How to Fix Attempt to Invoke Interface Method on a Null Object Reference

Attempt to invoke interface method '____' on a null object reference

How are you creating this Adapter? You have two different constructor methods for the Adapter, one contains a number of arguments and the other just the listener.

You should include the listener as another argument in the first constructor if you need to have access to it for all uses of this Adapter and delete the second constructor, otherwise using either of the constructors will leave the Adapter in an incomplete state.

If not all uses of the Adapter will need to include the listener, then create a separate method instead of a separate constructor to assign the listener to the Adapter class.

public setAdapter(GroupMethods listener){
    this.listener = listener;
}

Also, your RecyclerView is not extending from RecyclerView.Adapter as it probably should, check the examples available in the RecyclerView documentation

Attempt to invoke interface method on a null object reference and I don't know how to solve it

I solved my problem by replacing getTargetFragment() with getParentFragment() in these lines

@Override
    public void onAttach(@NonNull Context context) {
        super.onAttach(context);
        mListener = (CreatePelangganListener) getParentFragment();
    }

Attempt to invoke interface method on a null object reference while implement interface

Make sure to initialize the listener when AppCompatDialogFragment attach.

@Override
public void onAttach(Context context) {
    super.onAttach(context);
    listner = (ResetDialogListener) context
}

How to fix Attempt to invoke interface method on a null object reference

I fixed my problem by adding :

         ConnectFragment connectFragment = new ConnectFragment();

    FragmentManager fragmentManager = getSupportFragmentManager();
    fragmentManager.beginTransaction().replace(R.id.content_frame, connectFragment).commit();
    setActivityListener(connectFragment); // her i called the setter and i passed the object of ConnectFragment as parameter.

Android:NullPointerException: Attempt to invoke interface method on a null object reference

You are trying to access item before it is inflated, instance will be available after inflation

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    SharedPreferences sharedPreferences = PreferenceManager.getDefaultSharedPreferences(getApplicationContext());
    boolean do_inc_summary = sharedPreferences.getBoolean("summary", false);
    //inflate first
    getMenuInflater().inflate(R.menu.menu_main, menu);
    //then access
    if (do_inc_summary) {
        menu.findItem(R.id.action_most_recent_summary).setVisible(true);
    }
    return true;
}

java.lang.NullPointerException: Attempt to invoke interface method 'void onclicklistner.onItemClick(int, android.view.View) on a null object reference

You are using the listener before initialization. This is why you are getting nullpointerexception.

public class support_fragment extends Fragment {

FloatingActionButton create_support;
ticketsListAdapter.onclicklistner listner;
RecyclerView recyclerView;
ticketsListAdapter ticketsListAdapter;


@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_support, container, false);

    create_support = view.findViewById(R.id.create_support);
    recyclerView = view.findViewById(R.id.support_recycler);
    recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));
    recyclerView.setHasFixedSize(false);
    
    //initialize the listner before using!
    listner = new ticketsListAdapter.onclicklistner() {
        @Override
        public void onItemClick(int position, View v) {
            Toast.makeText(getContext(), "Test", Toast.LENGTH_SHORT).show();
        }
    };

    
    ticketsListAdapter = new ticketsListAdapter(getContext(), listner);
    recyclerView.setAdapter(ticketsListAdapter);

    create_support.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(getContext(), Support_main_activity.class);
            startActivity(i);
        }
    });
    /*
    listner = new ticketsListAdapter.onclicklistner() {
        @Override
        public void onItemClick(int position, View v) {
            Toast.makeText(getContext(), "Test", Toast.LENGTH_SHORT).show();
        }
    };
    */
    return view;
 }
}

What is a NullPointerException, and how do I fix it?

There are two overarching types of variables in Java:

1. Primitives: variables that contain data. If you want to manipulate the data in a primitive variable you can manipulate that variable directly. By convention primitive types start with a lowercase letter. For example variables of type int or char are primitives.

2. References: variables that contain the memory address of an Object i.e. variables that refer to an Object. If you want to manipulate the Object that a reference variable refers to you must dereference it. Dereferencing usually entails using . to access a method or field, or using [ to index an array. By convention reference types are usually denoted with a type that starts in uppercase. For example variables of type Object are references.

Consider the following code where you declare a variable of primitive type int and don't initialize it:

int x;
int y = x + x;

These two lines will crash the program because no value is specified for x and we are trying to use x's value to specify y. All primitives have to be initialized to a usable value before they are manipulated.

Now here is where things get interesting. Reference variables can be set to null which means "I am referencing nothing". You can get a null value in a reference variable if you explicitly set it that way, or a reference variable is uninitialized and the compiler does not catch it (Java will automatically set the variable to null).

If a reference variable is set to null either explicitly by you or through Java automatically, and you attempt to dereference it you get a NullPointerException.

The NullPointerException (NPE) typically occurs when you declare a variable but did not create an object and assign it to the variable before trying to use the contents of the variable. So you have a reference to something that does not actually exist.

Take the following code:

Integer num;
num = new Integer(10);

The first line declares a variable named num, but it does not actually contain a reference value yet. Since you have not yet said what to point to, Java sets it to null.

In the second line, the new keyword is used to instantiate (or create) an object of type Integer, and the reference variable num is assigned to that Integer object.

If you attempt to dereference num before creating the object you get a NullPointerException. In the most trivial cases, the compiler will catch the problem and let you know that "num may not have been initialized," but sometimes you may write code that does not directly create the object.

For instance, you may have a method as follows:

public void doSomething(SomeObject obj) {
   // Do something to obj, assumes obj is not null
   obj.myMethod();
}

In which case, you are not creating the object obj, but rather assuming that it was created before the doSomething() method was called. Note, it is possible to call the method like this:

doSomething(null);

In which case, obj is null, and the statement obj.myMethod() will throw a NullPointerException.

If the method is intended to do something to the passed-in object as the above method does, it is appropriate to throw the NullPointerException because it's a programmer error and the programmer will need that information for debugging purposes.

In addition to NullPointerExceptions thrown as a result of the method's logic, you can also check the method arguments for null values and throw NPEs explicitly by adding something like the following near the beginning of a method:

// Throws an NPE with a custom error message if obj is null
Objects.requireNonNull(obj, "obj must not be null");

Note that it's helpful to say in your error message clearly which object cannot be null. The advantage of validating this is that 1) you can return your own clearer error messages and 2) for the rest of the method you know that unless obj is reassigned, it is not null and can be dereferenced safely.

Alternatively, there may be cases where the purpose of the method is not solely to operate on the passed in object, and therefore a null parameter may be acceptable. In this case, you would need to check for a null parameter and behave differently. You should also explain this in the documentation. For example, doSomething() could be written as:

/**
  * @param obj An optional foo for ____. May be null, in which case
  *  the result will be ____.
  */
public void doSomething(SomeObject obj) {
    if(obj == null) {
       // Do something
    } else {
       // Do something else
    }
}

Finally, How to pinpoint the exception & cause using Stack Trace

What methods/tools can be used to determine the cause so that you stop
the exception from causing the program to terminate prematurely?

Sonar with find bugs can detect NPE.
Can sonar catch null pointer exceptions caused by JVM Dynamically

Now Java 14 has added a new language feature to show the root cause of NullPointerException. This language feature has been part of SAP commercial JVM since 2006.

In Java 14, the following is a sample NullPointerException Exception message:

in thread "main" java.lang.NullPointerException: Cannot invoke "java.util.List.size()" because "list" is null

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