創建片段時在相對布局中務實地創建 Cardview（在片段的“oncreate()”方法內）

Question

在我的應用程序中，單擊按鈕時會生成一個新片段。

在activity_main.xml文件中有LinearLayout 。 我想在創建片段時同時以語法方式將Cardview添加到該LinearLayout中。 所以我將卡片視圖創建代碼放在片段的oncreate()方法中。

沒有任何錯誤顯示。 但是當我單擊主活動中的按鈕時，應用程序崩潰了。

下面是片段代碼。

package com.example.infinitybrowser;
import android.content.Context;
import android.graphics.Color;
import android.os.Bundle;
import androidx.annotation.Nullable;
import androidx.cardview.widget.CardView;
import androidx.fragment.app.Fragment;
import android.util.AttributeSet;
import android.util.TypedValue;
import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.RelativeLayout;
import android.widget.TextView;

import static com.example.infinitybrowser.R.id.recycle_container;

public class frag_one extends Fragment
{
Button new_tab;
TextView test_tab_no;
private Context context;
LinearLayout.LayoutParams layoutparams;
RelativeLayout relativeLayout;
LinearLayout linearLayout;
TextView textview;
private AttributeSet attrs;
View v;
ViewGroup container;


@Override
public void onCreate(Bundle savedInstanceState)
{
    super.onCreate (savedInstanceState);
    // "recycle_container" is the linearlayout in activity_main.xml file
     // Here "view.findViewById" showing error also??
    linearLayout = (LinearLayout) v.findViewById (recycle_container);
    //linearLayout = (LinearLayout)MainActivity.v;
    context = context.getApplicationContext ();
    context= container.getContext();
    CardView tab_card = new CardView(context);
    layoutparams = new LinearLayout.LayoutParams (RelativeLayout.LayoutParams.WRAP_CONTENT, RelativeLayout.LayoutParams.WRAP_CONTENT);
    tab_card.setLayoutParams(layoutparams);
    tab_card.setRadius(15);
    tab_card.setPadding(25, 25, 25, 25);
    tab_card.setCardBackgroundColor(Color.MAGENTA);
    tab_card.setMaxCardElevation(30);
    tab_card.setMaxCardElevation(6);
    textview = new TextView (context);
    textview.setLayoutParams(layoutparams);
    textview.setText("CardView Programmatically\n");
    textview.setTextSize(TypedValue.COMPLEX_UNIT_DIP, 25);
    textview.setTextColor(Color.WHITE);
    textview.setPadding(25,25,25,25);
    textview.setGravity(Gravity.CENTER);
    tab_card.addView(textview);
    linearLayout.addView(tab_card);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState)
{
    
    // Inflate the layout for this fragment
    final View view = inflater.inflate(R.layout.fragment_frag_one,container,false);        
    return view;
}
}

但是當我單擊按鈕時，這是在 logcat 錯誤下方；

Process: com.example.infinitybrowser, PID: 30120
java.lang.NullPointerException
    at com.example.infinitybrowser.frag_one.onCreate(frag_one.java:45)
    at androidx.fragment.app.Fragment.performCreate(Fragment.java:2586)
    at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:838)
    at androidx.fragment.app.FragmentTransition.addToFirstInLastOut(FragmentTransition.java:1197)
    at androidx.fragment.app.FragmentTransition.calculateFragments(FragmentTransition.java:1080)
    at androidx.fragment.app.FragmentTransition.startTransitions(FragmentTransition.java:119)
    at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManagerImpl.java:1866)
    at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManagerImpl.java:1824)
    at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManagerImpl.java:1727)
    at androidx.fragment.app.FragmentManagerImpl$2.run(FragmentManagerImpl.java:150)
    at android.os.Handler.handleCallback(Handler.java:808)
    at android.os.Handler.dispatchMessage(Handler.java:103)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:5318)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:515)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:838)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:654)
    at dalvik.system.NativeStart.main(Native Method)

Answer 1

您的堆棧跟蹤解釋了NullPointerException ，主要發生在我們嘗試訪問任何 null object 的屬性時：

在包含context= container.getContext();的代碼行中還沒有做 memory 初始化。 所以，你需要解決這個問題並繼續你的工作。

Answer 2

NPE occering 因為布局尚未膨脹並且您正在對其執行一些操作。

執行創建視圖時創建的 cardView

public class DemoFragment extends Fragment {
Button new_tab;
TextView test_tab_no;
private Context context;
LinearLayout.LayoutParams layoutparams;
RelativeLayout relativeLayout;
LinearLayout linearLayout;
TextView textview;
private AttributeSet attrs;
View v;
ViewGroup container;

@Override
public void onCreate(Bundle savedInstanceState)
{
    super.onCreate (savedInstanceState);
    // "recycle_container" is the linearlayout in activity_main.xml file
    // Here "view.findViewById" showing error also??
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState)
{
    // Inflate the layout for this fragment
    final View view = inflater.inflate(R.layout.fragment_frag_one,container,false);
    return view;
}

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    linearLayout = (LinearLayout) v.findViewById (recycle_container);
    //linearLayout = (LinearLayout)MainActivity.v;
    context = context.getApplicationContext ();
    context= container.getContext();
    CardView tab_card = new CardView(context);
    layoutparams = new LinearLayout.LayoutParams (RelativeLayout.LayoutParams.WRAP_CONTENT, RelativeLayout.LayoutParams.WRAP_CONTENT);
    tab_card.setLayoutParams(layoutparams);
    tab_card.setRadius(15);
    tab_card.setPadding(25, 25, 25, 25);
    tab_card.setCardBackgroundColor(Color.MAGENTA);
    tab_card.setMaxCardElevation(30);
    tab_card.setMaxCardElevation(6);
    textview = new TextView (context);
    textview.setLayoutParams(layoutparams);
    textview.setText("CardView Programmatically\n");
    textview.setTextSize(TypedValue.COMPLEX_UNIT_DIP, 25);
    textview.setTextColor(Color.WHITE);
    textview.setPadding(25,25,25,25);
    textview.setGravity(Gravity.CENTER);
    tab_card.addView(textview);
    linearLayout.addView(tab_card);
}

}