在活动

Question

实施ExpenseFB的我的班级Parcelable包含Map UserFB（也实施Parcelable）：
ExpenseFB：

public class ExpenseFB implements Parcelable {

private String id;
private String name;
private String description;
private String whopaidID;
private String whopaidName;
private Double amount;
private Map<String, UserFB> partecipants;
// setters and getters...
@Override
public int describeContents() {
    return 0;
}

@Override
public void writeToParcel(Parcel dest, int flags) {
    dest.writeString(id);
    dest.writeString(name);
    dest.writeString(description);
    dest.writeString(whopaidID);
    dest.writeString(whopaidName);
    dest.writeMap(partecipants);
}


protected ExpenseFB(Parcel in) {
    id = in.readString();
    name = in.readString();
    description = in.readString();
    whopaidID = in.readString();
    whopaidName = in.readString();
    in.readMap(partecipants,UserFB.class.getClassLoader());
}

public static final Creator<ExpenseFB> CREATOR = new Creator<ExpenseFB>() {
    @Override
    public ExpenseFB createFromParcel(Parcel in) {
        return new ExpenseFB(in);
    }

    @Override
    public ExpenseFB[] newArray(int size) {
        return new ExpenseFB[size];
    }
};
}

UserFB：

public class UserFB implements Parcelable{

private String id;
private String name;
private String email;
private Map<String, GroupFB> groups;
private Map<String, UserFB> friends;
// setters and getters
@Override
public int describeContents() {
    return 0;
}

@Override
public void writeToParcel(Parcel dest, int flags) {
    dest.writeString(id);
    dest.writeString(name);
    dest.writeString(email);
    dest.writeMap(groups);
    dest.writeMap(friends);
}

protected UserFB(Parcel in) {
    id = in.readString();
    name = in.readString();
    email = in.readString();
    in.readMap(groups,GroupFB.class.getClassLoader());
    in.readMap(friends,UserFB.class.getClassLoader());
}

public static final Creator<UserFB> CREATOR = new Creator<UserFB>() {
    @Override
    public UserFB createFromParcel(Parcel in) {
        return new UserFB(in);
    }

    @Override
    public UserFB[] newArray(int size) {
        return new UserFB[size];
    }
};
}

我想通过添加对象在两个活动之间传递ExpenseFB对象 ExpenseFB意图：

intent.putExtra("id", expenseFB);

在调试模式下，当我在第二个活动中执行getIntent().getParcelableExtra("id")时，当尝试在readMap()地图上执行partecipants方法时，会引发以下异常：

 ... Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.Object java.util.Map.put(java.lang.Object, java.lang.Object)' on a null object reference

我看到第一个活动中的partecipants地图已填满：我认为问题出在 writeMap（）方法中。
是否存在传递包含地图的Parcelable对象的标准或更好方法？
 我是否要调用另一种方法来包裹地图？
我不想使用Serializable对象，因为我读到他们的表现更差。

Answer 1

问题在于readMap()用于将数据从Parcel读取到和现有Map 。在致电Map之前，您尚未创建readMap()，因此您获得了NullPointerException。

您可以通过在声明地图时初始化地图来解决此问题：

private Map<String, GroupFB> groups = new HashMap<String, GroupFB>();
private Map<String, UserFB> friends = new HashMap<String, UserFB>();

或者，您可以在Map构造函数中创建空UserFB，如下所示：

protected UserFB(Parcel in) {
    id = in.readString();
    name = in.readString();
    email = in.readString();
    groups = new HashMap<String, GroupFB>();
    in.readMap(groups,GroupFB.class.getClassLoader());
    friends = new HashMap<String, UserFB>()
    in.readMap(friends,UserFB.class.getClassLoader());
}

Answer 2

你明白了，但我认为你需要知道如何将Map<>写入parcelable

粘贴writeParcel()方法

@Override
public void writeToParcel(Parcel dest, int flags) {
    dest.writeInt(this.groups.size());
    for (Map.Entry<String, GroupFb> entry : this.groups.entrySet()) {
        dest.writeString(entry.getKey());
        dest.writeParcelable(entry.getValue(), flags);
    }
}

protected UserFB (Parcel in) {
    int groupsSize = in.readInt();
    this.groups = new HashMap<String, GroupFb>(groupsSize);
    for (int i = 0; i < groupsSize; i++) {
        String key = in.readString();
        GroupFb value = in.readParcelable(GroupFb.class.getClassLoader());
        this.groups.put(key, value);
    }
}

对另一个Map<>也这样做。