如何解决此错误：'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

Question

我正在尝试编写一个计数器应用程序，其中按下“按钮”会增加计数器并将其显示在 TextView 中。我遵循与我的在线老师完全相同的程序。 但是，代码没有运行。

XML 代码：

<?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"
    android:background="@drawable/wall_grad"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView_heading"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="208dp"
        android:text="The counter app"
        android:textSize="24sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.497"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/textView_counter"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="84dp"
        android:text="0"
        android:textSize="25sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView_heading" />

    <Button
        android:id="@+id/button_counter"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button"
        android:textSize="22sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView_counter"
        app:layout_constraintVertical_bias="0.198" />
</androidx.constraintlayout.widget.ConstraintLayout>

Java 代码：

package com.example.counter_08_01_23;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    TextView heading1 , counter_text1;
    Button btn1;
    int counter = 0;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        heading1 = findViewById(R.id.textView_heading);
        counter_text1 = findViewById(R.id.textView_counter);
        btn1 = findViewById(R.id.button_counter);

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Adding functionality

        btn1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                counter_text1.setText(inc_counter());
            }
        });
    }

    public int inc_counter(){
        return ++counter;
    }

}

日志如下：

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$OnClickListener)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3197) at android.app .ActivityThread.handleLaunchActivity(ActivityThread.java:3334) at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:113) at android.app. servertransaction.TransactionExecutor.execute（TransactionExecutor.java:71）在 android.app。 ActivityThread$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) 在 android.app.Activity.performCreate(Activity.java:7376) 在 android.app.Activity.performCreate(Activity.886925324) 888:7367) 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$H.handleMessage(ActivityThread.java:2025) at android.os.Handler.dispatchMessage(Handler.java:106) at android.os.Looper.loop(Looper.java:226) at android. app.ActivityThread.main(ActivityThread.java:7191) 在 java.lang.reflect.Method.invoke(Native Method) 在 8816 8970007088.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:499) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:942) I/Process: Sending signal. PID：22639 SIG：9

Answer 1

在设置内容下初始化视图

  super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);


heading1 = findViewById(R.id.textView_heading);
counter_text1 = findViewById(R.id.textView_counter);
 btn1 = findViewById(R.id.button_counter);

您可以使用视图绑定，使您的工作更轻松

Answer 2

- 在这里，你只需要上下改变线。 因为您首先声明findViewById()来膨胀 XML 布局。

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main); // Raise your layout to Java class level.

    // Then find Your view from xml
    btn1 = findViewById(R.id.button_counter);
    ...
}