为什么我在这些元素上获得 NullPointer

问：

我在我的按钮和文本视图上收到一个空指针异常，我真的不知道为什么， 对我来说，代码没有错

setcontentview 正在链接正确的布局

Java 代码

public class ClientSmart extends AppCompatActivity {
    Button PostButton123;
    TextView SmsText;
    String email;
    String clientId;
    String pwd;
    String URL2;
    String LogoUrl;
    StringRequest stringRequest;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_client_smart);

        PostButton123.findViewById(R.id.buttontest);
        SmsText.findViewById(R.id.editTextTest);
        email=getSharedPreferences("MyPrefs", MODE_PRIVATE).getString("name", "");

        clientId=getSharedPreferences("MyPrefs", MODE_PRIVATE).getString("id", "");

        pwd=getSharedPreferences("MyPrefs", MODE_PRIVATE).getString("password", "");


        Uri.Builder TimeLogUrl=new Uri.Builder();

        TimeLogUrl.scheme("https")
                .authority("www.jdjdjd.com")
                .appendPath("jsj")
                .appendQueryParameter("ClientID", clientId)
                .appendQueryParameter("Username", email)
                .appendQueryParameter("Pwd", pwd);


        URL2=TimeLogUrl.build().toString();


        PostButton123.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {

                try {
                    RequestQueue requestQueue=Volley.newRequestQueue(ClientSmart.this);
                    String URL=URL2;
                    JSONObject jsonBody=new JSONObject();
                    jsonBody.put("Title", "Android Volley Demo");
                    jsonBody.put("Author", SmsText.getText().toString());
                    final String requestBody=jsonBody.toString();

                    stringRequest=new StringRequest(Request.Method.POST, URL, new Response.Listener<String>() {
                        @Override
                        public void onResponse(String response) {

                            Log.i("VOLLEY", response);
                        }
                    }, new Response.ErrorListener() {
                        @Override
                        public void onErrorResponse(VolleyError error) {
                            Log.e("VOLLEY", error.toString());
                        }
                    }) {
                        @Override
                        public String getBodyContentType() {
                            return "application/json; charset=utf-8";
                        }

                        @Override
                        public byte[] getBody() throws AuthFailureError {
                            try {
                                return requestBody == null ? null : requestBody.getBytes("utf-8");
                            } catch (UnsupportedEncodingException uee) {
                                VolleyLog.wtf("Unsupported Encoding while trying to get the bytes of %s using %s", requestBody, "utf-8");
                                return null;
                            }
                        }

                        @Override
                        protected Response<String> parseNetworkResponse(NetworkResponse response) {
                            String responseString="";
                            if (response != null) {
                                responseString=String.valueOf(response.statusCode);
                                // can get more details such as response.headers
                            }
                            return Response.success(responseString, HttpHeaderParser.parseCacheHeaders(response));
                        }
                    };

                    requestQueue.add(stringRequest);
                } catch (JSONException e) {
                    e.printStackTrace();
                }


                Toasty.success(ClientSmart.this,stringRequest.toString(),Toasty.LENGTH_LONG).show();
            }


        });

    }

}

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="com.smartpractice.myapplication.ClientSmart">

    <Button
        android:id="@+id/buttontest"
        android:layout_width="280dp"
        android:layout_height="273dp"
        android:layout_marginEnd="8dp"
        android:text="Button"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

    <EditText
        android:id="@+id/editTextTest"
        android:layout_width="512dp"
        android:layout_height="422dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginBottom="8dp"
        android:ems="10"
        android:gravity="start|top"
        android:inputType="textMultiLine"
        app:layout_constraintBottom_toTopOf="@+id/buttontest"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

我试过了

多次重启 Android Studio

清洁项目

重建项目

完全重写代码 创建新活动并重写代码

错误日志

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.smartpractice.smartpracticesmartapp, PID: 7022
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.smartpractice.smartpracticesmartapp/com.smartpractice.myapplication.ClientSmart}: 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:3430)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3614)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:86)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2199)
        at android.os.Handler.dispatchMessage(Handler.java:112)
        at android.os.Looper.loop(Looper.java:216)
        at android.app.ActivityThread.main(ActivityThread.java:7625)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:524)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:987)
     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.smartpractice.myapplication.ClientSmart.onCreate(ClientSmart.java:50)
        at android.app.Activity.performCreate(Activity.java:7458)
        at android.app.Activity.performCreate(Activity.java:7448)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1286)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3409)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3614) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:86) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2199) 
        at android.os.Handler.dispatchMessage(Handler.java:112) 
        at android.os.Looper.loop(Looper.java:216) 
        at android.app.ActivityThread.main(ActivityThread.java:7625) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:524) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:987)