英文:
SharedPreference in a separate class?
问题
我正在尝试创建一个包含保存和加载对象ArrayList到SharedPreferences的方法的类。然而，当我尝试加载之前保存的数据时，就好像它没有保存过一样。我正在使用Google的Gson库来帮助处理。
**Instance_Read_Write.java**
public class Instance_Read_Write {
public void saveData(Context context, ArrayList<Example> Examples){
SharedPreferences sharedPreferences = context.getSharedPreferences("instanceKey",Context.MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPreferences.edit();
Gson gson = new Gson();
String json = gson.toJson(Examples);
editor.putString("list", json);
editor.apply();
}
public void loadData(Context context, ArrayList<Example> Examples) {
SharedPreferences sharedPreferences = context.getSharedPreferences("instanceKey",Context.MODE_PRIVATE);
Gson gson = new Gson();
String json = sharedPreferences.getString("list", null);
Type type = new TypeToken<ArrayList<Example>>() {}.getType();
Examples = gson.fromJson(json, type);
if (Examples == null) {
Examples = new ArrayList<>();
}
}
}
这是我如何引用它们的方式。
MainActivity.java
Instance_Read_Write instance_read_write = new Instance_Read_Write();
instance_read_write.loadData(context, Examples);
Examples.add(new Example("test"));
// ...
instance_read_write.saveData(context, Examples);
有办法使这个工作吗？或者也许有更好的方法来做到这一点吗？
<details>
<summary>英文:</summary>
I am attempting to create a class that will contain methods for saving and loading an ArrayList of objects into SharedPreferences. However, when I attempt to load back data I had previously saved. It is as if it has not saved. I am using Googles Gson Libraries to help.
**Instance_Read_Write.java**
public class Instance_Read_Write {
public void saveData(Context context, ArrayList<Example> Examples){
SharedPreferences sharedPreferences = context.getSharedPreferences("instanceKey",Context.MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPreferences.edit();
Gson gson = new Gson();
String json = gson.toJson(Examples);
editor.putString("list", json);
editor.apply();
}
public void loadData(Context context, ArrayList<Example> Examples) {
SharedPreferences sharedPreferences = context.getSharedPreferences("instanceKey",Context.MODE_PRIVATE);
Gson gson = new Gson();
String json = sharedPreferences.getString("list", null);
Type type = new TypeToken<ArrayList<Example>>() {}.getType();
Examples = gson.fromJson(json, type);
if (Examples == null) {
Examples = new ArrayList<>();
}
}
}
and this is how I am referencing them.
**MainActivity.java**
Instance_Read_Write instance_read_write = new Instance_Read_Write();
instance_read_write.loadData(context,Examples);
Examples.add(new Example("test"));
...
...
...
instance_read_write.saveData(context,Examples);
Is there a way to make this work? or perhaps a better way to do this?
</details>
# 答案1
**得分**: 0
将"Instead of the Token for the Type, put"翻译为：
用于类型的令牌，替换为
<details>
<summary>英文:</summary>
Instead of the Token for the Type, put
Examples = gson.fromJson(json, Example.class);
Tell me if it works
</details>
# 答案2
**得分**: 0
这解决了您发布的代码中的一个错误，即在`loadData`方法中处理`Examples`参数的方法。
由于您的`loadData`方法的意图似乎是始终生成一个ArrayList<Example>，因此我建议返回该实例。或者，您可以首先创建一个空的ArrayList<Example>，将其传递进去，然后对其进行操作。
因此，使用返回ArrayList<Example>的方法：
```java
public ArrayList<Example> loadData(Context context) {
ArrayList<Example> result;
SharedPreferences sharedPreferences = context.getSharedPreferences("instanceKey", Context.MODE_PRIVATE);
Gson gson = new Gson();
String json = sharedPreferences.getString("list", null);
Type type = new TypeToken<ArrayList<Example>>() {}.getType();
result = gson.fromJson(json, type);
if (result == null) {
result = new ArrayList<>();
}
return result;
}
// ... 调用代码（在MainActivity中）如下：
Examples = instance_read_write.loadData(context);
不想引发传值 vs 传引用的讨论，似乎您期望通过在方法体中对其赋值来更新Example实际参数 - 但实际上不会这样 - 您只能对对象进行操作以影响它：https://stackoverflow.com/a/40523/2711811。
为了完成这个主题，这里有一个解决方案，可以继续传入Examples对象的意图：
// 在调用方法中...
Examples = new ArrayList<Example>;
instance_read_write.loadData(context, Examples);
//...在类Instance_Read_Write中...
public void loadData(Context context, ArrayList<Example> Examples) {
SharedPreferences sharedPreferences = context.getSharedPreferences("instanceKey", Context.MODE_PRIVATE);
Gson gson = new Gson();
String json = sharedPreferences.getString("list", null);
Type type = new TypeToken<ArrayList<Example>>() {}.getType();
ArrayList<Example> t = gson.fromJson(json, type);
if (t != null) {
Examples.addAll(t);
}
}
<details>
<summary>英文:</summary>
This addresses one error in your posted code namely the treatment of the `Examples` parameter in the method `loadData` method.
Since it seems your intention of the `loadData` is to always produce an ArrayList<Example> then I'd recommend returning the instance. Alternatively, you would have to first create an empty ArrayList<Example> and pass that in and then operate on it.
So using the approach of returning the ArrayList<Example>:
public ArrayList<Example> loadData(Context context) {
ArrayList<Example> result;
SharedPreferences sharedPreferences = context.getSharedPreferences("instanceKey",Context.MODE_PRIVATE);
Gson gson = new Gson();
String json = sharedPreferences.getString("list", null);
Type type = new TypeToken<ArrayList<Example>>() {}.getType();
result = gson.fromJson(json, type);
if (result == null) {
result = new ArrayList<>();
}
return result;
}
// ... and the calling code (in MainActivity) looks like:
Examples = instance_read_write.loadData(context);
Not to stoke up a pass-by-value vs pass-by-reference discussion - it appears you are expecting `Example` actual parameter to be updated by assigning it in the body of the method - which it will not - you can only operate on the object to affect it: https://stackoverflow.com/a/40523/2711811.
To complete the topic, here's a solution which would continue your intention of passing in the `Examples` object:
// In calling method...
Examples = new ArrayList<Example>;
instance_read_write.loadData(context,Examples);
//...in class Instance_Read_Write...
public void loadData(Context context, ArrayList<Example> Examples) {
SharedPreferences sharedPreferences = context.getSharedPreferences("instanceKey",Context.MODE_PRIVATE);
Gson gson = new Gson();
String json = sharedPreferences.getString("list", null);
Type type = new TypeToken<ArrayList<Example>>() {}.getType();
ArrayList<Example> t = gson.fromJson(json, type);
if (t != null) {
Examples.addAll(t);
}
}
</details>
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