在后面的代码中更改任何内容时出现 NullPointerException

Question

所以在学习中 android 我的开端很糟糕。我的 XML Activity 文件如下：

<?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/text1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="100dp"
        android:layout_marginTop="100dp"
        android:gravity="center"
        android:text="Hello World!"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

我的Java文件如下：

public class MainActivity extends AppCompatActivity {
    @SuppressLint("SetTextI18n")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        TextView text = (TextView) findViewById(R.id.text1);

        setContentView(R.layout.activity_main);
        text.setText("Lets try this out");


    }
}

问题的logcat是：

2021-08-27 18:15:20.960 6550-6550/com.example.letstrythis E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.letstrythis, PID: 6550
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.letstrythis/com.example.letstrythis.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$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.letstrythis.MainActivity.onCreate(MainActivity.java:17)
        at android.app.Activity.performCreate(Activity.java:7994)
        at android.app.Activity.performCreate(Activity.java:7978)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 
2021-08-27 18:15:21.027 6550-6550/com.example.letstrythis I/Process: Sending signal. PID: 6550 SIG: 9

我尝试了几种格式化方法，但我看不出有什么不同，总是一样

Answer 1

我认为问题在于您在调用 setContentView() 之前调用了 findViewById()；参见 this answer。 setContentView() 将 XML 文件链接到这个特定的 activity，因此在调用它之前，findViewById() 不知道在哪里寻找 XML 元素，因此 returns 空。尝试将行 setContentView(R.layout.activity_main); 移动到行 TextView text = (TextView) findViewById(R.id.text1); 上方，这应该可以解决问题。

在后面的代码中更改任何内容时出现 NullPointerException

NullPointerException when changing anything in the code behind

java

android

nullpointerexception