Android Studio按钮关联空指针异常崩溃问题求助

定义Button变量后应用持续崩溃，错误提示空对象引用，但Ctrl点击按钮ID能正常定位到布局中的按钮。

原MainActivity.java代码 #

package com.example.datasaving;

import androidx.appcompat.app.AppCompatActivity;

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

public class MainActivity extends AppCompatActivity {


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        Button button = this.findViewById(R.id.SAVE);
        TextView InputID = findViewById(R.id.textView);
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String msg = InputID.getText().toString();
                Toast.makeText(MainActivity.this, "", Toast.LENGTH_SHORT).show();
            }
        });
    }
}

activity_main.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"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/SAVE"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/save"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:hint="@string/enter_your_gmail"
        android:textColorHint="#757575"
        app:layout_constraintBottom_toTopOf="@+id/SAVE"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

运行错误日志 #

FATAL EXCEPTION: main
Process: com.example.datasaving, PID: 6961
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.datasaving/com.example.datasaving.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3921)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:4078)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:103)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2423)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loopOnce(Looper.java:233)
    at android.os.Looper.loop(Looper.java:334)
    at android.app.ActivityThread.main(ActivityThread.java:8348)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:582)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1065)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at com.example.datasaving.MainActivity.onCreate(MainActivity.java:20)
    at android.app.Activity.performCreate(Activity.java:8363)
    at android.app.Activity.performCreate(Activity.java:8341)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1329)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3894)
    ... 12 more

这是代码执行顺序错误导致的空指针异常，和Android Studio无关。

在Android开发中，findViewById()必须在setContentView()之后调用——setContentView()是将布局文件加载到Activity的核心步骤，只有布局加载完成后，系统才能通过控件ID找到对应的视图实例。原代码中先调用findViewById()获取控件，之后才执行super.onCreate(savedInstanceState)和setContentView()，此时布局未加载，findViewById()返回null，后续调用button.setOnClickListener()就会触发空指针异常。

修正后的MainActivity.java代码 #

package com.example.datasaving;

import androidx.appcompat.app.AppCompatActivity;

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

public class MainActivity extends AppCompatActivity {


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // 先执行父类初始化与布局加载
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        // 布局加载完成后再获取控件实例
        Button button = this.findViewById(R.id.SAVE);
        TextView InputID = findViewById(R.id.textView);
        
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String msg = InputID.getText().toString();
                Toast.makeText(MainActivity.this, msg, Toast.LENGTH_SHORT).show();
            }
        });
    }
}

注：顺便将原代码中空内容的Toast改为显示输入文本，让功能更完整。

内容的提问来源于stack exchange，提问作者Divyansh undley