class1. how can you explain this?
an explanation of my doubt
I'm working on a game and I'm trying to add a JPanel to the JFrame. I have a separate class which extends JPanel and I create a new object which again, extends JPanel.
However, I get this error: "The method add(Component) in the type Container is not applicable for the arguments (mainMenu)" (mainMenu is the class which extends JPanel).
It suggests adding an argument to match 'add(Component, Object)'
I am at a loss in trying to fix this especially because later in my code, I use the same add argument to add an object whose class extends a JButton. Any help is greatly appreciated.
I have tried creating a regular JPanel and adding that which worked.
(JFrame class)
public class gardenRunner extends JFrame {
public static void run(String[] args)
{
new gardenRunner()
}
public void gardenRunner()
{
mainMenu m = new mainMenu();
add(m); <-- gives error
}
(JPanel class)
public class mainMenu extends JPanel {
public mainMenu()
{
super();
setSize(60, 60);
}
I would expect this to add a JPanel to my JFrame but this results in an error.
Thanks for the help, turns out Eclipse was acting a little buggy and this worked when I deleted my JPanel class and made a new one. Thanks again.
Consider the following code:
// ======== Abstract class ========
public abstract class Creatures {
public abstract void loseEnergy();
public void execute()
{
loseEnergy();
}
}
// ======== Animals ========
public class Animals : Creatures
{
public override void loseEnergy(){}
}
public class Birds : Animals
{
public override void loseEnergy(){}
}
// ======== Human ========
public class Human : Creatures
{
public override void loseEnergy(){}
}
public class Male : Human
{
public override void loseEnergy(){}
}
public class Female : Human
{
public override void loseEnergy(){}
}
[ This code was based on the code by Jayson suggested here: "Base class methods calling derived class methods ?" ]
In the given code example, I would like to have the runtime executing EACH derived class object's certain method, in this case, which is 'loseEnergy()', however, I could not find the solution.
How do I approach this problem?
What can be useful to know or to try.. in order to solve this issue?
Your help is very much appreciated!
Thank you!
Kind regards,
Segara
P.S. Some search I have done so far:
"How to call overriden methods in all derived classes"
"Collection of derived classes that have generic base class"
"How to call derived function using base class object"
"Call method of the derived class through reflection possible or no"
EDIT:
I decided to stick to the idea I had before which is to have some list that would contain the objects of the classes that have 'loseEnergy()' method. Having such list I will be able to call every object's method 'loseEnergy()', which is what I wanted.
Question can be closed.
Thank you.
I didn't really understand your problem but anyway i can try to give you some means to use abstract classes :
If you use a abstract method, you SHOULD override it in a subclasses (like a method declared in an interface)
If you want that all inherited class use a same method, you can implement it in the abstract class ; all subclasses will use the method you implements if you don't override it, you've have to not declare it in the subclasses (extends < ABS_CLASS > is good enough)
If you want use a method of the abstract class which is override in the sub class you can use the keyword super .
I hope it will help you.
if you mean that you want the calls: female.loseEnergy() -> human.loseEnergy() -> creature.loseEnergy(), call the base method in the first line of the overriden one
public class Female : Human
{
public override void loseEnergy()
{
base.loseEnergy();
// do stuff
}
}
In the Greenfoot environment that you mention in the post above, the act() method is called only on actors which have been added into the "world". Internally, this adds them into a list. The simulation process iterates through the list and calls act() on each object in turn. Objects that are not "in the world" are not known to the system and so do not have their act method called. There is no magic here going on here.
If you wanted similar behaviour but without manually adding objects into a list, you could possibly have the base class constructor add new objects into a global list. I don't know C# so I don't know precisely how to do this, but I cannot imagine it would be difficult.
Right I created a new thread from a static function from the same class.
Inside the same class I try to call a delegate to update the GUI.
I get a compiler error saying:
Invalid delegate initializer - an object is needed in addition to a function.
At &MainUi::AddListItemMethod.
delegate void AddListItem(void);
public: void AddListItemMethod(String^ myString)
{
ListView1->Items->Add(myString);
}
private: static void SecondThread()
{
AddListItem^ del = gcnew AddListItem(&MainUI::AddListItemMethod);
del->Invoke("test");
}
I don't know why it doesn't work. I also tried this and still failed. Any help please?
Invoke(gcnew AddListItem(MainUI::&AddListItemMethod), "test");
Either You have to make Listview1 static to work or you should create an instance/object of MainUI class to access a non static method of that class.
Thank you and Happy coding.
Hi I'm completely new to android programming and use AIDE via tablet.
I'm trying to create a very basic program with a Spinner box that gives output on the selection Ive made via an TextView or System.Out.printIn. (Perhaps the next step up from Hello world - if you will)
For some reason that I cannot fathom,the compiler refuses to recognise the OnClickListener and gives the error message 'Unknown method OnClickListener in Android.Widget.Spinner'
When I have already checked this in the imports.
As a matter of interest I have changed the name of the Spinner and the error seems to dissapear, the problem then is the Spinner name. I have tried several variations on this, and have came to the conclusion that the best option for me is to create a variable just after Main Acivity, and before the layout is declared.
I have also disabled one of the overrides in order to resolve my problem
has anyone got an idea what the problem could be?
package com.BGilbert.AUC;
import android.app.*;
import android.os.*;
import android.widget.*;
import android.view.View.OnClickListener;
import android.widget.Spinner.*;
import android.view.*;
public class MainActivity extends Activity {;
String Fbstring;
OnClickListener Myonclick;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
this.setContentView(R.layout.main);
final Spinner Fbspinner=(Spinner)findViewById(R.id.Spinner);
// The problem is with this line. OnClickListener just wont be
// recognised
Fbspinner.OnClickListener(Myonclick);
}
// Override previously disabled
#Override
public void Onselect(AdapterView<?> parent,View V, int pos, long id) {
Fbstring = parent.getItemAtPosition(pos).toString();
System.out.println(Fbstring);
}
#Override
public void onNothingSelected(AdapterView<?> arg0) {
}
}
You can't set an onClickListener on a spinner, only on it's views which is too advanced for you at the moment. Instead, use an onItemSelectedListener.
public class MainActivity extends Activity extends Activity implements OnItemSelectedListener {
public void onItemSelected(AdapterView<?> parent, View view,
int pos, long id) {
...
...
}
You should read the documentation first http://developer.android.com/guide/topics/ui/controls/spinner.html
Also try to use standard naming conventions:
http://www.oracle.com/technetwork/java/codeconv-138413.html
http://www.oracle.com/technetwork/java/javase/documentation/codeconventions-135099.html#367
Finally, you have many problems in this code, e.g.
public class MainActivity extends Activity {;
Note the semicolon at the end.
Get your code compiling first, then come back with your next question.
Good luck