Question

我正在尝试在ArrayList中保存一个SharedPreferences，但出现错误

java.lang.ClassCastException：无法将java.util.ArrayList强制转换为


com.example.host.myapplication.adapter.DataModel     这是我的代码示例。

DataModel.java

public class DataModel extends ArrayList<String> {

    public String name;
    public boolean checked;

    public DataModel(String name, boolean checked) {
        this.name = name;
        this.checked = checked;
    }
}

PickCategoryActivity

//...
 adapter = new CustomCategoryAdapter(getArrayList("CATEGORY"), getApplicationContext());
//... 
public void saveArrayList(ArrayList<DataModel> list, String key){
    SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this.getApplicationContext());
    SharedPreferences.Editor editor = prefs.edit();
    Gson gson = new Gson();
    String json = gson.toJson(list);
    editor.putString(key, json);
    editor.apply(); 
}

public ArrayList<DataModel> getArrayList(String key){
    SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this.getApplicationContext());
    Gson gson = new Gson();
    String json = prefs.getString(key, null);
    Type type = new TypeToken<ArrayList<DataModel>>() {}.getType();
    return gson.fromJson(json, type);
}

CustomCategoryAdapter.java

private ArrayList dataSet;
    Context mContext;

    // View lookup cache
    private static class ViewHolder {
        TextView txtName;
        CheckBox checkBox;
    }

    public CustomCategoryAdapter(ArrayList<DataModel> data, Context context) {
        super(context, R.layout.activity_pick_category_row_list, data);
        this.dataSet = data;
        this.mContext = context;

    }

    @Override
    public DataModel getItem(int position) {
        return (DataModel) dataSet.get(position); // i get an error here
    }
//...

错误logcat：

java.lang.ClassCastException：无法将java.util.ArrayList强制转换为 com.example.host.myapplication.adapter.DataModel 在com.example.host.myapplication.adapter.CustomCategoryAdapter.getItem（CustomCategoryAdapter.java:45）在com.example.host.myapplication.adapter.CustomCategoryAdapter.getView（CustomCategoryAdapter.java:69）

Answer 1

问题出在您的CustomCategoryAdapter

您已将dataSet声明为ArrayList，但在您的CustomCategoryAdapter构造函数中，参数是ArrayList<DataModel>

使用此

ArrayList<DataModel> dataSet

代替此

private ArrayList dataSet;

Answer 2

我正在尝试在ArrayList中保存一个SharedPreferences，但是我得到了   错误


java.lang.ClassCastException：无法将java.util.ArrayList强制转换为


com.example.host.myapplication.adapter.DataModel这是我的代码示例。

那是因为您试图将DataModel转换为ArrayList。

您需要像这样将DataModel更改为简单的pojo：

public class DataModel {

    public String name;
    public boolean checked;

    public DataModel(String name, boolean checked) {
        this.name = name;
        this.checked = checked;
    }
}

无需扩展ArrayList，因为您的模型用于单个数据项。

然后，您需要在以下内容上将saveArrayList和getArrayList方法修改为更具可读性的方法，例如saveCategories和getCategories()：

private static final CATEGORIES_KEY = "CATEGORIES";
public void saveCategories(List<DataModel> list){
    SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this.getApplicationContext());
    SharedPreferences.Editor editor = prefs.edit();
    Gson gson = new Gson();
    String json = gson.toJson(list);
    editor.putString(CATEGORIES_KEY, json);
    editor.apply(); 
}

public List<DataModel> getCategories(){
    SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this.getApplicationContext());

    List<Model> models;
    Gson gson = new Gson();
    String json = prefs.getString(CATEGORIES_KEY, null);
    Type type = new TypeToken<List<DataModel>>() {}.getType();

   try {
     models = gson.fromJson(json, type);
   } catch (IllegalStateException | JsonSyntaxException exception) {
     // You need to catch the error here
     Log.d("CONVERT", exception);
   }

   return models;
}

您会看到键被更改为常量并在方法内部移动。这是因为我们需要向方法调用者隐藏细节。适配器不知道详细信息。

然后您可以使用获取类别列表：

adapter = new CustomCategoryAdapter(getCategories(), getApplicationContext());

Answer 3

SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this.getApplicationContext()); 
String json = new Gson().toJson(yourlist);
prefs .edit().putString("testArray",json).commit();

尝试commit（）;会将其首选项同步写到持久性存储中

将arrayList保存到SharedPreferences

3 个答案: