Converting java util Properties to HashMap String String

Question

Properties properties   new Properties    Map lt String  String gt  map   new HashMap lt String  String gt  properties     why wrong    java util Properties is a implementation of java util Map  And java util HashMap s constructor receives a Map type param  So  why must it be converted explicitly

User · Answer

First thing       Properties class is based on Hashtable and not Hashmap  Properties class basically extends Hashtable   There is no such constructor in HashMap class which takes a properties object and return you a hashmap object  So what you are doing is NOT correct  You should be able to cast the object of properties to hashtable reference

User · Answer

How about this      Map properties   new Properties       Map lt String  String gt  map   new HashMap lt String  String gt  properties     Will cause a warning  but works without iterations

User · Answer

The efficient way to do that is just to cast to a generic Map as follows   Properties props   new Properties     Map lt String  String gt  map    Map props    This will convert a Map lt Object  Object gt  to a raw Map  which is  ok  for the compiler  only warning   Once we have a raw Map it will cast to Map lt String  String gt  which it also will be  ok   another warning   You can ignore them with annotation  SuppressWarnings    unchecked    rawtypes      This will work because in the JVM the object doesn t really have a generic type  Generic types are just a trick that verifies things at compile time   If some key or value is not a String it will produce a ClassCastException error  With current Properties implementation this is very unlikely to happen  as long as you don t use the mutable call methods from the super Hashtable lt Object Object gt  of Properties   So  if don t do nasty things with your Properties instance this is the way to go

User · Answer

i use this           for  Map Entry lt Object  Object gt  entry properties entrySet          map put  String  entry getKey     String  entry getValue

User · Answer

The problem is that Properties implements Map lt Object  Object gt   whereas the HashMap constructor expects a Map lt   extends String    extends String gt    This answer explains this  quite counter-intuitive  decision  In short  before Java 5  Properties implemented Map  as there were no generics back then   This meant that you could put any Object in a Properties object  This is still in the documenation      Because Properties inherits from Hashtable  the put and putAll methods   can be applied to a Properties object  Their use is strongly   discouraged as they allow the caller to insert entries whose keys or   values are not Strings  The setProperty method should be used instead    To maintain compatibility with this  the designers had no other choice but to make it inherit Map lt Object  Object gt  in Java 5  It s an unfortunate result of the strive for full backwards compatibility which makes new code unnecessarily convoluted   If you only ever use string properties in your Properties object  you should be able to get away with an unchecked cast in your constructor   Map lt String  String gt  map   new HashMap lt String  String gt    Map lt String  String gt   properties     or without any copies   Map lt String  String gt  map    Map lt String  String gt   properties

User · Answer

This is because Properties extends Hashtable lt Object  Object gt   which  in turn  implements Map lt Object  Object gt    You attempt to feed that into a Map lt String  String gt   It is therefore incompatible   You need to feed string properties one by one into your map     For instance   for  final String name  properties stringPropertyNames        map put name  properties getProperty name

User · Answer

this is only because the constructor of HashMap requires an arg of Map generic type and Properties implements Map    This will work  though with a warning      Properties properties   new Properties        Map lt String  String gt  map   new HashMap properties

User · Answer

You could use Google Guava s   com google common collect Maps fromProperties Properties

User · Answer

If you know that your Properties object only contains  lt String  String gt  entries  you can resort to a raw type   Properties properties   new Properties    Map lt String  String gt  map   new HashMap lt String  String gt   Map  properties

User · Answer

I would use following Guava API  com google common collect Maps fromProperties  Properties properties   new Properties    Map lt String  String gt  map   Maps fromProperties properties

User · Answer

When I see Spring framework source code I find this way Properties props   getPropertiesFromSomeWhere        change properties to map Map lt String String gt  map   new HashMap props

User · Answer

You can use this   Map lt String  String gt  map   new HashMap lt  gt      props forEach  key  value  - gt  map put key toString    value toString

User · Answer

Properties implements Map lt Object  Object gt  - not Map lt String  String gt    You re trying to call this constructor   public HashMap Map lt   extends K   extends V gt  m        with K and V both as String   But Map lt Object  Object gt  isn t a Map lt   extends String    extends String gt     it can contain non-string keys and values   This would work   Map lt Object  Object gt  map   new HashMap lt Object  Object gt           but it wouldn t be as useful to you   Fundamentally  Properties should never have been made a subclass of HashTable    that s the problem  Since v1  it s always been able to store non-String keys and values  despite that being against the intention  If composition had been used instead  the API could have only worked with string keys values  and all would have been well   You may want something like this   Map lt String  String gt  map   new HashMap lt String  String gt     for  String key   properties stringPropertyNames          map put key  properties getProperty key

User · Answer

The Java 8 way   properties entrySet   stream   collect      Collectors toMap           e - gt  e getKey   toString             e - gt  e getValue   toString

[java] Converting java.util.Properties to HashMap<String,String>

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