Question

我创建了我的所有应用，但有一个ID在usersList = (ListView)findViewById(R.id.userList);这个地方给了我问题。我的布局中有这个id，但它现在正在获取它。一切都很好。我试图改变id，但它也没有用。

Users.java：

public class Users extends AppCompatActivity {
    ListView usersList;
    TextView noUsersText;
    ArrayList<String> al = new ArrayList<>();
    int totalUsers = 0;
    ProgressDialog pd;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_users);

        usersList = (ListView)findViewById(R.id.userList);
        noUsersText = (TextView)findViewById(R.id.noUsersText);

        pd = new ProgressDialog(Users.this);
        pd.setMessage("Loading...");
        pd.show();

        String url = "https://androidchatapp-76776.firebaseio.com/users.json";

        StringRequest request = new StringRequest(Request.Method.GET, url, new Response.Listener<String>(){
            @Override
            public void onResponse(String s) {
                doOnSuccess(s);
            }
        },new Response.ErrorListener(){
            @Override
            public void onErrorResponse(VolleyError volleyError) {
                System.out.println("" + volleyError);
            }
        });

        RequestQueue rQueue = Volley.newRequestQueue(Users.this);
        rQueue.add(request);

        usersList.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                UserDetails.chatWith = al.get(position);
                startActivity(new Intent(Users.this, ChatActivity.class));
            }
        });
    }

    public void doOnSuccess(String s){
        try {
            JSONObject obj = new JSONObject(s);

            Iterator i = obj.keys();
            String key = "";

            while(i.hasNext()){
                key = i.next().toString();

                if(!key.equals(UserDetails.username)) {
                    al.add(key);
                }

                totalUsers++;
            }

        } catch (JSONException e) {
            e.printStackTrace();
        }

        if(totalUsers <=1){
            noUsersText.setVisibility(View.VISIBLE);
            usersList.setVisibility(View.GONE);
        }
        else{
            noUsersText.setVisibility(View.GONE);
            usersList.setVisibility(View.VISIBLE);
            usersList.setAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, al));
        }

        pd.dismiss();
    }
}

activity_users：

<?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: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"
    tools:context="eras.ppt.ras.Users"
    android:orientation="vertical">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="No users found!"
        android:id="@+id/noUsersText"
        android:visibility="gone"/>

    <ListView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/usersList"/>
</LinearLayout>

Answer 1

改变这个：

 usersList = (ListView)findViewById(R.id.userList);

到此：

 usersList = (ListView)findViewById(R.id.usersList);

您忘记了列表视图的ID名称中的s，应该是usersList

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