CODEWITHSUNDEEP
androidkotlin
Gordon Oppenheimer
Gordon Oppenheimer

Reputation: 143

null object reference error on Synthetic Binding from within Class

I'm a beginner with Android Studio and Kotlin. I get the following error when trying to change the text of a TextView using Synthetic Binding from within my class. 

Unable to start activity ComponentInfo{com.oppenheimer.myapplication/com.oppenheimer.myapplication.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference

The code up to "Some other text" works but when I try with "Very new phrase" from within the class it crashes. It seems to be a common error but I'm not finding other posts that match my issue.

Here is my code

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/MyText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

MainActivity.kt

package com.oppenheimer.myapplication

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        MyText.text = "Some other text"
        val myClass = MyClass()
        myClass.MyFunction()

    }
}

MyClass.kt

package com.oppenheimer.myapplication

import androidx.appcompat.app.AppCompatActivity
import kotlinx.android.synthetic.main.activity_main.*

class MyClass : AppCompatActivity() {

    fun MyFunction () {

       MyText.text = "Very new phrase"
    }
}

Full Error

2020-05-02 13:13:50.784 3632-3632/com.oppenheimer.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.oppenheimer.myapplication, PID: 3632
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.oppenheimer.myapplication/com.oppenheimer.myapplication.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
        at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:163)
        at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:174)
        at android.content.Context.obtainStyledAttributes(Context.java:738)
        at androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor(AppCompatDelegateImpl.java:692)
        at androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor(AppCompatDelegateImpl.java:659)
        at androidx.appcompat.app.AppCompatDelegateImpl.findViewById(AppCompatDelegateImpl.java:479)
        at androidx.appcompat.app.AppCompatActivity.findViewById(AppCompatActivity.java:214)
        at com.oppenheimer.myapplication.MyClass._$_findCachedViewById(Unknown Source:25)
        at com.oppenheimer.myapplication.MyClass.MyFunction(MyClass.kt:10)
        at com.oppenheimer.myapplication.MainActivity.onCreate(MainActivity.kt:18)
        at android.app.Activity.performCreate(Activity.java:7802)
        at android.app.Activity.performCreate(Activity.java:7791)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
        at android.os.Handler.dispatchMessage(Handler.java:107) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7356) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930) 

How do I change the layout from within a class? What is the "right" way to do this? Should I Pass the TextView into the class?

Thanks

Upvotes: 1

Views: 774

Answers (2)

Ajeeli
Ajeeli

Reputation: 373

If you really need to, you could access MyText and alter its value by passing a reference of it to MyClass and to MyFunction() as a parameter.

MainActivity.kt

package com.oppenheimer.myapplication

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.TextView
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        MyText.text = "Some other text"
        val textView = findViewById<TextView>(R.id.MyText)
        val myClass = MyClass(textView)
        myClass.MyFunction(textView)
    }
}

MyClass.kt

package com.oppenheimer.myapplication

import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity

class MyClass(textView: TextView) : AppCompatActivity() {

    fun MyFunction (textView: TextView) {
        textView.text = "Very new phrase"
    }
}

Please note also that following naming conventions is good software practice and therefore all function names should start with a small letter, myFunction().

Upvotes: 1

Ajeeli
Ajeeli

Reputation: 373

Generally speaking, you should not be trying to access a layout item from within another class, which is why you get an error.

Move it to the MainActivty's onCreate() function:

MainActivity.kt

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        MyText.text = "Some other text"
        val myClass = MyClass()
        myClass.MyFunction()
        MyText.text = "Very new phrase"

    }
}

MyClass.kt

class MyClass : AppCompatActivity() {

    fun MyFunction () {
        }
}

Upvotes: 1

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