Cannot instantiate the type List Product

Question

I have the following code   List lt Product gt  product   new List lt Product gt       The error   Cannot instantiate the type List lt Product gt    Product is an Entity in my EJB project  Why I m getting this error

User · Answer

List is an interface  You need a specific class in the end so  either try  List l   new ArrayList      or  List l   new LinkedList      Whichever suit your needs

User · Answer

Interfaces can not be directly instantiated  you should instantiate classes that implements such Interfaces   Try this   NameValuePair   params   new BasicNameValuePair             new BasicNameValuePair  param1   param1           new BasicNameValuePair  param2   param2

User · Answer

List is an interface  Interfaces cannot be instantiated  Only concrete types can be instantiated  You probably want to use an ArrayList  which is an implementation of the List interface   List lt Product gt  products   new ArrayList lt Product gt

User · Answer

Use a concrete list type  e g  ArrayList instead of just List

User · Answer

List can be instantiated by any class implementing the interface By this way Java provides us polymorphic behaviour See the example below   List lt String gt  list   new ArrayList lt String gt       Instead of instantiating an ArrayList directly I am using a List to refer to ArrayList object so that we are using only the List interface methods and do not care about its actual implementation   Examples of classes implementing List are ArrayList LinkedList Vector You probably want to create a List depending upon your requirements   Example - a LinkedList is more useful when you hve to do a number of inertion or deletions  Arraylist is more performance intensive as it is backed by a fixed size array and array contents have to be changed by moving or regrowing the array   Again using a List we can simply change our object instantiation without changing any code further in your programs   Suppose we are using ArrayList lt String gt  value   new ArrayList lt String gt      we may use a specific method of ArrrayList and out code will not be robust  By using List lt String gt  value   new ArrayList lt String gt      we are making sure we are using only List interface methods  and if we want to change it to a LinkedList we simply have to change the code    List lt String gt  value   new ArrayList lt String gt        ------ your code uses List interface methods       value   new LinkedList lt String gt        -----your code still uses List interface methods and we do not have to change anything---- and we dont have to change anything in our code further  By the way a LinkedList also works a Deque which obviously also you cannot instantiate as it is also an interface

[java] Cannot instantiate the type List<Product>

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