Java OCP: "classes inheriting the interface vs without an instance of the interface"

[Nico van de Kamp]

Hello,
I'm busy with studying the Java OCP 17 by the Java Study Guide. In chapter 7 table 7.2 Interface member Acces, I do not understand exactly what is meant by:

column header 3: "Accessible from methods in classes inheriting the interface" (see attachement)
column header 4: "Accessible without an instance of the interface?" (see attachement)

Column 3:  what is exactly meant by "inheriting the interface"? Is that the keyword implements?
Is this what they are saying here?


Colunm 4: And if the class doesn't implement the interface, than by instantiate the class there is not any "instance" of the interface, but you can access the constant variable (because public static final) and the static method by interface name.
Is this what they are saying here?

Can anyone confirm this or am I complete wrong?

[Mike Simmons]

Nico van de Kamp wrote:Column 3:  what is exactly meant by "inheriting the interface"? Is that the keyword implements?
Is this what they are saying here?

Pretty much, yes - either implements or extends, or some combination of those.  You could have an interface A with method foo(), and interface B that extends A, and class C that implements B, and class D that extends C... and you could access method foo() from any of those.  If that's too confusing, then just take my answer as "yes".

Nico van de Kamp wrote:Colunm 4: And if the class doesn't implement the interface, than by instantiate the class there is not any "instance" of the interface, but you can access the constant variable (because public static final) and the static method by interface name.
Is this what they are saying here?

Yes.

[Nico van de Kamp]

Ok thanks, yesterday I've made some code examples which I will put here, but I think I understand it.

The interface

interface MemberAccess {
    public static final int CONSTANT_VARIABLE = 10;

    void abstracMethod();

    static void staticMethod() {
        System.out.println("static method");
    }

    default void voidMethod() {
        System.out.println("default method");
    }

   // (private method is without discussion)
}

Code according 'Column 3':

public class Table72InterfaceMemberAccess implements MemberAccess{ // the interface MemberAccess is "inherited" here due the keyword implements

    public static void main(String[] args) {
        Table72InterfaceMemberAccess t = new Table72InterfaceMemberAccess();
        t.accessInterfaceMethods();
    }

    public void accessInterfaceMethods() {
        System.out.println(MemberAccess.CONSTANT_VARIABLE);   // access to the static constant variable
        MemberAccess.staticMethod();                       // access to the static method
        abstracMethod();                                // access to the (implented) abstract  method
        MemberAccess.super.voidMethod();                     // access to the default method 
    }

    @Override
    public void abstracMethod() {
        System.out.println("This is the abstract method implemented");
    }
}

Code according 'Column 4':

public class Table72InterfaceMemberAccess { // without "inheriting" of the interface MemberAccess due the keyword implements

    public static void main(String[] args) {
        Table72InterfaceMemberAccess t = new Table72InterfaceMemberAccess();
        t.accessInterfaceMethods();
    }

    public void accessInterfaceMethods() {                       // Than only access to the two static interface members by the interface name,
        System.out.println(MemberAccess.CONSTANT_VARIABLE);        // in table the text "(interface name required) is missing as by the static method
        MemberAccess.staticMethod();                  
    }
}

[Nico van de Kamp]

Ok thanks, yesterday I've made some code examples which I will put here, but I think I understand it.

The interface

interface MemberAccess {
    public static final int CONSTANT_VARIABLE = 10;

    void abstracMethod();

    static void staticMethod() {
        System.out.println("static method");
    }

    default void voidMethod() {
        System.out.println("default method");
    }

   // (private method is without discussion)
}

Code according 'Column 3':

public class Table72InterfaceMemberAccess implements MemberAccess{ // the interface MemberAccess is "inherited" here due the keyword implements

    public static void main(String[] args) {
        Table72InterfaceMemberAccess t = new Table72InterfaceMemberAccess();
        t.accessInterfaceMethods();
    }

    public void accessInterfaceMethods() {
        System.out.println(MemberAccess.CONSTANT_VARIABLE);   // access to the static constant variable
        MemberAccess.staticMethod();                       // access to the static method
        abstracMethod();                                // access to the (implented) abstract  method
        MemberAccess.super.voidMethod();                     // access to the default method 
    }

    @Override
    public void abstracMethod() {
        System.out.println("This is the abstract method implemented");
    }
}

Code according 'Column 4':

public class Table72InterfaceMemberAccess { // without "inheriting" of the interface MemberAccess due the keyword implements

    public static void main(String[] args) {
        Table72InterfaceMemberAccess t = new Table72InterfaceMemberAccess();
        t.accessInterfaceMethods();
    }

    public void accessInterfaceMethods() {                       // Than only access to the two static interface members by the interface name,
        System.out.println(MemberAccess.CONSTANT_VARIABLE);        // in table the text "(interface name required) is missing as by the static method
        MemberAccess.staticMethod();                  
    }
}