Java 登录到另一个活动的页面赢得'；行不通
所以我刚刚完成我正在开发的第一个应用程序，在确保一切正常后，我添加了一个新的活动作为登录活动，从android清单中更改了它，这样它将首先启动它，我复制粘贴了我从另一个应用程序中尝试制作的代码，并简单地更改了intent参数，这样它将转到我在该项目中创建的活动。然而，该应用程序不断崩溃，在查看了所有日志后，它说在某一行中有一个问题，该行是空的。我会把代码贴在这里，也许你可以看一下Java 登录到另一个活动的页面赢得'；行不通,java,android,Java,Android,所以我刚刚完成我正在开发的第一个应用程序，在确保一切正常后，我添加了一个新的活动作为登录活动，从android清单中更改了它，这样它将首先启动它，我复制粘贴了我从另一个应用程序中尝试制作的代码，并简单地更改了intent参数，这样它将转到我在该项目中创建的活动。然而，该应用程序不断崩溃，在查看了所有日志后，它说在某一行中有一个问题，该行是空的。我会把代码贴在这里，也许你可以看一下 public class LoginActivity extends AppCompatActivity {
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;
}
}
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)
<?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>
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“生成-->清理项目并运行”吗？您应该添加错误堆栈跟踪。我在代码
login=（按钮）findviewbyd（R.id.Button）下面添加了它findViewById