我正在开发和Android应用程序，我已经使用共享首选项实现了所有处理的类。这个类拥有获取首选项的通用方法，它是通用的，因为它根据参数的不同返回不同类型的数据类型。我存储到共享prefs并从共享prefs检索的数据类型是整数，字符串，布尔值和List（使用JSON将列表序列化/反序列化到字符串中）。 好吧一切都很好，我可以通过这种方式从共享偏好成功获得int，string和boolean，但不是List，在那个cas我得到一个：
java.lang.ClassCastException: java.lang.String cannot be cast to java.util.List
我忘了告诉该方法被声明为返回一个Object，当它被调用时，返回的值被强制转换为正确的类型。 无论如何我不明白为什么它说不能投射字符串，它甚至不返回一个字符串？这是一个正确的方法，我的意思是返回一个对象然后再投射它是完全正确的。
以下获取共享偏好的完整方法。谢谢你的建议！
public Object getPrefs(String sharedPreferences, String key, int type,
Context context, Object defaultObject) {
// A String indicating an error occurred while retrieving shared
// preferences
final String ERROR = "ERROR";
// Set shared preferences from context
sharedPref = context.getSharedPreferences(sharedPreferences,
Context.MODE_PRIVATE);
switch (type) {
case (0): // <-- Integer
// Check that defaultObject is of correct instance else collect
// "hardcoded" default value of 0
if (defaultObject instanceof Integer) {
this.logger.logCatTxt(
this.logger.getINFO(),
this.LOG_TAG + ":getPrefs()",
"Integer with value"
+ Integer.toString(sharedPref.getInt(key,
(Integer) defaultObject))
+ " retrieved from shared preferences: "
+ sharedPreferences + ", with key: " + key
+ ", type: " + Integer.toString(type)
+ ", context: " + context.toString()
+ " and default value: "
+ Integer.toString((Integer) defaultObject));
return sharedPref.getInt(key, (Integer) defaultObject);
} else {
this.logger
.logCatTxt(
this.logger.getERROR(),
this.LOG_TAG + ":getPrefs()",
"Default value couldn't be set because of instance mismatch, hardcoded default value of 0 is used. However Integer with value"
+ Integer.toString(sharedPref.getInt(
key, 0))
+ " retrieved from shared preferences: "
+ sharedPreferences
+ ", with key: "
+ key
+ ", type: "
+ Integer.toString(type)
+ ", context: "
+ context.toString()
+ " and default value: "
+ Integer
.toString((Integer) defaultObject));
return sharedPref.getInt(key, 0);
}
case (1): // <-- String
// Check that defaultObject is of correct instance else collect
// "hardcoded" default value of ""
if (defaultObject instanceof String) {
this.logger.logCatTxt(
this.logger.getINFO(),
this.LOG_TAG + ":getPrefs()",
"String with value"
+ sharedPref.getString(key,
(String) defaultObject)
+ " retrieved from shared preferences: "
+ sharedPreferences + ", with key: " + key
+ ", type: " + Integer.toString(type)
+ " and context: " + context.toString()
+ " and default value: "
+ (String) defaultObject);
return sharedPref.getString(key, (String) defaultObject);
} else {
this.logger
.logCatTxt(
this.logger.getERROR(),
this.LOG_TAG + ":getPrefs()",
"Default value couldn't be set because of instance mismatch, hardcoded default value of \"\" is used. However String with value"
+ sharedPref.getString(key, "")
+ " retrieved from shared preferences: "
+ sharedPreferences
+ ", with key: "
+ key
+ ", type: "
+ Integer.toString(type)
+ " and context: " + context.toString());
return sharedPref.getString(key, "");
}
case (2): // <-- Boolean
// Check that defaultObject is of correct instance else collect
// "hardcoded" default value of false
if (defaultObject instanceof Boolean) {
this.logger.logCatTxt(
this.logger.getINFO(),
this.LOG_TAG + ":getPrefs()",
"Boolean with value"
+ sharedPref.getBoolean(key,
(Boolean) defaultObject)
+ " retrieved from shared preferences: "
+ sharedPreferences + ", with key: " + key
+ ", type: " + Integer.toString(type)
+ " and context: " + context.toString()
+ " and default value: "
+ (Boolean) defaultObject);
return sharedPref.getBoolean(key, (Boolean) defaultObject);
} else {
this.logger
.logCatTxt(
this.logger.getERROR(),
this.LOG_TAG + ":getPrefs()",
"Default value couldn't be set because of instance mismatch, hardcoded default value of false is used. However Boolean with value"
+ sharedPref.getBoolean(key, false)
+ " retrieved from shared preferences: "
+ sharedPreferences
+ ", with key: "
+ key
+ ", type: "
+ Integer.toString(type)
+ " and context: " + context.toString());
return sharedPref.getBoolean(key, false);
}
case (3): // <-- List
// Retrieve secondaryListenNumbers to json string and clear
// secondaryListenNumbers List just to be sure that it's empty
String json = sharedPref.getString(key, "");
// List of Strings containing
List<String> list = new ArrayList<String>();
// If json string is not empty
if (json != "") {
try {
// Create a JSONArray from json string and retrieve strings
// from it and and them to secondaryListenNumbers List
JSONArray a = new JSONArray(json);
for (int i = 0; i < a.length(); i++) {
String secondaryListenNumber = a.optString(i);
list.add(secondaryListenNumber);
}
this.logger.logCatTxt(this.logger.getINFO(), this.LOG_TAG
+ ":getPrefs()", "List<String> with value(s)" + json
+ " retrieved from shared preferences: "
+ sharedPreferences + ", with key: " + key
+ ", type: " + Integer.toString(type)
+ " and context: " + context.toString());
// Return the list
return list;
} catch (JSONException e) {
e.printStackTrace();
this.logger.logCatTxt(this.logger.getERROR(), this.LOG_TAG
+ ":getPrefs()",
"Failed to retrieve List<String> from shared preferences: "
+ sharedPreferences + ", with key: " + key
+ ", type: " + Integer.toString(type)
+ " and context: " + context.toString(), e);
}
}
break;
default:
this.logger.logCatTxt(this.logger.getWARN(), this.LOG_TAG
+ ":getPrefs()",
"Unsupported data type was givien as parameter. Shared preferences: "
+ sharedPreferences + ", with key: " + key
+ ", type: " + Integer.toString(type)
+ " and context: " + context.toString());
}
// We should never reach this far but if we do an error has occurred and
// we return the ERROR string
return ERROR;
}
答案 0 :(得分：4)
是的，您的方法可以返回String：
return ERROR;
声明为String。
final String ERROR = "ERROR";
此外，这可能会返回String：
return sharedPref.getString(key, (String) defaultObject);
可能相关：
if (json != "") {
请勿将字符串值与==或!=进行比较。使用String#equals，即
if (!json.equals("")) {
甚至是这样：
if (json.isEmpty()) {
如果出现错误，在Java中返回错误String并不是一个好的设计。
如果出现错误，最好抛出某种Exception，并让调用方法捕获异常并处理错误。
答案 1 :(得分：1)
您正在比较一个字符串与不相等的运算符：if (json != "")您应该使用if (!"".equals(json))。
如果json为空，则返回ERROR witch是一个String。如果您尝试强制转换为List，则会导致异常。
答案 2 :(得分：1)
java.lang.String cannot be cast to java.util.List
错误字符串说：您正在尝试将String强制转换为List。 使用调试器找到它。
答案 3 :(得分：1)
如果返回类型为Object，则在转换为其他类型时使用instanseof是个好主意。同样如前所述，不要返回错误字符串，最好抛出Exception