I have an ArrayList with custom objects. Each custom object contains a variety of strings and numbers. I need the array to stick around even if the user leaves the activity and then wants to come back at a later time, however I don't need the array available after the application has been closed completely. I save a lot of other objects this way by using the SharedPreferences but I can't figure out how to save my entire array this way. Is this possible? Maybe SharedPreferences isn't the way to go about this? Is there a simpler method?


当前回答

还有Kotlin:

fun SharedPreferences.Editor.putIntegerArrayList(key: String, list: ArrayList<Int>?): SharedPreferences.Editor {
    putString(key, list?.joinToString(",") ?: "")
    return this
}

fun SharedPreferences.getIntegerArrayList(key: String, defValue: ArrayList<Int>?): ArrayList<Int>? {
    val value = getString(key, null)
    if (value.isNullOrBlank())
        return defValue
    return ArrayList (value.split(",").map { it.toInt() }) 
}

其他回答

以防有人需要保存列表的列表,即list < list < String > >。我是这样做的:

序列化

Gson gson = new Gson();
// Save the size of the array
sharedPreferencesEditor.putInt("ArraySize", myArray.size());

for (int i=0; i<myArray.size(); i++) {
  String key = "Array"+i;
  String json = gson.toJson(myArray.get(i));
  sharedPreferencesEditor.putString(key, json);
}

sharedPreferencesEditor.commit();

反序列化

// Get the size of the array to be deserialized. In my case, the default number should be 3
int arraySize = sharedPreferences.getInt("ArraySize",3);

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

for (int i=0; i<arraySize; i++) {
  String key = "Array"+i;
  String json = sharedPreferences.getString(key, null);
  List<String> arrayTemp= gson.fromJson(json, List.class);
  myArray.add(arrayTemp);
}

// My array may also include components with empty strings. 
// Gson makes them null values and it is not possible 
// to deserialize them as empty strings. 
// The following takes care of that:

for (int i=0; i<myArray.size();i++) {
   if (myArray.get(i) == null) {
      List<String> emptyComponent = new ArrayList<String>() {
         { add(""); }
      };
      myArray.set(i,emptyComponent);
   }
}

您可以将其转换为JSON字符串,并将字符串存储在SharedPreferences中。

在API 11之后,SharedPreferences Editor接受set。您可以将List转换为HashSet或类似的东西,并像这样存储它。当你把它读回来时,把它转换成一个数组列表,如果需要的话对它排序,你就可以开始了。

//Retrieve the values
Set<String> set = myScores.getStringSet("key", null);

//Set the values
Set<String> set = new HashSet<String>();
set.addAll(listOfExistingScores);
scoreEditor.putStringSet("key", set);
scoreEditor.commit();

你也可以序列化你的数组列表,然后将它保存/读取到SharedPreferences。解决方案如下:

编辑: 下面是将ArrayList作为一个序列化对象保存到SharedPreferences的解决方案,然后从SharedPreferences中读取它。

因为API只支持在SharedPreferences中存储和检索字符串(在API 11之后,它更简单),我们必须序列化和反序列化ArrayList对象,它将任务列表变成一个字符串。

在TaskManagerApplication类的addTask()方法中,我们必须获取共享首选项的实例,然后使用putString()方法存储序列化的ArrayList:

public void addTask(Task t) {
  if (null == currentTasks) {
    currentTasks = new ArrayList<task>();
  }
  currentTasks.add(t);
 
  // save the task list to preference
  SharedPreferences prefs = getSharedPreferences(SHARED_PREFS_FILE, Context.MODE_PRIVATE);
  Editor editor = prefs.edit();
  try {
    editor.putString(TASKS, ObjectSerializer.serialize(currentTasks));
  } catch (IOException e) {
    e.printStackTrace();
  }
  editor.commit();
}

类似地,我们必须从onCreate()方法中的首选项中检索任务列表:

public void onCreate() {
  super.onCreate();
  if (null == currentTasks) {
    currentTasks = new ArrayList<task>();
  }
 
  // load tasks from preference
  SharedPreferences prefs = getSharedPreferences(SHARED_PREFS_FILE, Context.MODE_PRIVATE);
 
  try {
    currentTasks = (ArrayList<task>) ObjectSerializer.deserialize(prefs.getString(TASKS, ObjectSerializer.serialize(new ArrayList<task>())));
  } catch (IOException e) {
    e.printStackTrace();
  } catch (ClassNotFoundException e) {
    e.printStackTrace();
  }
}

您可以从Apache Pig项目ObjectSerializer.java中获得ObjectSerializer类

下面的代码是公认的答案,为新手(我)多写几行,例如。说明了如何将set类型对象转换回arrayList,并提供了关于`之前操作的附加指导。putStringSet'和。getstringset '。(谢谢你邪恶)

// shared preferences
   private SharedPreferences preferences;
   private SharedPreferences.Editor nsuserdefaults;

// setup persistent data
        preferences = this.getSharedPreferences("MyPreferences", MainActivity.MODE_PRIVATE);
        nsuserdefaults = preferences.edit();

        arrayOfMemberUrlsUserIsFollowing = new ArrayList<String>();
        //Retrieve followers from sharedPreferences
        Set<String> set = preferences.getStringSet("following", null);

        if (set == null) {
            // lazy instantiate array
            arrayOfMemberUrlsUserIsFollowing = new ArrayList<String>();
        } else {
            // there is data from previous run
            arrayOfMemberUrlsUserIsFollowing = new ArrayList<>(set);
        }

// convert arraylist to set, and save arrayOfMemberUrlsUserIsFollowing to nsuserdefaults
                Set<String> set = new HashSet<String>();
                set.addAll(arrayOfMemberUrlsUserIsFollowing);
                nsuserdefaults.putStringSet("following", set);
                nsuserdefaults.commit();

使用Kotlin,对于简单的数组和列表,你可以这样做:

class MyPrefs(context: Context) {
    val prefs = context.getSharedPreferences("x.y.z.PREFS_FILENAME", 0)
    var listOfFloats: List<Float>
        get() = prefs.getString("listOfFloats", "").split(",").map { it.toFloat() }
        set(value) = prefs.edit().putString("listOfFloats", value.joinToString(",")).apply()
}

然后轻松访问首选项:

MyPrefs(context).listOfFloats = ....
val list = MyPrefs(context).listOfFloats