java - 当类扩展了LinearLayout时如何将类作为 Activity 调用

Question

正如我的标题所说的那样，我的应用程序在Android Studio(JAVA)中遇到问题，当类扩展了LinearLayout时，如何将类作为 Activity 调用？

我的课是:
public class CustomCalendar extends LinearLayout {
我尝试调用的代码:

 Intent customCalendar = new Intent(MainActivity.this, CustomCalendarActivity.class);
 startActivity(customCalendar);

我试图做到这一点:

 public class CustomCalendarActivity extends AppCompatActivity {


    CustomCalendar customCalendar;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        customCalendar = (CustomCalendar) findViewById(R.id.kalendoriaus_virsus);
        customCalendar.SetUpCalendar();
    }
}

我的崩溃是这样的:

 java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.kalendorius.CustomCalendar.SetUpCalendar()' on a null object reference
        at com.example.kalendorius.MainActivity$2.onClick(MainActivity.java:75)

75行是:

 startActivity(customCalendar);

如何设置日历:

我的kalendorius.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/kalendoriaus_virsus"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="@drawable/soft_blue_green"
        android:orientation="horizontal"
        android:paddingTop="8dp"
        android:paddingBottom="8dp">

        <ImageButton
            android:id="@+id/atgalBtn"
            android:layout_width="40dp"
            android:layout_height="40dp"
            android:layout_margin="10dp"
            android:background="@drawable/back" />

        <TextView
            android:id="@+id/dabartineData"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="4dp"
            android:layout_weight="3"
            android:gravity="center"
            android:text="Data"
            android:textColor="#ffffff"
            android:textSize="18sp"
            android:textStyle="bold" />

那只是我使用那个kalendorius_virsus的一点代码。

完成。我找出了问题所在。我只是忘了为数据库构造函数生成:D谢谢所有尝试提供帮助的人。

Answer 1

问题是
customCalendar =(CustomCalendar)findViewById(R.id.kalendoriaus_virsus);

kalendorias.xml中ID为kalendorias_virsus的groupView LinearLayout不是CustomCalendar，因此始终返回null。

您需要了解如何正确使用Costum View

基本示例CustomView->

public class CustomView extends LinearLayout {

    public CustomView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        View view =  LayoutInflater.from(getContext()).inflate(
                R.layout.costum, null);

        this.addView(view);
    }

    public void setCustomText(String text){
        TextView textview = (TextView) findViewById(R.id.textViewId);
        textview.setText(text);
    }
}

布局custom.xml->

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical">

    <TextView
        android:id="@+id/textViewId"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:text="@string/app_name"
        android:gravity="center">

    </TextView>
</LinearLayout>

activity.xml->

<LinearLayout 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"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <com.example.myapplication.CustomView
        android:id="@+id/customViewId"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

</LinearLayout>

MainActivity类->

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        CustomView customView = findViewById(R.id.customViewId);
        customView.setCustomText("Welcome ");
    }
}