default methods in interface but only static final fields

Question

I understand that all fields in an Inteface is implicitly static and final. And this made sense before Java 8.

But with the introduction of default methods, interfaces also have all the capabilities of an abstract class. And hence non-static and non-final fields are also necessary.

But when I tried declaring a field normally, it became static and final by default.

Is there a way to declare a non-static and non-final field in Interface in Java 8.

Or am I totally misunderstanding something here???

No. Nonstatic fields are the primary remaining difference between interfaces and abstract classes. — Louis Wasserman, Jun 30 '15 at 17:36

Crazyjavahacking · Accepted Answer · 2015-06-30T17:24:04.387

13

All fields in interfaces in Java are public static final.

Even after addition of default methods, it still does not make any sense to introduce mutable fields into the interfaces.

Default methods were added because of interface evolution reasons. You can add a new default method to the interface, but it only makes sense if the implementation uses already defined methods in the interface:

public interface DefaultMethods {

    public int getValue();

    public default int getValueIncremented() {
        if (UtilityMethod.helper()) { // never executed, just to demonstrate possibilities
            "string".charAt(0); // does nothing, just to show you can call instance methods
            return 0;
        }

        return 1 + getValue();
    }

    public static class UtilityMethod {

        public static boolean helper() {
            return false;
        }
    }
}

edited Jun 30 '15 at 17:24

answered Jun 30 '15 at 17:11

Crazyjavahacking

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1

So do you mean default methods purpose is to only include variations of existing methods and not introduce totally new methods which need to operate on object state??? – Codebender Jun 30 '15 at 17:19
2

Well that was the purpose. You still cannot add mutable fields into interfaces. Default methods can call static methods and instance methods on other objects, and all abstract and default methods defined in the interface. Look at the code I just updated. – Crazyjavahacking Jun 30 '15 at 17:25
@AbishekManoharan: Yup, pretty much. – Louis Wasserman Jun 30 '15 at 17:37

score 3 · Answer 2 · answered Jun 30 '15 at 17:08

3

No - in Java 8 all fields are static and final as in previous Java versions.

Having state (fields) in an interface would raise issues, in particular with relation to the diamond problem.

See also this entry that clarifies the difference between behaviour and state inheritance.

answered Jun 30 '15 at 17:08

assylias

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There were no fields before Java 8 :) – Dmitry Ginzburg Jun 30 '15 at 17:11
There were fields in interfaces from Java inception. – Crazyjavahacking Jun 30 '15 at 17:11
@DmitryGinzburg What do you mean? Java has always allowed to declare fields in interfaces (which happen to be automatically static and final). – assylias Jun 30 '15 at 17:12
2

@assylias If they could avoid the problem with methods, why not extend it to fields? – Chetan Kinger Jun 30 '15 at 17:12
@assylias ooops, some temporary madness. – Dmitry Ginzburg Jun 30 '15 at 17:13
As Chetan has metioned, the diamond problem was avoided for methods I guess.. And the same approach could be used for fields also... And default methods wont always be useful if it can only operate on local variables... – Codebender Jun 30 '15 at 17:16

score 0 · Answer 3 · answered Nov 30 '18 at 05:03

I strongly do not recommend to do that. Use abstract classes instead. But if you really need it you can do some workaround trick with some wrapper class. Here is an example:

public interface TestInterface {

    StringBuilder best = new StringBuilder();

    default void test() {
        best.append("ok");
        System.out.print(best.toString());
    }
}

M.Paniccia · Answer 4 · 2020-06-09T20:36:16.220

-2

No - In java 8 you can't have not public static final fields in interfaces, unfortunately.

But I hope in the future this will be added to the language.

Scala allows interface with not static fields, and this opens up so many more possibilities for code reuse and composition than available in Java 8

edited Jun 09 '20 at 20:36

answered Nov 14 '17 at 09:41

M.Paniccia

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You can have fields in Interfaces but only public, static and final fields are allowed. Refer https://docs.oracle.com/javase/specs/jls/se7/html/jls-9.html#jls-9.3 – Rohit Jun 08 '20 at 08:52
I meant fields that are not public static final. I should have made that more clear apologies. I will update my answer. – M.Paniccia Jun 09 '20 at 20:57

default methods in interface but only static final fields

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