Java 启动应用程序时出现致命异常

我是android studio的新手。我正在创建一个应用程序，使用socket在PC和手机之间传递数据。 我已经将基于命令的程序与android相结合。 在这种情况下，客户端将号码发送到服务器，服务器将检查它是否为prime，根据它，服务器将向客户端发送回复。但是，我在启动主活动时遇到致命异常

Logcat是：

08-18 11:59:26.977 1728-1728/com.example.admin.test_purpose W/dalvikvm: threadid=1: thread exiting with uncaught exception (group=0xb4e68288)
08-18 11:59:26.977 1728-1728/com.example.admin.test_purpose E/AndroidRuntime: 
FATAL EXCEPTION: main java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.admin.test_purpose/com.example.admin.test_purpose.MainActivity}: java.lang.NullPointerException

 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1983)
   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
 at android.app.ActivityThread.access$600(ActivityThread.java:130)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4745)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at android.app.Activity.findViewById(Activity.java:1825)
at com.example.admin.test_purpose.MainActivity.<init>(MainActivity.java:15)
at java.lang.Class.newInstanceImpl(Native Method)
at java.lang.Class.newInstance(Class.java:1319)
at android.app.Instrumentation.newActivity(Instrumentation.java:1053)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1974)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084) 
at android.app.ActivityThread.access$600(ActivityThread.java:130) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195) 
at android.os.Handler.dispatchMessage(Handler.java:99) 
at android.os.Looper.loop(Looper.java:137) 
at android.app.ActivityThread.main(ActivityThread.java:4745) 
at java.lang.reflect.Method.invokeNative(Native Method)
 at java.lang.reflect.Method.invoke(Method.java:511) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
at dalvik.system.NativeStart.main(Native Method) 

activity_main.xml

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

    <EditText
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/editText"
        android:layout_gravity="center_horizontal"
        android:hint="@string/user_value"
        android:layout_marginTop="15dp" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/submit"
        android:id="@+id/button"
        android:layout_gravity="center_horizontal"
        android:layout_marginTop="15dp" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:id="@+id/textView"
        android:layout_gravity="center_horizontal"
        android:layout_marginTop="20dp" />
</LinearLayout>

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.admin.test_purpose">
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
    <uses-permission android:name="android.permission.ACCESS_WIFI_STATE"/>
    <uses-permission android:name="android.permission.INTERNET"/>
</manifest>

---

更新您的主要活动

EditText user_num;
TextView reply;
Button btnsbm;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    user_num = (EditText)findViewById(R.id.editText);
    reply  = (TextView)findViewById(R.id.textView);
    btnsbm = (Button)findViewById(R.id.button);
    onclicklistener();
}

---

更新您的主要活动

EditText user_num;
TextView reply;
Button btnsbm;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    user_num = (EditText)findViewById(R.id.editText);
    reply  = (TextView)findViewById(R.id.textView);
    btnsbm = (Button)findViewById(R.id.button);
    onclicklistener();
}

---

问题在于这三条线：

EditText user_num = (EditText)findViewById(R.id.editText);
TextView reply  = (TextView)findViewById(R.id.textView);
Button btnsbm = (Button)findViewById(R.id.button);

问题是，在视图初始化之前，您正在调用

findViewById（）

将

findViewById

-组件移动到

onCreate（）

，然后它应该可以工作：

EditText user_num;
TextView reply;
Button btnsbm;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    user_num = (EditText)findViewById(R.id.editText);
    reply  = (TextView)findViewById(R.id.textView);
    btnsbm = (Button)findViewById(R.id.button);
    onclicklistener();
}

---

问题在于这三条线：

EditText user_num = (EditText)findViewById(R.id.editText);
TextView reply  = (TextView)findViewById(R.id.textView);
Button btnsbm = (Button)findViewById(R.id.button);

问题是，在视图初始化之前，您正在调用

findViewById（）

将

findViewById

-组件移动到

onCreate（）

，然后它应该可以工作：

EditText user_num;
TextView reply;
Button btnsbm;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    user_num = (EditText)findViewById(R.id.editText);
    reply  = (TextView)findViewById(R.id.textView);
    btnsbm = (Button)findViewById(R.id.button);
    onclicklistener();
}

---

将您的

findViewByid

移动到

setContentView（布局）

后

setContentView（布局）

移动您的

findViewByid

后

setContentView（布局）

谢谢合作伙伴：）工作正常。。。！！！谢谢伙计：）它工作了。。。！！！