how to solve this error : 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

huangapple
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英文:

how to solve this error : 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

问题

XML 代码和 Java 代码中的注释将不被翻译，以下是翻译好的内容：

  1. XML 代码：
  3. <?xml version="1.0" encoding="utf-8"?>
  4. <androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
  5.     xmlns:app="http://schemas.android.com/apk/res-auto"
  6.     xmlns:tools="http://schemas.android.com/tools"
  7.     android:layout_width="match_parent"
  8.     android:layout_height="match_parent"
  9.     android:background="@drawable/wall_grad"
  10.     tools:context=".MainActivity">
  12.     <TextView
  13.         android:id="@+id/textView_heading"
  14.         android:layout_width="wrap_content"
  15.         android:layout_height="wrap_content"
  16.         android:layout_marginTop="208dp"
  17.         android:text="The counter app"
  18.         android:textSize="24sp"
  19.         app:layout_constraintEnd_toEndOf="parent"
  20.         app:layout_constraintHorizontal_bias="0.497"
  21.         app:layout_constraintStart_toStartOf="parent"
  22.         app:layout_constraintTop_toTopOf="parent" />
  24.     <TextView
  25.         android:id="@+id/textView_counter"
  26.         android:layout_width="wrap_content"
  27.         android:layout_height="wrap_content"
  28.         android:layout_marginTop="84dp"
  29.         android:text="0"
  30.         android:textSize="25sp"
  31.         app:layout_constraintEnd_toEndOf="parent"
  32.         app:layout_constraintHorizontal_bias="0.498"
  33.         app:layout_constraintStart_toStartOf="parent"
  34.         app:layout_constraintTop_toBottomOf="@+id/textView_heading" />
  36.     <Button
  37.         android:id="@+id/button_counter"
  38.         android:layout_width="wrap_content"
  39.         android:layout_height="wrap_content"
  40.         android:text="Button"
  41.         android:textSize="22sp"
  42.         app:layout_constraintBottom_toBottomOf="parent"
  43.         app:layout_constraintEnd_toEndOf="parent"
  44.         app:layout_constraintHorizontal_bias="0.498"
  45.         app:layout_constraintStart_toStartOf="parent"
  46.         app:layout_constraintTop_toBottomOf="@+id/textView_counter"
  47.         app:layout_constraintVertical_bias="0.198" />
  48. </androidx.constraintlayout.widget.ConstraintLayout>
  1. Java 代码：
  3. package com.example.counter_08_01_23;
  5. import androidx.appcompat.app.AppCompatActivity;
  7. import android.os.Bundle;
  8. import android.view.View;
  9. import android.widget.Button;
  10. import android.widget.TextView;
  12. public class MainActivity extends AppCompatActivity {
  14.     TextView heading1, counter_text1;
  15.     Button btn1;
  16.     int counter = 0;
  18.     @Override
  19.     protected void onCreate(Bundle savedInstanceState) {
  21.         heading1 = findViewById(R.id.textView_heading);
  22.         counter_text1 = findViewById(R.id.textView_counter);
  23.         btn1 = findViewById(R.id.button_counter);
  25.         super.onCreate(savedInstanceState);
  26.         setContentView(R.layout.activity_main);
  28.         // 添加功能
  30.         btn1.setOnClickListener(new View.OnClickListener() {
  31.             @Override
  32.             public void onClick(View view) {
  33.                 counter_text1.setText(inc_counter());
  34.             }
  35.         });
  36.     }
  38.     public int inc_counter() {
  39.         return ++counter;
  40.     }
  42. }

日志信息：

  1. > E/AndroidRuntime: FATAL EXCEPTION: main
  2.     Process: com.example.counter_08_01_23, PID: 22639
  3.     java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.counter_08_01_23/com.example.counter_08_01_23.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
  4.         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3197)
  5.         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3334)
  6.         at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
  7.         at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:113)
  8.         at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:71)
  9.         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2025)
  10.         at android.os.Handler.dispatchMessage(Handler.java:106)
  11.         at android.os.Looper.loop(Looper.java:226)
  12.         at android.app.ActivityThread.main(ActivityThread.java:7191)
  13.         at java.lang.reflect.Method.invoke(Native Method)
  14.         at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:499)
  15.         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:942)
  16.      Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
  17.         at com.example.counter_08_01_23.MainActivity.onCreate(MainActivity.java:28)
  18.         at android.app.Activity.performCreate(Activity.java:7376)
  19.         at android.app.Activity.performCreate(Activity.java:7367)
  20.         at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
  21.         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3177)
  22.         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3334)
  23.         at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
  24.         at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:113)
  25.         at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:71)
  26.         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2025)
  27.         at android.os.Handler.dispatchMessage(Handler.java:106)
  28.         at android.os.Looper.loop(Looper.java:226)
  29.         at android.app.ActivityThread.main(ActivityThread.java:7191)
  30.         at java.lang.reflect.Method.invoke(Native Method)
  31.         at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:499)
  32.         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:942)
  33. I/Process: Sending signal. PID: 22639 SIG: 9

请注意，上述日志中的错误信息指出按钮对象为空，因此需要确保在 setContentView(R.layout.activity_main); 之后初始化按钮对象。

英文:

I am trying to code a counter app in which pressing the "Button" increments the counter and displays it in the TextView. I followed the exact same procedure as my online teacher. But, the code is not running.

XML code:

  1. &lt;?xml version=&quot;1.0&quot; encoding=&quot;utf-8&quot;?&gt;
  2. &lt;androidx.constraintlayout.widget.ConstraintLayout xmlns:android=&quot;http://schemas.android.com/apk/res/android&quot;
  3.     xmlns:app=&quot;http://schemas.android.com/apk/res-auto&quot;
  4.     xmlns:tools=&quot;http://schemas.android.com/tools&quot;
  5.     android:layout_width=&quot;match_parent&quot;
  6.     android:layout_height=&quot;match_parent&quot;
  7.     android:background=&quot;@drawable/wall_grad&quot;
  8.     tools:context=&quot;.MainActivity&quot;&gt;
  10.     &lt;TextView
  11.         android:id=&quot;@+id/textView_heading&quot;
  12.         android:layout_width=&quot;wrap_content&quot;
  13.         android:layout_height=&quot;wrap_content&quot;
  14.         android:layout_marginTop=&quot;208dp&quot;
  15.         android:text=&quot;The counter app&quot;
  16.         android:textSize=&quot;24sp&quot;
  17.         app:layout_constraintEnd_toEndOf=&quot;parent&quot;
  18.         app:layout_constraintHorizontal_bias=&quot;0.497&quot;
  19.         app:layout_constraintStart_toStartOf=&quot;parent&quot;
  20.         app:layout_constraintTop_toTopOf=&quot;parent&quot; /&gt;
  22.     &lt;TextView
  23.         android:id=&quot;@+id/textView_counter&quot;
  24.         android:layout_width=&quot;wrap_content&quot;
  25.         android:layout_height=&quot;wrap_content&quot;
  26.         android:layout_marginTop=&quot;84dp&quot;
  27.         android:text=&quot;0&quot;
  28.         android:textSize=&quot;25sp&quot;
  29.         app:layout_constraintEnd_toEndOf=&quot;parent&quot;
  30.         app:layout_constraintHorizontal_bias=&quot;0.498&quot;
  31.         app:layout_constraintStart_toStartOf=&quot;parent&quot;
  32.         app:layout_constraintTop_toBottomOf=&quot;@+id/textView_heading&quot; /&gt;
  34.     &lt;Button
  35.         android:id=&quot;@+id/button_counter&quot;
  36.         android:layout_width=&quot;wrap_content&quot;
  37.         android:layout_height=&quot;wrap_content&quot;
  38.         android:text=&quot;Button&quot;
  39.         android:textSize=&quot;22sp&quot;
  40.         app:layout_constraintBottom_toBottomOf=&quot;parent&quot;
  41.         app:layout_constraintEnd_toEndOf=&quot;parent&quot;
  42.         app:layout_constraintHorizontal_bias=&quot;0.498&quot;
  43.         app:layout_constraintStart_toStartOf=&quot;parent&quot;
  44.         app:layout_constraintTop_toBottomOf=&quot;@+id/textView_counter&quot;
  45.         app:layout_constraintVertical_bias=&quot;0.198&quot; /&gt;
  46. &lt;/androidx.constraintlayout.widget.ConstraintLayout&gt;

Java Code:

  1. package com.example.counter_08_01_23;
  3. import androidx.appcompat.app.AppCompatActivity;
  5. import android.os.Bundle;
  6. import android.view.View;
  7. import android.widget.Button;
  8. import android.widget.TextView;
  10. public class MainActivity extends AppCompatActivity {
  12.     TextView heading1 , counter_text1;
  13.     Button btn1;
  14.     int counter = 0;
  16.     @Override
  17.     protected void onCreate(Bundle savedInstanceState) {
  19.         heading1 = findViewById(R.id.textView_heading);
  20.         counter_text1 = findViewById(R.id.textView_counter);
  21.         btn1 = findViewById(R.id.button_counter);
  23.         super.onCreate(savedInstanceState);
  24.         setContentView(R.layout.activity_main);
  26.         // Adding functionality
  28.         btn1.setOnClickListener(new View.OnClickListener() {
  29.             @Override
  30.             public void onClick(View view) {
  31.                 counter_text1.setText(inc_counter());
  32.             }
  33.         });
  34.     }
  36.     public int inc_counter(){
  37.         return ++counter;
  38.     }
  40. }

The Log is as follows :
>
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.counter_08_01_23, PID: 22639
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.counter_08_01_23/com.example.counter_08_01_23.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3197)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3334)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:113)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:71)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2025)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:226)
at android.app.ActivityThread.main(ActivityThread.java:7191)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:499)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:942)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.example.counter_08_01_23.MainActivity.onCreate(MainActivity.java:28)
at android.app.Activity.performCreate(Activity.java:7376)
at android.app.Activity.performCreate(Activity.java:7367)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3177)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3334) 
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:113) 
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:71) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2025) 
at android.os.Handler.dispatchMessage(Handler.java:106) 
at android.os.Looper.loop(Looper.java:226) 
at android.app.ActivityThread.main(ActivityThread.java:7191) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:499) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:942) 
I/Process: Sending signal. PID: 22639 SIG: 9

答案1

得分: 2

初始化视图并设置内容

  1. super.onCreate(savedInstanceState);
  2. setContentView(R.layout.activity_main);
  4. heading1 = findViewById(R.id.textView_heading);
  5. counter_text1 = findViewById(R.id.textView_counter);
  6. btn1 = findViewById(R.id.button_counter);
  8. 您可以使用视图绑定来使您的工作更加轻松
英文:

Initialise views below set content

  1.   super.onCreate(savedInstanceState);
  2. setContentView(R.layout.activity_main);
  5. heading1 = findViewById(R.id.textView_heading);
  6. counter_text1 = findViewById(R.id.textView_counter);
  7.  btn1 = findViewById(R.id.button_counter);

You can use view binding which make your work more easier

答案2

得分: 0

- 在这里，你只需要将上下的行交换一下顺序。因为你首先声明了findViewById()，然后才充气XML布局。

  1. @Override
  2. protected void onCreate(Bundle savedInstanceState) {
  3.     super.onCreate(savedInstanceState);
  4.     
  5.     // 提升你的布局到Java类级别。
  6.     setContentView(R.layout.activity_main);
  8.     // 然后从XML中查找你的视图
  9.     btn1 = findViewById(R.id.button_counter);
  10.     ...
  11. }
英文:

- Here, you need to change just the line up and down. Because you declare findViewById() first of inflating XML Layout.

  1. @Override
  2. protected void onCreate(Bundle savedInstanceState) {
  3.     super.onCreate(savedInstanceState);
  4.     setContentView(R.layout.activity_main); // Raise your layout to Java class level.
  6.     // Then find Your view from xml
  7.     btn1 = findViewById(R.id.button_counter);
  8.     ...
  9. }

huangapple
  • 本文由 发表于 2023年1月9日 10:19:58
  • 转载请务必保留本文链接：https://go.coder-hub.com/75052648.html
  • android
  • java
  • xml
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