How to convert List<Object> into List<MyClass>?

Question

in Java i have one List of Objects from my database and i want to convert this objects into my class objects but i cant find the solution.

My List of objects is SalvaguardasAGR and i want to convert this objects to List<AGRSalvaguardasInforme>.

I tried with

List<AGRSalvaguardasInforme> InformeFinal = new ArrayList<AGRSalvaguardasInforme>(SalvaguardasAGR);

as i see in How to cast List<Object> to List<MyClass> and How to Convert List<String> to List<Object> but throw this exception:

    25-sep-2014 17:40:47 org.apache.catalina.core.StandardWrapperValve invoke
GRAVE: El Servlet.service() para el servlet [procop2front] en el contexto con ruta [/CDNMS] lanzó la excepción [java.lang.Error: Unresolved compilation problem: 
    The constructor ArrayList<AGRSalvaguardasInforme>(List<Object>) is undefined
] con causa raíz
java.lang.Error: Unresolved compilation problem: 
        The constructor ArrayList<AGRSalvaguardasInforme>(List<Object>) is undefined at com.dominion.procop.agr.struts.actions.AGRInformes.mostrarInformeActivosAGR(AGRInformes.java:1140)

I tried a simple List<AGRSalvaguardasInforme> InformeFinal = (AGRSalvaguardasInforme)SalvaguardasAGR; too but still not working.

What im doing wrong? how i can convert a List of objects into a List of AGRSalvaguardasInforme?

Thank you in advance.

Solution 1

Did you try to double-cast it like in answers to How to cast List<Object> to List<MyClass> question?

List<Object> list = getList();
List<DesiredType> castedList = (List<DesiredType>) (List) list;

Be careful though, because calls to castedList.get(i) or other methods may throw ClassCastException if it turns out that list contains element which is not of DesiredType.

That's why I would really recommend to try and get List<DesiredType> instance right-away - where do you get instance of List<Object> from? Is it going from your own code (code which you can modify) or from another library/code which you cannot modify?

Solution 2

Java 8:

List<AGRSalvaguardasInforme> InformeFinal = SalvaguardasAGR.stream().map(x -> (AGRSalvaguardasInforme)x).collect(Collectors.toList());

This:

• treats the elements of the list as a stream, in order
• transforms each element of the stream by casting it (actually, this doesn't do any transformation at all, but it does check to make sure the object is in the right class and throws a ClassCastException if not)
• collects the resulting items into a new list

Note: I've tested this and it works.

Comments

[Pracede]

Do you having an exception ? If yes post stacktrace please

[S. Moreno]

Added the stacktrace

[Kalpesh Soni]

give code for mostrarInformeActivosAGR

[user3062946]

The current problem is that he's trying to create a new List with a List as a constructor parameter, the ArrayList constructor does not support handing in a List.

[Kalpesh Soni]

this kind of casting does not work, e.g. List<Object> is not List<String>

[Kalpesh Soni]

yes, and to add - if possible do not use List<Object> create an interface for getters use List<MyInterface>

[user3062946]

That is true, but the implication he seems to be making is that the Objects in his original list ARE instances of 'AGRSalvaguardasInforme', and he just wanted a way to cast the list into the correct type.

[ajb]

I believe you're wrong about ClassCastException. The code you write does not check each element of the list to make sure it's the right type--I just tested it. That means doing this could possibly create a List<DesiredType> that has objects that aren't DesiredType in it, defeating Java's whole type system. ClassCastExceptions could come later, at totally unexpected places.

[Yuriy Nakonechnyy]

Looks totally cool in Java 8 - I thought about posting similar answer using Guava, but skipped that because of ugly anonymous classes :)

[S. Moreno]

@Yura is from my own code. In this line is where returns a Vector<E> (and i cast, but i dont know why ignore that). and the error when i try to convert my objects list into myclass list: java.lang.ClassCastException: java.util.Vector cannot be cast to procop.agr.util.AGRSalvaguardasInforme

[Yuriy Nakonechnyy]

@S.Moreno could you please add code snippet producing java.util.Vector instance which you try to cast to List<DesiredType>? I'm sure there's decent typesafe solution to your problem :)