关闭应用程序后如何保存arraylist

Question

如何在应用程序关闭后保存arraylist。这是我的环球班

public class GlobalClass extends Application {

ArrayList<Trip> allTrips = new ArrayList<>();

public ArrayList<String> allTripsString() {
    ArrayList<String> allTripsString = new ArrayList<String>();
    for (Trip trip : allTrips){
        allTripsString.add(trip.getName());
    }
    return allTripsString;
    }
}

这是我的旅行班

public class Trip {

/** A map with category as key and the associed list of items as value */
Map<String,List<Item>> expanses;

private String name;
public ArrayList<String> ExpensesCategory = new ArrayList<>();
public ArrayList<String> Adults = new ArrayList<>();
public ArrayList<String> Children = new ArrayList<>();
public ArrayList<String> ShoppingName = new ArrayList<>();
public ArrayList<Double> ShoppingPrice = new ArrayList<>();
public ArrayList<String> ShoppingCategory = new ArrayList<>();
public double budget = 0;

/** An item in the expanses list */
static class Item {
    final String name;
    final double cost;
    final String Category;
    public Item(String name, double cost, String Category) {
        this.name = name;
        this.cost = cost;
        this.Category = Category;
    }
    @Override public String toString() {
        return this.name + " (" + this.cost + "$)";
    }
    public String getName(){
        return name;
    }
    public String getCategory(){
        return Category;
    }
    public double getCost(){
        return cost;
    }
}

public Trip(String name) {
    this.name = name;
    this.expanses = new HashMap<String,List<Item>>();
    for (String cat: ExpensesCategory) { // init the categories with empty lists
        this.expanses.put(cat, new ArrayList<Item>());
    }
}

public String getName(){
    return name;
}

/** Register a new expanse to the trip. */
public void add(String item, double cost, String category) {
    List<Item> list = this.expanses.get(category);
    if (list == null)
        throw new IllegalArgumentException("Category '"+category+"' does not exist.");
    list.add( new Item(item, cost, category) );
}

/** Get the expanses, given a category.
 * @return  a fresh ArrayList containing the category elements, or null if the category does not exists
 */
public List<Item> getItems(String category) {
    List<Item> list = this.expanses.get(category);
    if (list == null)
        return null;
    return new ArrayList<Item>(list);
}

/** Get the expanses, given a category.
 * @return  a fresh ArrayList containing all the elements
 */
public List<Item> getItems() {
    List<Item> list = new ArrayList<Item>();
    for (List<Item> l: this.expanses.values()) // fill with each category items
        list.addAll(l);
    return list;
}

public ArrayList<String> getItemsString() {
    List<Item> list = new ArrayList<Item>();
    for (List<Item> l: this.expanses.values()) // fill with each category items
        list.addAll(l);
    ArrayList<String> listString = new ArrayList<String>();
    for (Item item : list){
        listString.add(item.getName());
    }
    return listString;
}

public ArrayList<Double> getItemsCost(){
    List<Item> list = new ArrayList<Item>();
    for (List<Item> l: this.expanses.values()) // fill with each category items
        list.addAll(l);
    ArrayList<Double> listDouble = new ArrayList<>();
    for (Item item : list){
        listDouble.add(item.getCost());
    }
    return listDouble;
}

public ArrayList<String> getItemsCategory() {
    List<Item> list = new ArrayList<Item>();
    for (List<Item> l: this.expanses.values()) // fill with each category items
        list.addAll(l);
    ArrayList<String> listString = new ArrayList<String>();
    for (Item item : list){
        listString.add(item.getCategory());
    }
    return listString;
}

/** Get the total cost, given a category. */
public double getCost(String category) {
    List<Item> list = this.expanses.get(category);
    if (list == null)
        return -1;
    double cost = 0;
    for (Item item: list)
        cost += item.cost;
    return cost;
}

/** Get the total cost. */
public double getCost() {
    double cost = 0;
    for (List<Item> l: this.expanses.values())
        for (Item item: l)
            cost += item.cost;
    cost *= 1000;
    cost = (int)(cost);
    cost /= 1000;
    return cost;
    }
}

我想保存数组列表，当我关闭和打开应用程序的数组列表（所有旅行）被保存。如何做到这一点？

我想使用共享首选项，但我不知道如何做到这一点，因为它不是带有Trip的字符串it数组列表。有人能帮我在共享首选项中保存arraylist吗？

Answer 1

您需要将数据保存到数据库中，或者您可以按照此问题进行操作

Answer 2

我认为最简单的方法是将数据保存在SharedReferences中，并从SharedReferences填充arraylist。由于SharedPreferences只存储基元类型，您可以使用Gson将对象转换为字符串，然后将其存储在SharedPreference中。希望有帮助！。

Answer 3

共享首选项允许您保存一组字符串，因此您的最佳方案是将每个对象转换为字符串，然后将此字符串集合保存为共享首选项，然后在需要时可以从这些字符串重建对象。

或者您可以将其保存在文件系统或数据库中，但如果您只想这样做，SharedReferences会更轻松。无论哪种方式，您都需要构造/解构列表中保存的内容。

//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();

编辑

简单例子

public void saveToSharedPreferences(Context context, List<String> list){

    Set<String> set =
            list.stream()
            .collect(Collectors.toSet()); // Returns in this case a Set, if you need Iterable or Collection it is also available.

    PreferenceManager.getDefaultSharedPreferences(context) // Get Default preferences, you could use other.
            .edit()
            .putStringSet("myKey", set) // Save the Set of Strings (all trips but as Strings) with the key "myKey", this key is up to you.
            .apply();                   // When possible commit the changes to SharePreferences.
}

public void saveToSharedPreferences(Context context, List<Trip> list){
    List<String> stringList = list.stream()
            .map(Trip::getName) // This uses this method to transform a Trip into a String
                                // you could use other method to transform into String but if all you need to save is the name this suffices
                                // examples: .map( trip -> trip.getName()), .map(trip -> trip.toString()), etc. you choose how to save.
                                // It should save every state that a Trip has, so when you are reading the saved Strings
                                // you will be able to reconstruct a Trip with its state from each String.
            .collect(Collectors.toList());
    saveToSharedPreferences(context, stringList);
}

注意，我没有测试这个，但它应该是可以的（目前我没有Android Studio，所以可能会有一些小错误，但从这个解决方案，你应该能够完成这个。如果需要，我可以给你完整的解决方案工作。到目前为止，我无法访问我的IDE和工作站。