我正在创建一个带有自定义适配器的ListView。 在客户适配器类中，我将Context对象传递给它的超类，并在为Adapter类创建对象时将Context对象传递给MainActivity 。
它显示以下错误
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mylistpractice/com.mylistpractice.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference现在，我从MainActivity传递参数时传递getApplicationContext()而不是Context对象，并且它不显示任何错误。
我对传递Context对象和传递getApplicationContext()之间的区别感到困惑。
但它没有在ListView中显示任何数据。
这是Adapter类
public class Myadapter extends ArrayAdapter<Data> { ArrayList<Data> arraydata; Context context; public Myadapter(ArrayList<Data> arraydata,Context context) { super(context,R.layout.content,arraydata); this.arraydata=arraydata; this.context=context; } @Override public int getCount() { return 0; } @Override public long getItemId(int position) { return 0; } @Override public View getView(int position, View convertView, ViewGroup parent) { Data data=getItem(position); Viewholder viewholder; View vi=convertView; if(vi==null) { viewholder=new Viewholder(); LayoutInflater inflater = LayoutInflater.from(getContext()); vi = inflater.inflate(R.layout.content, parent, false); viewholder.name=(TextView)vi.findViewById(R.id.name); viewholder.id=(TextView)vi.findViewById(R.id.id); vi.setTag(viewholder); } else { viewholder=(Viewholder)vi.getTag(); } viewholder.name.setText(data.getName()); viewholder.id.setText(data.getId()); return vi; } public class Viewholder { TextView id; TextView name; } }这是我的MainActivity
public class MainActivity extends AppCompatActivity { ListView list; ArrayList<Data> arraydata=new ArrayList<>(); Myadapter myadapter; Context context; Data data; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); list=(ListView)findViewById(R.id.list); setList(); myadapter = new Myadapter(arraydata,MainActivity.this); list.setAdapter(myadapter); } public void setList() { for(int i=0;i<15;i++) { String s,id; data = new Data(); data.setName("Farhana:::"+i); s = data.getName(); data.setId("Id:::"+i); id = data.getId(); arraydata.add(data); Log.e("DAtA:--",s); Log.e("ID:--",id); } } }I'm creating a ListView with a custom adapter. In the customer adapter class, I'm passing a Context object to its super class and also passing a Context object to the MainActivity while creating the object for the Adapter class.
It shows the following error
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mylistpractice/com.mylistpractice.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object referenceNow I pass getApplicationContext() rather than the Context object while passing the argument from MainActivity and it doesn't show any error.
I am confused about the difference between passing a Context object and passing getApplicationContext().
But it's not showing any data in the ListView.
This is the Adapter class
public class Myadapter extends ArrayAdapter<Data> { ArrayList<Data> arraydata; Context context; public Myadapter(ArrayList<Data> arraydata,Context context) { super(context,R.layout.content,arraydata); this.arraydata=arraydata; this.context=context; } @Override public int getCount() { return 0; } @Override public long getItemId(int position) { return 0; } @Override public View getView(int position, View convertView, ViewGroup parent) { Data data=getItem(position); Viewholder viewholder; View vi=convertView; if(vi==null) { viewholder=new Viewholder(); LayoutInflater inflater = LayoutInflater.from(getContext()); vi = inflater.inflate(R.layout.content, parent, false); viewholder.name=(TextView)vi.findViewById(R.id.name); viewholder.id=(TextView)vi.findViewById(R.id.id); vi.setTag(viewholder); } else { viewholder=(Viewholder)vi.getTag(); } viewholder.name.setText(data.getName()); viewholder.id.setText(data.getId()); return vi; } public class Viewholder { TextView id; TextView name; } }this is my MainActivity
public class MainActivity extends AppCompatActivity { ListView list; ArrayList<Data> arraydata=new ArrayList<>(); Myadapter myadapter; Context context; Data data; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); list=(ListView)findViewById(R.id.list); setList(); myadapter = new Myadapter(arraydata,MainActivity.this); list.setAdapter(myadapter); } public void setList() { for(int i=0;i<15;i++) { String s,id; data = new Data(); data.setName("Farhana:::"+i); s = data.getName(); data.setId("Id:::"+i); id = data.getId(); arraydata.add(data); Log.e("DAtA:--",s); Log.e("ID:--",id); } } }最满意答案
但没有在listview中显示任何数据
因为你告诉它不要创建任何视图。
@Override public int getCount() { return 0; }您正在使用ArrayAdapter。 您不需要实现getCount ，但如果这样做，则返回列表的大小
@Override public int getCount() { return arraydata == null ? 0 : arraydata.size(); }无论如何，关于你的问题，倾向于使用MainActivity.this就像你为Context（活动是上下文）所做的那样。
如果您的错误在这里
LayoutInflater inflater = LayoutInflater.from(getContext());然后，这可能是
LayoutInflater inflater = LayoutInflater.from(this.context);你从构造函数中获取了上下文
but not showing anydata in listview
Because you told it not to create any views.
@Override public int getCount() { return 0; }You are using an ArrayAdapter. You don't need to implement getCount, but if you do, return the size of the list
@Override public int getCount() { return arraydata == null ? 0 : arraydata.size(); }Anyways, regarding your question, tend to use MainActivity.this like you have done for the Context (Activities are Contexts).
If your error is here
LayoutInflater inflater = LayoutInflater.from(getContext());Then, that could instead be
LayoutInflater inflater = LayoutInflater.from(this.context);which you got the context from the constructor
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