Why is my App crashing with a NullPointerException in my ButtonManager class?

Question

I'm making a small Quiz Game and I didn't want to make every function for the Buttons in every Activity, because don't repeat yourself. But when I launch my App it crashes with a Null Pointer Exception.

I tried the Solution at the Bottom and aswell created a function in the ButtonManager class where I had as constructor parameter a String and converted it in the function named above.

This is what my ButtonManager class looks like:

class ButtonManager(buttonName: Int) : AppCompatActivity() {

    val button: Button = findViewById(buttonName)

    fun quitGame(){

        finish()
    }
}

This is what my functions call looks like in my Activity:

ButtonManager(R.id.quitGameButton).button.setOnClickListener {
    ButtonManager(R.id.quitGameButton).quitGame()
}

This is the Error Code:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference

if there is no layout visible then you cant use "findviewbyId" since there are no views yet. — ivan, May 24 '19 at 19:09
you could pass a reference of your button (instead of its name) to your buttonmanager class. — ivan, May 24 '19 at 19:17
It's a unresolved reference, because it wants to know where the button is. — SuperSaiyanJSTN, May 24 '19 at 19:23
its not needed to know where it is, you just need to know when the button was pressed. — ivan, May 24 '19 at 19:26
val button: Button = findViewById(buttonName) I can't do it this way. Maybe I'm retarded but I don't know how you mean it — SuperSaiyanJSTN, May 24 '19 at 19:28
Possible duplicate of [What is a NullPointerException, and how do I fix it?](https://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — Martin Zeitler, May 24 '19 at 20:26

score 0 · Answer 1 · answered May 24 '19 at 20:39

Looks like you are missing some fundamental concepts for android.

I assume you just want to add a clicklistener to your button. In that case you don't need your Button manager class at all.

In your Activity you can simply add:

class MyActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?){

        setContentView(R.layout.your_layout) // xml layout with a Button with android:I'd=" @+I'd/quitGameButton"

        val button: Button = findViewById(R.id.quitGameButton)
        button.onClickListener{
                finish()
        }
    }
}

No i don't want to add a click listener to my Button – SuperSaiyanJSTN May 24 '19 at 20:49 — SuperSaiyanJSTN, May 24 '19 at 20:49

Why is my App crashing with a NullPointerException in my ButtonManager class?

1 Answers1