另一个活动的登录页面不起作用

Question

所以我刚刚完成了我正在开发的第一个应用程序，在确保一切正常后，我添加了一个新活动作为登录活动，从 android 清单中更改了它，所以它会首先启动它，然后我复制粘贴了一个我从另一个我试图制作的应用程序中获得的代码只是更改了意图参数，因此它将 go 用于我在这个项目中创建的活动。 但是，该应用程序不断崩溃，并且在仔细查看日志后，它说在某个为空的行中存在问题。 我把代码贴在这里，也许你可以看一下

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;
    }
}

错误堆栈跟踪：

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 清单文件是否如下所示：

<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>

您可以单击 Build --> Clean Project 并运行吗？