Changing text value with Kotlin in Android Studio-CodePudding

New to Kotlin, I've followed the guide on how to set up a basic "Press the button and it changes text" However Whenever I press the button, the app crashes and in the debugger I get

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.myapplication, PID: 14051 java.lang.IllegalStateException: Could not execute method for android:onClick at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:414) at android.view.View.performClick(View.java:7448) at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1119) at android.view.View.performClickInternal(View.java:7425) at android.view.View.access$3600(View.java:810) at android.view.View$PerformClick.run(View.java:28305) at android.os.Handler.handleCallback(Handler.java:938) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:223) at android.app.ActivityThread.main(ActivityThread.java:7656) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) Caused by: java.lang.reflect.InvocationTargetException at java.lang.reflect.Method.invoke(Native Method) at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:409) at android.view.View.performClick(View.java:7448) at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1119) at android.view.View.performClickInternal(View.java:7425) at android.view.View.access$3600(View.java:810) at android.view.View$PerformClick.run(View.java:28305) at android.os.Handler.handleCallback(Handler.java:938) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:223) at android.app.ActivityThread.main(ActivityThread.java:7656) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference at com.example.myapplication.MainActivity.One(MainActivity.kt:16) at java.lang.reflect.Method.invoke(Native Method) at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:409) at android.view.View.performClick(View.java:7448) at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1119) at android.view.View.performClickInternal(View.java:7425) at android.view.View.access$3600(View.java:810) at android.view.View$PerformClick.run(View.java:28305) at android.os.Handler.handleCallback(Handler.java:938) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:223) at android.app.ActivityThread.main(ActivityThread.java:7656) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

Here is my current MainActivity.KT:

  package com.example.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)
    }

    fun One(view: android.view.View) {
       val onetext = view.findViewById<TextView>(R.id.textView)
        onetext.text = "Hello"





    }
}

And my 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/textView"
        android:layout_width="168dp"
        android:layout_height="90dp"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@ id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="One"
        android:text="Button"
        tools:layout_editor_absoluteX="163dp"
        tools:layout_editor_absoluteY="462dp" />

</androidx.constraintlayout.widget.ConstraintLayout>

Thank you!

CodePudding user response：

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference at com.example.myapplication.MainActivity.One(MainActivity.kt:16)

This is the problem. onetext is null, which means view.findViewById<TextView>(R.id.textView) didn't find a view with textView as id inside of view.

The view passed in the function is the button, which has no textView as a child. You need to search for the textView in a view higher in the hierarchy.

In the specific example, one way of doing that is by omitting the view receiver, like so:

package com.example.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)
    }

    fun One(view: android.view.View) {
       val onetext = findViewById<TextView>(R.id.textView)
       onetext.text = "Hello"
    }
}

This way it will search for the textView id in the Activity's view, which is the activity_main layout.

CodePudding user response：

I believe the problem has already been solved, but I noticed that in the xml file the button is not properly docked, putting:

app:layout_constraintTop_toBottomOf="@id/textView"

the button would be below the TextView component and putting:

app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"

the button would be centered on the screen...

Button code would look like this

<Button
    android:id="@ id/button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:onClick="One"
    android:text="Button"
    app:layout_constraintTop_toBottomOf="@id/textView"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"/>