Android程序运行报错：findViewById触发NullPointerException求助

程序启动后直接终止，错误源自MainActivity第19行的findViewById调用，触发空指针异常。

Logcat错误日志 #

---------------------------- PROCESS STARTED (10563) for package com.example.test0 ----------------------------
2023-05-23 15:20:13.344 10563-10563 AndroidRuntime          com.example.test0                    E  FATAL EXCEPTION: main
Process: com.example.test0, PID: 10563
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.test0/com.example.test0.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.Button.findViewById(int)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2835)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2913)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1615)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6687)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:810)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.Button.findViewById(int)' on a null object reference
at com.example.test0.MainActivity.onCreate(MainActivity.java:19)
at android.app.Activity.performCreate(Activity.java:7028)
at android.app.Activity.performCreate(Activity.java:7019)

MainActivity代码 #

public class MainActivity extends AppCompatActivity {
Button button;
EditText editText;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    button.findViewById(R.id.button);  //<<<<<<==== line19
    editText.findViewById(R.id.editText);
}
}

相关说明 #

• 布局截图显示界面包含Button和EditText控件，对应id分别为button和editText
• 已尝试更换多种模拟器及真机测试，问题仍存在

解决方案 #

错误原因是button和editText变量未初始化（默认值为null），直接调用它们的findViewById方法必然触发空指针异常。正确做法是调用Activity的findViewById方法获取控件实例，并赋值给对应变量：

修改onCreate方法中的代码：

button = findViewById(R.id.button);
editText = findViewById(R.id.editText);

这样就能正确获取到布局中的控件，避免空指针异常。

内容的提问来源于stack exchange，提问作者M.R.E