Android Studio中的NPE错误

我在Android Studio中遇到以下错误，无法解决：

07-06 19:44:41.798 4491-4491/com.example.first_app D/AndroidRuntime: Shutting down VM
07-06 19:44:41.799 4491-4491/com.example.first_app E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.first_app, PID: 4491
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.first_app/com.example.first_app.welcome}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.view.View.toString()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.view.View.toString()' on a null object reference
at com.example.first_app.welcome.onCreate(welcome.java:22)
at android.app.Activity.performCreate(Activity.java:6237)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
at android.app.ActivityThread.-wrap11(ActivityThread.java) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:148) 
at android.app.ActivityThread.main(ActivityThread.java:5417) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 
07-06 19:44:44.696 4491-4497/com.example.first_app W/art: Suspending all threads took: 46.612ms
07-06 19:44:46.517 4491-4491/com.example.first_app I/Process: Sending signal. PID: 4491 SIG: 9

这就是我所犯的错误

---

welcome.java的代码

public class welcome extends AppCompatActivity {


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.welcome);
        TextView my_msg;
        String name;
        my_msg= (TextView)findViewById(R.id.textView3);
        name= "Welcome " +findViewById(R.id.textView2).toString();
        my_msg.setText(name);
    }
}

---

welcome.XML的XML代码…请参考它..它仅在屏幕上显示消息“welcome Name”。。该名称由用户通过activity_form.java输入

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:weightSum="1"
    android:orientation="vertical">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="0dp"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Large Text"
        android:id="@+id/textView3"
        android:layout_gravity="center"
        android:layout_weight="1" />

</LinearLayout>

---

首先解决异常的方法是

键入cast

视图，然后从该视图中选择

getText

(TextView) findViewById(R.id.textView2).getText().toString();

所以最后的字符串看起来像

name= "Welcome " + (TextView) findViewById(R.id.textView2).getText().toString();

建议

---

对

view

s使用正确的命名约定，而不是命名

view1

view2

etc

java.lang.NullPointerException:尝试在com.example.first\u app.welcome.onCreate（welcome.java:22）的空对象引用上调用虚拟方法“java.lang.String android.view.view.toString（）

查看

com.example.第22行。首先是应用程序。欢迎

先生请详细说明..如何做..实际上我是初学者我们无法详细说明“问题”，也没有信息