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我是 Android 的初学者，如果这是一个非常简单的修复，请原谅我。我已经查看了此处关于同一 NullPointerException 问题的其他帖子，但是我仍然无法在我的代码中找到错误的来源。
我有一个非常简单的项目，它有 Main java 类和一个 fragment 类。当用户单击单选按钮时，主要 Activity 的背景颜色必须更改，但我不断收到:
java.lang.NullPointerException: Attempt to invoke interface method 'OnColorChangeListener.colorChanged(java.lang.String)' on a null object reference.
Activity5.java:
public class Activity5 extends AppCompatActivity implements ColorFragment.OnColorChangeListener {
LinearLayout linearLayout;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_5);
linearLayout = (LinearLayout)findViewById(R.id.main_layout_id);
FragmentManager fragmentManager = getFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
ColorFragment colorFragment = new ColorFragment();
fragmentTransaction.add(R.id.fragment_container, colorFragment);
fragmentTransaction.commit();
}
@Override
public void colorChanged(String colorname) {
if(colorname.equals("RED")) {
linearLayout.setBackgroundColor(Color.RED);
}
else if(colorname.equals("GREEN")) {
linearLayout.setBackgroundColor(Color.GREEN);
}
else if(colorname.equals("BLUE")){
linearLayout.setBackgroundColor(Color.BLUE);
}
else if(colorname.equals("MAGENTA")) {
linearLayout.setBackgroundColor(0xffff00ff);
}
else if(colorname.equals("YELLOW")) {
linearLayout.setBackgroundColor(0xffffff00);
}
}
}
现在是 fragment 类:
public class ColorFragment extends Fragment {
OnColorChangeListener onColorChangeListener;
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View view;
view = inflater.inflate(R.layout.color_fragment_layout, container, false);
RadioGroup radioButtonGroup = (RadioGroup)view.findViewById(R.id.color_radio_group);
radioButtonGroup.setOnCheckedChangeListener(new RadioGroup.OnCheckedChangeListener(){
@Override
public void onCheckedChanged(RadioGroup group, int checkIdOfButton) {
switch (checkIdOfButton){
case R.id.red_id:
onColorChangeListener.colorChanged("RED");
break;
case R.id.green_id:
onColorChangeListener.colorChanged("GREEN");
break;
case R.id.blue_id:
onColorChangeListener.colorChanged("BLUE");
break;
case R.id.magenta_id:
onColorChangeListener.colorChanged("MAGENTA");
break;
case R.id.yellow_id:
onColorChangeListener.colorChanged("YELLOW");
break;
}
}
});
return view;
}
@Override
public void onAttach(Context context) {
super.onAttach(context);
try {
onColorChangeListener = (OnColorChangeListener) context;
}
catch catch (Exception e){}
}
}
public interface OnColorChangeListener
{
public void colorChanged(String colorname);
}
}
这是 XML Activity :
<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:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:orientation="vertical"
app:layout_behavior="@string/appbar_scrolling_view_behavior"
tools:context="com.example.user.seriousapp.Activity5"
tools:showIn="@layout/activity_5"
android:id="@+id/main_layout_id">
<TextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:textAppearance="?android:attr/textAppearanceLarge"
android:text="Fragment Communication Example"
android:id="@+id/textView2"
android:textColor="#000000"/>
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="300dp"
android:id="@+id/fragment_container"
android:layout_marginTop="60dp"
android:layout_marginLeft="20dp"
android:layout_marginRight="20dp"
android:gravity="center"></RelativeLayout>
</LinearLayout>
最后是 XML fragment :
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="250dp"
android:background="#000000">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceLarge"
android:text="Select a Color"
android:textColor="#ffffff"
android:id="@+id/textView3"
android:layout_gravity="center_horizontal" />
<RadioGroup
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="25dp"
android:layout_marginLeft="25dp"
android:layout_marginRight="25dp"
android:orientation="vertical"
android:id="@+id/color_radio_group">
<RadioButton
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="RED"
android:textColor="#ffffff"
android:buttonTint="#ffffff"
android:id="@+id/red_id" />
<RadioButton
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="GREEN"
android:textColor="#ffffff"
android:buttonTint="#ffffff"
android:id="@+id/green_id" />
<RadioButton
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="BLUE"
android:textColor="#ffffff"
android:buttonTint="#ffffff"
android:id="@+id/blue_id" />
<RadioButton
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="MAGENTA"
android:textColor="#ffffff"
android:buttonTint="#ffffff"
android:id="@+id/magenta_id" />
<RadioButton
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="YELLOW"
android:textColor="#ffffff"
android:buttonTint="#ffffff"
android:id="@+id/yellow_id" />
</RadioGroup>
</LinearLayout>