另一個活動的登錄頁面不起作用

Question

所以我剛剛完成了我正在開發的第一個應用程序，在確保一切正常后，我添加了一個新活動作為登錄活動，從 android 清單中更改了它，所以它會首先啟動它，然后我復制粘貼了一個我從另一個我試圖制作的應用程序中獲得的代碼只是更改了意圖參數，因此它將 go 用於我在這個項目中創建的活動。 但是，該應用程序不斷崩潰，並且在仔細查看日志后，它說在某個為空的行中存在問題。 我把代碼貼在這里，也許你可以看一下

public class LoginActivity extends AppCompatActivity {

    private Button login;
    private EditText user;
    private EditText password;

    private String Username = "Admin";
    private String UserPassword = "12345";
    boolean isValid = false;



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

        user = findViewById(R.id.etUser);
        password =findViewById(R.id.etPassword);

        login= (Button) findViewById(R.id.button);
        login.setOnClickListener(new View.OnClickListener(){
            //the problem seems to be here according to the log
            public void onClick(View v){

                String inputUser= user.getText().toString();
                String inputPassword =password.getText().toString();

                if(inputUser.isEmpty() || inputPassword.isEmpty())
                {
                    Toast.makeText(LoginActivity.this, "Va rugam sa introduceti toate datele", Toast.LENGTH_SHORT).show();
                }else{

                    isValid = validate(inputUser , inputPassword);

                    if(!isValid){


                        Toast.makeText(LoginActivity.this, "Date Incorecte!", Toast.LENGTH_SHORT).show();



                    }else {

                        Toast.makeText(LoginActivity.this, "Ati reusit sa va logati", Toast.LENGTH_SHORT).show();
                        Intent intent = new Intent(LoginActivity.this, MainActivity.class);
                        startActivity(intent);


                    }



                }



            }

        });
    }

    private boolean validate (String name, String password){

        if(name.equals(Username) && password.equals(UserPassword)){
            return true;
        }
        return false;
    }
}

錯誤堆棧跟蹤：

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.mircea.bookrecyclerview, PID: 14617
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mircea.bookrecyclerview/com.mircea.bookrecyclerview.LoginActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2778)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.mircea.bookrecyclerview.LoginActivity.onCreate(LoginActivity.java:34)
        at android.app.Activity.performCreate(Activity.java:7009)
        at android.app.Activity.performCreate(Activity.java:7000)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1214)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2731)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6494) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807) 

布局文件：

<?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">

    <EditText
        android:id="@+id/etUser"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:hint="Enter User"
        android:inputType="textPersonName"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button"
        android:layout_width="163dp"
        android:layout_height="48dp"
        android:text="Log In"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.691" />

    <EditText
        android:id="@+id/etPassword"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:hint="Enter Password"
        android:inputType="textPassword"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.502"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.581" />

</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

您的 LoginActivity 清單文件是否如下所示：

<activity
    android:name=".LoginActivity"
    android:label="@string/app_name"
    android:theme="@style/AppTheme.NoActionBar">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>

您可以單擊 Build --> Clean Project 並運行嗎？