java - 如何将 ArrayList 转换为 ArrayList？

Question

我目前正在尝试为 SQLite 数据库的 ListView 创建搜索功能。但要做到这一点，我需要将数组列表“userList”中的数据库值转换为字符串，以便可以在菜单中使用它？我需要 userList 是一个字符串数组，以便我可以在搜索功能中使用它们。

编辑:我可以不这样做而只将“Id Entry”变量放入数组列表中吗？我怎样才能做到这一点？

这是我的代码:

public class ViewListContents extends AppCompatActivity {


    DatabaseHelper deviceDB;
    ArrayList<User> userList;
    //ArrayList<String> listItem;

    ListView listView;
    User user;


    ArrayAdapter adapter1;
    ArrayList<String> list;


    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.viewcontents_layout); //sets layout

        deviceDB = new DatabaseHelper(this); //initialises DB for database class
        userList = new ArrayList<>();
        Cursor data = deviceDB.getListContents();


        list = new ArrayList<>();
        //FiveColumn_ListAdapter adapter1 = new FiveColumn_ListAdapter(this,R.layout.list_adapter_view,list);


        int numRows = data.getCount();
        if (numRows == 0 ){
            Toast.makeText(ViewListContents.this,"The database was empty",Toast.LENGTH_LONG).show();
        }
        else {
            while (data.moveToNext()){
                user = new User(data.getString(0),data.getString(1),data.getString(2),data.getString(3),data.getString(4),data.getString(5)); //retrieving columns from data
                userList.add(user); //userlist array takes items of type user
            }
        }

        FiveColumn_ListAdapter adapter = new FiveColumn_ListAdapter(this,R.layout.list_adapter_view,userList);
        listView = (ListView) findViewById(R.id.ListView);
        listView.setAdapter(adapter);




    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.menu, menu);

        MenuItem searchItem = menu.findItem(R.id.item_search);
        SearchView searchView = (SearchView) MenuItemCompat.getActionView(searchItem);

        searchView.setOnQueryTextListener(new SearchView.OnQueryTextListener() {
            @Override
            public boolean onQueryTextSubmit(String s) {



                return false;
            }

            @Override
            public boolean onQueryTextChange(String s) {

                //ArrayList<String> List = new ArrayList<>();

                for (User user : userList){
                    if (user.
                }

                return true;
            }
        });




        return super.onCreateOptionsMenu(menu);
    }

}

User 是我为获取此处的变量而创建的自定义类:

public User (String fID_Entry, String fName, String fAddress, String fLatitude, String fLongtitude, String fTime){
        ID_Entry = fID_Entry;
        Name = fName;
        Address = fAddress;
        Latitude = fLatitude;
        Longtitude = fLongtitude;
        Time = fTime;

    }

    public String getID_Entry() {
        return ID_Entry;
    }

    public String getName() {
        return Name;
    }

    public String getAddress() {

        return Address;
    }

    public String getLatitude() {

        return Latitude;
    }

    public String getLongtitude() {

        return Longtitude;
    }
    public String getTime() {
        return Time;
    }
}

Answer 1

重写 toString 方法或创建您自己的方法来获取 User 类中的类 String 表示形式，然后您可以执行以下操作:

List<String> usersStrings = userList.stream().map(User::representation).collect(Collectors.toList());

或者例如:

ArrayList<String> idList = new ArrayList<>();
for (int i = 0; i <  user_list.size(); i++) {
    // This line depend on how you want to represent your user string
    idList.add(user_list.get(i).getID_Entry());
}

我还更改了您的字段名称。它们应该始终以小​​写字母开头，以免与类混淆。而且也不需要前缀，因为大多数 IDE 都会为您着色。尝试对字段和方法坚持使用驼峰命名法，对类使用帕斯卡命名法。

https://en.wikipedia.org/wiki/Camel_case

class User {

        String idEntry;
        String name;
        String address;
        String latitude;
        String longtitude;
        String time;

        public String representation() {
            // This could be implemented as you want
            return name + " " + idEntry;
        }

        public User (String fID_Entry, String fName, String fAddress, String fLatitude, String fLongtitude, String fTime){
            idEntry = fID_Entry;
            name = fName;
            address = fAddress;
            latitude = fLatitude;
            longtitude = fLongtitude;
            time = fTime;
        }

        public String getID_Entry() {
            return idEntry;
        }

        public String toString() {
            // This is also used when you will try to print the object
            // You can change how you want it to look
            return "User{" +
                    "idEntry='" + idEntry + '\'' +
                    ", name='" + name + '\'' +
                    ", address='" + address + '\'' +
                    ", latitude='" + latitude + '\'' +
                    ", longtitude='" + longtitude + '\'' +
                    ", time='" + time + '\'' +
                    '}';
        }

        public String getName() {
            return name;
        }

        public String getAddress() {
            return address;
        }

        public String getLatitude() {
            return latitude;
        }

        public String getLongtitude() {
            return longtitude;
        }

        public String getTime() {
            return time;
        }
    }

请注意，当调用第一个代码段中的代码时，您的用户将表示为他们的姓名 + ""+ id。如果您想要不同的东西，您可以调整表示方法。