Why can't static methods be abstract in Java

Question

The question is in Java why can't I define an abstract static method? for example

abstract class foo {
    abstract void bar( ); // <-- this is ok
    abstract static void bar2(); //<-- this isn't why?
}

Answer 1

Because abstract is a keyword which is applied over Abstract methods do not specify a body. And If we talk about static keyword it belongs to class area.

Answer 2

I also asked the same question , here is why

Since Abstract class says, it will not give implementation and allow subclass to give it

so Subclass has to override the methods of Superclass ,

RULE NO 1 - A static method cannot be overridden

Because static members and methods are compile time elements , that is why Overloading(Compile time Polymorphism) of static methods are allowed rather then Overriding (Runtime Polymorphism)

So , they cant be Abstract .

There is no thing like abstract static <--- Not allowed in Java Universe

Answer 3

Declaring a method as static means we can call that method by its class name and if that class is abstract as well, it makes no sense to call it as it does not contain any body, and hence we cannot declare a method both as static and abstract.

Answer 4

Poor language design. It would be much more effective to call directly a static abstract method than creating an instance just for using that abstract method. Especially true when using an abstract class as a workaround for enum inability to extend, which is another poor design example. Hope they solve those limitations in a next release.

Answer 5

A static method can be called without an instance of the class. In your example you can call foo.bar2(), but not foo.bar(), because for bar you need an instance. Following code would work:

foo var = new ImplementsFoo();
var.bar();

If you call a static method, it will be executed always the same code. In the above example, even if you redefine bar2 in ImplementsFoo, a call to var.bar2() would execute foo.bar2().

If bar2 now has no implementation (that's what abstract means), you can call a method without implementation. That's very harmful.

Answer 6

The idea of having an abstract static method would be that you can't use that particular abstract class directly for that method, but only the first derivative would be allowed to implement that static method (or for generics: the actual class of the generic you use).

That way, you could create for example a sortableObject abstract class or even interface with (auto-)abstract static methods, which defines the parameters of sort options:

public interface SortableObject {
    public [abstract] static String [] getSortableTypes();
    public String getSortableValueByType(String type);
}

Now you can define a sortable object that can be sorted by the main types which are the same for all these objects:

public class MyDataObject implements SortableObject {
    final static String [] SORT_TYPES = {
        "Name","Date of Birth"
    }
    static long newDataIndex = 0L ;

    String fullName ;
    String sortableDate ;
    long dataIndex = -1L ;
    public MyDataObject(String name, int year, int month, int day) {
        if(name == null || name.length() == 0) throw new IllegalArgumentException("Null/empty name not allowed.");
        if(!validateDate(year,month,day)) throw new IllegalArgumentException("Date parameters do not compose a legal date.");
        this.fullName = name ;
        this.sortableDate = MyUtils.createSortableDate(year,month,day);
        this.dataIndex = MyDataObject.newDataIndex++ ;
    }
    public String toString() {
        return ""+this.dataIndex+". "this.fullName+" ("+this.sortableDate+")";
    }

    // override SortableObject 
    public static String [] getSortableTypes() { return SORT_TYPES ; }
    public String getSortableValueByType(String type) {
        int index = MyUtils.getStringArrayIndex(SORT_TYPES, type);
        switch(index) {
             case 0: return this.name ;
             case 1: return this.sortableDate ;
        }
        return toString(); // in the order they were created when compared
    }
}

Now you can create a

public class SortableList<T extends SortableObject> 

that can retrieve the types, build a pop-up menu to select a type to sort on and resort the list by getting the data from that type, as well as hainv an add function that, when a sort type has been selected, can auto-sort new items in. Note that the instance of SortableList can directly access the static method of "T":

String [] MenuItems = T.getSortableTypes();

The problem with having to use an instance is that the SortableList may not have items yet, but already need to provide the preferred sorting.

Cheerio, Olaf.