Java 来自其他类的Arraylist为空
我正在尝试从其他类访问Arraylist。这可以工作,但Arraylist始终为空Java 来自其他类的Arraylist为空,java,class,arraylist,constructor,Java,Class,Arraylist,Constructor,我正在尝试从其他类访问Arraylist。这可以工作,但Arraylist始终为空 public class CategoryFragment extends Fragment { private List<Category> lsCategory; public CategoryFragment() { // Required empty public constructor } @Override public View onCreateView(LayoutInf
public class CategoryFragment extends Fragment {
private List<Category> lsCategory;
public CategoryFragment() {
// Required empty public constructor
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
lsCategory = new ArrayList<>();
lsCategory.add(new Category(1, getString(R.string.cat_school), R.drawable.cat_school_filled_48dp));
lsCategory.add(new Category(2, getString(R.string.cat_food), R.drawable.cat_food_filled_48dp));
lsCategory.add(new Category(3, getString(R.string.cat_sports), R.drawable.cat_sports_filled_48dp));
lsCategory.add(new Category(4, getString(R.string.cat_travel), R.drawable.cat_travel_filled_48dp));
lsCategory.add(new Category(5, getString(R.string.cat_hobbies), R.drawable.cat_sports_filled_48dp));
lsCategory.add(new Category(6, getString(R.string.cat_countries), R.drawable.cat_countries_filled_48dp));
lsCategory.add(new Category(7, getString(R.string.cat_numbers), R.drawable.cat_countries_filled_48dp));
lsCategory.add(new Category(8, getString(R.string.cat_animals), R.drawable.cat_animals_filled_48dp));
lsCategory.add(new Category(9, getString(R.string.cat_nature), R.drawable.cat_nature_filled_48dp));
View v = inflater.inflate(R.layout.fragment_category, container, false);
RecyclerView myRv = (RecyclerView) v.findViewById(R.id.main_recyclerView);
CategoryRecyclerViewAdapter myAdapter = new CategoryRecyclerViewAdapter(getActivity(), lsCategory);
myRv.setLayoutManager(new GridLayoutManager(getActivity(), 3));
myRv.setAdapter(myAdapter);
return v;
}
public List<Category> getLsCategory() {
return lsCategory;
}
公共类CategoryFragment扩展了片段{
私人名单类别;
公共类别碎片(){
//必需的空公共构造函数
}
@凌驾
创建视图上的公共视图(布局、充气机、视图组容器、,
Bundle savedInstanceState){
lsCategory=newarraylist();
lsCategory.add(新类别(1,getString(R.string.cat_school),R.drawable.cat_school_filled_48dp));
lsCategory.add(新类别(2,getString(R.string.cat_food),R.drawable.cat_food_filled_48dp));
lsCategory.add(新类别(3,getString(R.string.cat_sports),R.drawable.cat_sports_filled_48dp));
lsCategory.add(新类别(4,getString(R.string.cat_travel),R.drawable.cat_travel_filled_48dp));
lsCategory.add(新类别(5,getString(R.string.cat_嗜好),R.drawable.cat_体育填充(48dp));
lsCategory.add(新类别(6,getString(R.string.cat_countries),R.drawable.cat_countries_filled_48dp));
lsCategory.add(新类别(7,getString(R.string.cat_编号),R.drawable.cat_countries_filled_48dp));
lsCategory.add(新类别(8,getString(R.string.cat_animals),R.drawable.cat_animals_filled_48dp));
添加(新类别(9,getString(R.string.cat_nature),R.drawable.cat_nature_filled_48dp));
视图v=充气机。充气(R.layout.fragment_类别,容器,错误);
RecyclerView myRv=(RecyclerView)v.findViewById(R.id.main\u RecyclerView);
CategoryRecyclServiceAdapter myAdapter=新的CategoryRecyclServiceAdapter(getActivity(),lsCategory);
setLayoutManager(新的GridLayoutManager(getActivity(),3));
myRv.setAdapter(myAdapter);
返回v;
}
公共列表getLsCategory(){
返回类别;
}
}
在另一个类中,我尝试访问此Arraylist
public class FlashcardFragment extends Fragment {
private List<Flashcard> lsFlashcard;
private CategoryFragment categoryFragment;
public FlashcardFragment() {
// Required empty public constructor
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
lsFlashcard = new ArrayList<>();
categoryFragment = new CategoryFragment();
List<Category> calledList = categoryFragment.getLsCategory();
if(calledList != null && !calledList.isEmpty()){
for(int i = 0; i < calledList.size(); i++){
int number = calledList.get(i).getId();
if(number == 1){
lsFlashcard.add(new Flashcard(1, getString(R.string.school_backpack), R.drawable.school_backpack));
lsFlashcard.add(new Flashcard(2, getString(R.string.school_textbook), R.drawable.school_textbook));
lsFlashcard.add(new Flashcard(3, getString(R.string.school_book), R.drawable.school_book));
lsFlashcard.add(new Flashcard(4, getString(R.string.school_workbook), R.drawable.school_workbook));
lsFlashcard.add(new Flashcard(5, getString(R.string.school_file), R.drawable.school_file));
}
else if(number == 2){
lsFlashcard.add(new Flashcard(1, getString(R.string.school_backpack), R.drawable.school_backpack));
lsFlashcard.add(new Flashcard(2, getString(R.string.school_textbook), R.drawable.school_textbook));
lsFlashcard.add(new Flashcard(3, getString(R.string.school_book), R.drawable.school_book));
lsFlashcard.add(new Flashcard(4, getString(R.string.school_workbook), R.drawable.school_workbook));
lsFlashcard.add(new Flashcard(5, getString(R.string.school_file), R.drawable.school_file));
}
else{
lsFlashcard.add(new Flashcard(1, getString(R.string.school_backpack), R.drawable.school_backpack));
lsFlashcard.add(new Flashcard(2, getString(R.string.school_textbook), R.drawable.school_textbook));
lsFlashcard.add(new Flashcard(3, getString(R.string.school_book), R.drawable.school_book));
lsFlashcard.add(new Flashcard(4, getString(R.string.school_workbook), R.drawable.school_workbook));
lsFlashcard.add(new Flashcard(5, getString(R.string.school_file), R.drawable.school_file));
}
}
}
else{
}
View v = inflater.inflate(R.layout.fragment_flashcard, container, false);
RecyclerView myRv = (RecyclerView) v.findViewById(R.id.main_recyclerView);
FlashcardRecyclerViewAdapter myAdapter = new FlashcardRecyclerViewAdapter(getActivity(), lsFlashcard);
myRv.setLayoutManager(new GridLayoutManager(getActivity(), 3));
myRv.setAdapter(myAdapter);
return v;
}
公共类FlashcardFragment扩展了片段{
私人名单和信用卡;
私人类别分部类别分部;
公共FlashcardFragment(){
//必需的空公共构造函数
}
@凌驾
创建视图上的公共视图(布局、充气机、视图组容器、,
Bundle savedInstanceState){
lsFlashcard=newarraylist();
categoryFragment=新的categoryFragment();
List calledList=categoryFragment.getLsCategory();
if(calledList!=null&&!calledList.isEmpty()){
对于(int i=0;i
}
我得到的错误是:
java.lang.NullPointerException:尝试对空对象引用调用接口方法“boolean java.util.List.add(java.lang.Object)”,这是因为您正在创建
CategoryFragment
的新实例并调用其getLsCategory()代码>方法
categoryFragment = new CategoryFragment();
List<Category> calledList = categoryFragment.getLsCategory();
categoryFragment=新的categoryFragment();
List calledList=categoryFragment.getLsCategory();
这里
私有列表类别
是一个实例变量,因此类的每个实例都有自己的lsCategory
副本
如果希望此变量对所有实例都相同,请将其设置为staticprivate static List lsCategory
您正在创建一个新CategoryFragment()
,而不是调用带有填充列表的类别。