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Is AppCompatActivity object required for calling setSupportActionBar() method?

I went through a code where setSupportActionBar(toolbar) method was used without object of AppCompatActivity class. But when I went on implementing some code, I found it rejecting the way. It is important to use it as the toolbar I am passing as parameter to the above mentioned function is of type androidx.appcompat.widget.toolbar.
After implementing it, there is no compile time error, but facing error when starting the emulator:
"Caused by: java.lang.NullPointerException: Attempt to invoke virtual
method 'void
androidx.appcompat.app.AppCompatActivity.setSupportActionBar(androidx.appcompat.widget.Toolbar)'
on a null object reference'.

NullPointerException: 'void android.widget.Button.setText(java.lang.CharSequence)' on a null object reference [duplicate]

This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 1 year ago.
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setText(java.lang.CharSequence)' on a null object reference
at com.example.mentalhealth.diary.onCreate(diary.java:49)

In your diary.java, #line no:49, in the onCreate() method, it appears that you are using an object which is not instantiated.
Calling a method on that object (which is just declared , but not instantiated using new classname() syntax).
if your file is shared , at least the 49th line of your sourcecode, it would be easy to correctly mention .

method argument is not accessible by name inside method

Is it a bug in android studio or java, don't know. I was working on an astronomy application. I have created a method to get phase of moon. It takes two argument but only one argument is accessible by name. In debugger it says argument not found but when I assign to some other variable, it gets assigned. No runtime error in execution.
Argument that is not accessible is moon_degrees but when I assign it's value to variable moon_degree it works.
Don't know what's happening
Android studio Version 2.0
windows 7 X86

This may be a known bug:
https://issuetracker.google.com/issues/37019591
You could provide your case to the bug report to help to find the cause of this.

Your method argument is accessing inside method, but if you want to see inside method variable value in Debugger, please declare this inside variable Global.

JNI function error in android

I'm in trouble with this: http://imgur.com/hH0q3Tn
I'm using the VTK sample code and JNI isn't working and I don't know what to do. I compiled all the libraries and when I try the app on phone, the app stop.
What can I do?
Edit: Errors: http://pastebin.com/rf7vFVT8

The second arg for static native method should be jclass, not jobject.
See Are native Java methods equivalent to static Java methods? for more details.
Native Method Arguments
The JNI interface pointer is the first argument to native methods. The JNI interface pointer is of type JNIEnv. The second argument differs depending on whether the native method is static or nonstatic. The second argument to a nonstatic native method is a reference to the object. The second argument to a static native method is a reference to its Java class.

I solved.
The building of the library was incorrect and I followed this guide http://www.vtk.org/Wiki/VTK/Building/Linux

How Console reference variable refers to some other class method

I was strucked in a place that i cannot create new instance for Console class. So i took the source code of jdk and then look into it. Then i got cleared that it was declared as "public final class Console........"... So i understood that the final class cannot be instantiated. But that is not my actual doubt. Here in the SCJP Book, i came across a line of code that tells me as Console c = System.console()
Here we cannot create new instance for console class, so creating a reference variable c. Then What is that System.console actually is?? In the book it is told as
Keep in mind that it's possible for your Java program to be running in an environment that doesn't have access to a console object, so be sure that your invocation of System.console() actually returns a valid console reference and not null.
So then i entered to look up source code for System.console().
There i happen to see System as final class and console() as static method inside that.
So how can a Console reference object refer to that console method in system class..
What is the link between these two. I thought of a polymorphic reference.
But that is not because it doesn't pass IS-A Test. So please explain me in detail about this.
Hope my question is clear. Thanks in advance. !

the final class cannot be instantiated
That is wrong , final class cannot be subclassed. It can be instantiated using the new operator if its constructor is visible . I guess Console class has private constructor. Abstract classes cannot be instantited. See the JLS 8.1.1.2:
A class can be declared final if its definition is complete and no subclasses are desired or required.
It is a compile-time error if the name of a final class appears in the extends clause (§8.1.4) of another class declaration; this implies that a final class cannot have any subclasses.
System.console()
Returns the unique Console object associated with the current Java virtual machine, if any.
Whether a virtual machine has a console is dependent upon the underlying platform and also upon the manner in which the virtual machine is invoked. If the virtual machine is started from an interactive command line without redirecting the standard input and output streams then its console will exist and will typically be connected to the keyboard and display from which the virtual machine was launched. If the virtual machine is started automatically, for example by a background job scheduler, then it will typically not have a console.
If this virtual machine has a console then it is represented by a unique instance of this class which can be obtained by invoking the System.console() method.
If no console device is available then an invocation of that method will return null.
console() is some sort of a factory method which gives you back an object of Console associated with the current JVM . The System class has knowledge of the JVM in which it is running and it is the perfect candidate to give you back the Console object.
Console c = System.console();
System.console() gives us back a reference to the Console object and you are assigning the object reference to variable c. This way you can use that reference variable c to access the properties or methods of Console object.

I believe the mistake in your understanding is here:
So i understood that the final class cannot be instantiated
Final classes cannot be extended but can be instantiated. FYI it is the abstract class than cannot be instantiated.