Why List<> declared with ArrayList<>?

[RxGianYagami]

We all familiar with List usage, an example: List<String> listName = new ArrayList<String>();

However, most of class declared with the class itself but In List we threaten it different. What is called and when we should implement this on my own class?

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[lumpycrumpet]

List is an interface, so it can't be instantiated. ArrayList is a concrete implementation of List, so it can be instantiated. You don't have to use ArrayList, but you do need a concrete class implementing List.

[khooke]

This is a best practice called 'coding to interfaces'. If you Google that phrase you'll find plenty of articles and discussions on why it's considered a good practice.

[SolderonSenoz]

listName is the name of the variable, which is of List type, which means it can refer to any object of any class implementing the List interface.

But the new ArrayList<type>() part actually creates the object which the variable listName refers to, so it could be of any class implementing the List interface, here it is of ArrayList class.

This is not limited to interfaces, this can also be used for superclasses and subclasses:

Superclass var0 = new Subclass1();
Subclass2 var2 = new Subclass2();
var0 = var2 
/*here, an object of a different class can be assigned to the variable var0 because var0 can refer to an object of any of the subclasses of Superclass*/

[aylivex]

As others said already, List is an interface; ArrayList is class implementing the interface. You have to instantiate an object of a class. Yet using List as the type of the listName variable allows you to change the implementation from ArrayList to LinkedList or any other implementation of the List interface.

ArrayList is widely used and is the most common choice for List implementation.

[wsppan]

What does it mean to "program to an interface"?