Convert Map String Object to Map String String

Question

How can I convert  Map lt String Object gt  to Map lt String String gt     This does not work   Map lt String Object gt  map   new HashMap lt String Object gt       Object is containing String Map lt String String gt  newMap  new HashMap lt String String gt  map

User · Answer

While you can do this with brute casting and suppressed warnings

Map<String,Object> map = new HashMap<String,Object>();
// Two casts in a row.  Note no "new"!
@SuppressWarnings("unchecked")
Map<String,String> newMap = (HashMap<String,String>)(Map)map;

that's really missing the whole point. :)

An attempt to convert a narrow generic type to a broader generic type means you're using the wrong type in the first place.

As an analogy: Imagine you have a program that does volumous text processing. Imagine that you do first half of the processing using Objects (!!) and then decide to do the second half with correct-typing as a String, so you narrow-cast from Object to String. Fortunately, you can do this is java (easily in this case) - but it's just masking the fact you're using weak-typing in the first half. Bad practice, no argument.

No difference here (just harder to cast). You should always use strong typing. At minimum use some base type - then generics wildcards can be used ("? extends BaseType" or "? super BaseType") to give type-compatability and automatic casting. Even better, use the correct known type. Never use Object unless you have 100% generalised code that can really be used with any type.

Hope that helps! :) :)

Note: The generic strong typing and type-casting will only exist in .java code. After compilation to .class we are left with raw types (Map and HashMap) with no generic type parameters plus automatic type casting of keys and values. But it greatly helps because the .java code itself is strongly-typed and concise.

User · Answer

Generic types is a compile time abstraction  At runtime all maps will have the same type Map lt Object  Object gt   So if you are sure that values are strings  you can cheat on java compiler   Map lt String  Object gt  m1   new HashMap lt String  Object gt     Map lt String  String gt  m2    Map  m1    Copying keys and values from one collection to another is redundant  But this approach is still not good  because it violates generics type safety  May be you should reconsider your code to avoid such things

User · Answer

As you are casting from Object to String I recommend you catch and report  in some way  here I just print a message  which is generally bad  the exception       Map lt String Object gt  map   new HashMap lt String Object gt       Object is containing String     Map lt String String gt  newMap  new HashMap lt String String gt          for  Map Entry lt String  Object gt  entry   map entrySet              try              newMap put entry getKey     String  entry getValue                       catch ClassCastException e               System out println  ERROR    entry getKey     - gt    entry getValue                                     not added  as   entry getValue     is not a String

User · Answer

Not possible   This a little counter-intuitive       You re encountering the  Apple is-a fruit  but  Every Fruit is not an Apple    Go for creating a new map and checking with instance of with String

User · Answer

Now that we have Java 8 streams  we can add one more possible answer to the list    Assuming that each of the values actually are String objects  the cast to String should be safe  Otherwise some other mechanism for mapping the Objects to Strings may be used   Map lt String Object gt  map   new HashMap lt  gt     Map lt String String gt  newMap   map entrySet   stream         collect Collectors toMap Map Entry  getKey  e - gt   String e getValue

User · Answer

private Map lt String  String gt  convertAttributes final Map lt String  Object gt  attributes        final Map lt String  String gt  result   new HashMap lt String  String gt         for  final Map Entry lt String  Object gt  entry   attributes entrySet              result put entry getKey    String valueOf entry getValue                return result

User · Answer

There are two ways to do this  One is very simple but unsafe   Map lt String  Object gt  map   new HashMap lt String  Object gt     Map lt String  String gt  newMap   new HashMap lt String  String gt   Map map       unchecked warning   The other way has no compiler warnings and ensures type safety at runtime  which is more robust   After all  you can t guarantee the original map contains only String values  otherwise why wouldn t it be Map lt String  String gt  in the first place    Map lt String  Object gt  map   new HashMap lt String  Object gt     Map lt String  String gt  newMap   new HashMap lt String  String gt      SuppressWarnings  unchecked   Map lt String  Object gt  intermediate        Map Collections checkedMap newMap  String class  String class   intermediate putAll map

User · Answer

If your Objects are containing of Strings only  then you can do it like this   Map lt String Object gt  map   new HashMap lt String Object gt       Object is containing String Map lt String String gt  newMap  new HashMap lt String String gt     for  Map Entry lt String  Object gt  entry   map entrySet             if entry getValue   instanceof String               newMap put entry getKey     String  entry getValue                      If every Objects are not String then you can replace   String  entry getValue   into entry getValue   toString

User · Answer

Use the Java 8 way of converting a Map lt String  Object gt  to Map lt String  String gt   This solution handles null values   Map lt String  String gt  keysValuesStrings   keysValues entrySet   stream        filter entry - gt  entry getValue      null       collect Collectors toMap Entry  getKey  entry - gt  entry getValue   toString

User · Answer

The following will transform your existing entries   TransformedMap decorateTransform params  keyTransformer  valueTransformer    Where as   MapUtils transformedMap java util Map map  keyTransformer  valueTransformer    only transforms new entries into your map

User · Answer

Great solutions here  just one more option that taking into consideration handling of null values   Map lt String Object gt  map   new HashMap lt  gt      Map lt String String gt  stringifiedMap   map entrySet   stream                 filter m - gt  m getKey      null  amp  amp  m getValue     null                collect Collectors toMap Map Entry  getKey  e - gt   String e getValue

[java] Convert Map<String,Object> to Map<String,String>

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