java - 动态添加布局，以与其他布局重叠

Question

我正在为我的应用程序编写一个“欢迎屏幕”，该屏幕在应用程序首次启动时运行。我想要的:通过代码动态地重叠在根布局上的透明布局。像这样的图片:

但是当我运行我的应用程序时，它由于 NULLPOINTEREXCEPTION 而崩溃。即使我没有添加第二个布局

问题 1:为什么第一个布局不显示，并且应用崩溃？

问题2:如何显示第二种布局？

提前致谢......

身份验证.java

import component.Util;

public class Authentication extends Activity {

      private RelativeLayout rootLayout;
      public static Context CONTEXT;
      public Authentication(){
            CONTEXT=this;
    }

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    rootLayout=new RelativeLayout(CONTEXT);
    LinearLayout firstLayout=(LinearLayout) findViewById(R.layout.authentication);

    rootLayout.addView(firstLayout);
    setContentView(rootLayout);

    if(Util.isFirstLaunch(CONTEXT)){
        //Add the second layout
    }
  }
}

authentication.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/authentication"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:background="@drawable/background" >
<TextView
    android:id="@+id/textView_login"
    android:layout_width="210dp"
    android:layout_height="wrap_content"
    android:layout_marginTop="20dp"
    android:layout_marginBottom="12dp"
    android:layout_marginLeft="4dp"
    android:layout_marginRight="4dp"
    android:layout_gravity="center_horizontal"
    android:text="@string/loginText"
    android:textSize="18sp" />

<EditText
    android:id="@+id/editText_username"
    android:layout_width="210dp"
    android:layout_height="wrap_content"
    android:layout_marginTop="4dp"
    android:layout_marginBottom="4dp"
    android:layout_marginLeft="4dp"
    android:layout_marginRight="4dp"
    android:layout_gravity="center_horizontal"
    android:nextFocusDown="@+id/editText_password"
    android:inputType="textPersonName"
    android:hint="@string/username" />

<EditText
    android:id="@+id/editText_password"
    android:layout_width="210dp"
    android:layout_height="wrap_content"
    android:layout_marginTop="4dp"
    android:layout_marginBottom="12dp"
    android:layout_marginLeft="4dp"
    android:layout_marginRight="4dp"
    android:layout_gravity="center_horizontal"
    android:inputType="textPassword"
    android:hint="@string/password" />

<Button
    android:id="@+id/button_submit"
    android:layout_width="140dp"
    android:layout_height="wrap_content"
    android:layout_marginTop="14dp"
    android:layout_gravity="center_horizontal"
    android:background="@drawable/button"
    android:text="@string/submit"
    android:textColor="#e1ecff" />

</LinearLayout>

堆栈跟踪:

08-31 13:50:32.623: D/AndroidRuntime(5740): Shutting down VM
08-31 13:50:32.623: W/dalvikvm(5740): threadid=1: thread exiting with uncaught exception (group=0x40a71930)
08-31 13:50:32.643: E/AndroidRuntime(5740): FATAL EXCEPTION: main
08-31 13:50:32.643: E/AndroidRuntime(5740): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.shayan.filesharing/com.shayan.filesharing.Authentication}: java.lang.ClassCastException: android.widget.LinearLayout cannot be cast to android.widget.RelativeLayout
08-31 13:50:32.643: E/AndroidRuntime(5740):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
08-31 13:50:32.643: E/AndroidRuntime(5740):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
08-31 13:50:32.643: E/AndroidRuntime(5740):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
08-31 13:50:32.643: E/AndroidRuntime(5740):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
08-31 13:50:32.643: E/AndroidRuntime(5740):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-31 13:50:32.643: E/AndroidRuntime(5740):     at android.os.Looper.loop(Looper.java:137)
08-31 13:50:32.643: E/AndroidRuntime(5740):     at android.app.ActivityThread.main(ActivityThread.java:5041)
08-31 13:50:32.643: E/AndroidRuntime(5740):     at java.lang.reflect.Method.invokeNative(Native Method)
08-31 13:50:32.643: E/AndroidRuntime(5740):     at java.lang.reflect.Method.invoke(Method.java:511)
08-31 13:50:32.643: E/AndroidRuntime(5740):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
08-31 13:50:32.643: E/AndroidRuntime(5740):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
08-31 13:50:32.643: E/AndroidRuntime(5740):     at dalvik.system.NativeStart.main(Native Method)
08-31 13:50:32.643: E/AndroidRuntime(5740): Caused by: java.lang.ClassCastException: android.widget.LinearLayout cannot be cast to android.widget.RelativeLayout
08-31 13:50:32.643: E/AndroidRuntime(5740):     at com.shayan.filesharing.Authentication.onCreate(Authentication.java:22)
08-31 13:50:32.643: E/AndroidRuntime(5740):     at android.app.Activity.performCreate(Activity.java:5104)
08-31 13:50:32.643: E/AndroidRuntime(5740):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
08-31 13:50:32.643: E/AndroidRuntime(5740):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
08-31 13:50:32.643: E/AndroidRuntime(5740):     ... 11 more

Answer 1

首先，删除您的构造函数。不要在 Activity 上实现构造函数。您不需要它，并且您错误地实现了它(无法链接到父类(super class))。

其次，删除您的 CONTEXT 静态数据成员。不要将 Activity 放入静态数据成员中，否则会泄漏内存。在 onCreate() 方法中将所有对 CONTEXT 的引用替换为 this。