Instantiating a List in Java

Question

Trying to use the following code   List lt Integer gt  list   new List lt Integer gt       I get the following error message       java util List is abstract  cannot be instantiated   What does this mean and why can t I initialize a List the same way I would an ArrayList

User · Answer

A List in java is an interface that defines certain qualities a  list  must have  Specific list implementations  such as ArrayList implement this interface and flesh out how the various methods are to work  What are you trying to accomplish with this list  Most likely  one of the built-in lists will work for you

User · Answer

List is the interface  not a class so it can t be instantiated  ArrayList is most likely what you re after   ArrayList lt Integer gt  list   new ArrayList lt Integer gt      An interface in Java essentially defines a blueprint for the class - a class implementing an interface has to provide implementations of the methods the list defines  But the actual implementation is completely up to the implementing class  ArrayList in this case   The JDK also provides LinkedList - an alternative implementation that again conforms to the list interface  It works very differently to the ArrayList underneath and as such it tends to be more efficient at adding   removing items half way through the list  but for the vast majority of use cases it s less efficient  And of course if you wanted to define your own implementation it s perfectly possible   In short  you can t create a list because it s an interface containing no concrete code - that s the job of the classes that implement that list  of which ArrayList is the most used by far  and with good reason    It s also worth noting that in C  a List is a class  not an interface - that s IList  The same principle applies though  just with different names

User · Answer

List is an interface  not a concrete class  An interface is just a set of functions that a class can implement  it doesn t make any sense to instantiate an interface   ArrayList is a concrete class that happens to implement this interface and all of the methods in it

User · Answer

Use List lt Integer gt  list   new ArrayList lt Integer gt

User · Answer

In Java  List is an interface  That is  it cannot be instantiated directly   Instead you can use ArrayList which is an implementation of that interface that uses an array as its backing store  hence the name    Since ArrayList is a kind of List  you can easily upcast it   List lt T gt  mylist   new ArrayList lt T gt       This is in contrast with  NET  where  since version 2 0  List lt T gt  is the default implementation of the IList lt T gt  interface

User · Answer

A List isn t a real thing in Java  It s an interface  a way of defining how an object is allowed to interact with other objects  As such  it can t ever be instantiated  An ArrayList is an implementation of the List interface  as is a linked list  and so on  Use those instead

[java] "Instantiating" a List in Java?

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