How to use generics in a java interface?

Question

I'm trying to build a simple interface using generics, here's the code I'm trying to use:

public interface MyInterface<T>
{
   public ArrayList<T> items;
   //...
}

However I'm getting an error about accessing static variable T from a non static context, etc. How can I accomplish what I'm trying to do? This is how I envision using a class which implements this interface:

MyInterface<SomeObject> foo = new MyInterfaceImpl<>();

for (SomeObject bar: foo.items)
{
     bar.someMethod();
}

Is this possible to specify via interfaces?

Answer 1

All the variable declared in interface those are by-default static and final So your generic type T can not be applicable to ArrayList. so it is promoting error Cannot make a static reference to the non-static type T

From the Java Language Specification Sec. 8.1.2

• It is a compile-time error to refer to a type parameter of a generic class C anywhere in:
• the declaration of a static member of C (§8.3.1.1, §8.4.3.2, §8.5.1), or
• the declaration of a static member of any type declaration nested within C, or
• a static initializer of C (§8.7), or
• a static initializer of any class declaration nested within C.

Answer 2

The main problem is the data member, which is static by default when declared in an interface.

A static data member is not allowed to refer to generic parameters. This would break the type safety if it was allowed, since different instances should be allowed to have a different type argument, but the static member is shared by all of them, so it can't be linked to a particular type argument.

Answer 3

Basically an interface describes capabilities(aka what a class can do) not implementation(aka how an instance achieves those capabilities).

From this other stackoverflow question

Interface variables are static because Java interfaces cannot be instantiated in their own right; the value of the variable must be assigned in a static context in which no instance exists. The final modifier ensures the value assigned to the interface variable is a true constant that cannot be re-assigned by program code.

What you actually need here is a abstract base class

class abstract Base<T> implements MyInterface<T> {
    //btw it's recommended that you use interfaces as the declaring type not implementations
    public List<T> items = new ArrayList<T>;
}

class MyInterfaceImpl<T> extends Base<T> {
   //...
}

Abstract base classes have the advantage that they can include from 0 to 100% implementation including public/protect/private instance fields.

Answer 4

Following Eyal's explanation, do like this:

public interface MyInterface<T> {
  Collection<T> getItems();
}

Implementing classes will hold a dynamic data member:

public class MyImpl<T> implements MyInterface<T> {
  private Collection<T> items;
  public Collection<T> getItems() { return items; }
}

Answer 5

You have to do this:

MyInterface<SomeObject> foo = new MyInterfaceImpl<SomeObject>();

for (SomeObject bar: foo.items)
{
    bar.someMethod();
}

Note that MyInterfaceImpl has to be parametrized with appropriate class if it's generic or if it's not generic than no brackets would be needed there.