Android studio ，这不是从共享首选项中检索字符串并将其转换为哈希集的正确方法吗？<string> ?

Question

My code is:

HashSet<String> set = (HashSet<String>) sharedPreferences.getStringSet("notes" , null);

I got that exception:

java.lang.ClassCastException: java.lang.String cannot be cast to java.util.S

 Caused by: java.lang.ClassCastException: java.lang.String cannot be cast to java.util.Set
        at android.app.SharedPreferencesImpl.getStringSet(SharedPreferencesImpl.java:293)
        at com.example.notes.MainActivity.onCreate(MainActivity.java:60)

Answer 1

You cannot , under any circumstances, cast a String to a Set (or anything but a type that String directly implements, such as Object , Comparable , or Serializable ).

You must explicitly write code to convert the String to a Set , such as (for example) splitting it on comma characters and adding each element of the split to a set you create.