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Is the purpose of `abstract` keyword in a FunctionalInterface to distinguish the 'lambda' method from other abstract methods? [duplicate]

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Are all methods of interface abstract?
(7 answers)
Why Functional Interfaces in Java 8 have one Abstract Method?
(4 answers)
what is a abstract method on a interface in java [duplicate]
(4 answers)
When we add two abstract methods in an interface and implement just one method then why can't we implement the other method using lambda?
(1 answer)
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Is the purpose of abstract keyword in a FunctionalInterface to distinguish the 'lambda' method from other abstract methods?
Since methods that have no body in an interface are abstract, and Functional Interfaces that have only one method do not have the abstract keyword, I was wondering whether the real purpose of the abstract keyword is to mark or distinguish which among the 'abstract' methods in a Functional Interface will serve as the 'carrier' of the lambda expression.
Is my understanding correct?
This method does not have the abstract keyword but is a functional interface:
public interface AllTrades {
public boolean checkSalary(ATrade t);
}
now suppose the interface above had 2 methods, they are both abstract,
but the interface is no longer a FunctionalInterface
public interface AllTrades {
public boolean checkSalary(ATrade t);
public boolean checkSalary2(ATrade t);
}
Now if we mark which method is abstract, it is a Functional Interface again:
public interface AllTrades {
abstract public boolean checkSalary(ATrade t);
public boolean checkSalary2(ATrade t);
}
Is my understanding correct?

Although you can use abstract in an interface, it has no extra meaning, as all methods in an interface (except default, private or static methods) are by definition abstract. It has no extra meaning in a functional interface, and if you annotate your interface with #FunctionalInterface, both of your examples with two methods will fail to compile as they are equivalent.
To be complete, your first example does have an abstract method, and is a potential function interface. Your second and third example both have two abstract methods, and cannot be used as a function interface.
See also the Java Language Specification 17, section 9.4:
An interface method lacking a private, default, or static
modifier is implicitly abstract. Its body is represented by a
semicolon, not a block. It is permitted, but discouraged as a matter
of style, to redundantly specify the abstract modifier for such a
method declaration.
(emphasis mine)
and section 9.8:
A functional interface is an interface that is not declared sealed
and has just one abstract method (aside from the methods of
Object), and thus represents a single function contract.

All methods in interfaces are public and abstract by default, and the inclusion of either of those modifiers does not change the behavior of the code in any way. You can verify this by compiling the second and third code samples with the #FunctionalInterface annotation, which would result in a compile time error.
Edit:
From the Java Language Specification
9.8. Functional Interfaces
9.4.3. Interface Method Body
9.6.4.9. #FunctionalInterface

Why Override an Interface method? [duplicate]

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When do you use Java's #Override annotation and why?
(27 answers)
Should we #Override an interface's method implementation?
(15 answers)
Closed 4 years ago.
When extending an abstract class I understand why you would have to over ride some methods. But when it comes to interfaces I do not understand why it must be done. If all methods of an interface must be implemented, what is the use of overriding them?.
Im talking about the #Override annotation.

Using #Override is purely optional. You don't have to use it. If you do, using #Override when implementing an interface has more or less the same reasoning as using it when extending an abstract class. In a nutshell, it prevents stupid mistakes - if an interface is changed, it forces you to change your class accordingly, instead of just tacking on the "new" methods you suddenly need to implement.

Functional Interface vs Interfaces with single methods like Runnable [duplicate]

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What are functional interfaces used for in Java 8?
(11 answers)
Closed 4 years ago.
While going through Functional Interfaces, I am not able to understand how are they different from other interfaces with a single method, like Runnable.
We can use Runnable as we try to use other Functional interfaces.
Prior to Java 8, we already could create interfaces and anonymous objects for a single piece of functionality.
For example:
#FunctionalInterface
public interface ITrade {
public boolean check(Trade t);
}
How is this different from:
public interface ITrade {
public boolean check(Trade t);
}

There is no difference, the docs for FunctionalInterface state:
An informative annotation type used to indicate that an interface type declaration is intended to be a functional interface [emphasis added]
and
However, the compiler will treat any interface meeting the definition of a functional interface as a functional interface regardless of whether or not a FunctionalInterface annotation is present on the interface declaration.
So the annotation is only there to indicate that the developer intended the interface to be used as a functional interface.

What is the use of abstract class that implements an interface [duplicate]

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Why an abstract class implementing an interface can miss the declaration/implementation of one of the interface's methods?
(7 answers)
Closed 7 years ago.
While reading Herbert Schildt I came across partial implementation where overriding is'nt mandatory , But I fail to understand why do we implement such an interface where we don't override its methods :
interface CallBack{
void callback();
}
abstract class Incomplete implements Callback { //Legal
void someMethod();
}
Is there any practical use of such a class or it's just theoretical ?

One use case is a family of classes which all have to implement the callback interface in the same way. So it could be implemented in the abstract superclass and you don't have to handle it in every subclass.
Abstracted classes can't be instantiated, so you would make sure that all subclasses that you will instantiate in you system later handle the callbacks in the same way.

The name it self showing that it's abstract, it need not to implement. Where as the subclasses of that abstract class must and must full fill that definition and needs to implement that methods in that interface.

Every subclass of Incomplete now have to implement Callback.

Why define a abstract interface in Java [duplicate]

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Java abstract interface
(9 answers)
Closed 9 years ago.
As we know, interface is to define some method, but some interface defined as a abstract interface, but why ?
for example:
public abstract interface Test{
}
what is the advantage for using abstract to define a interface?

There is no need to add that abstract key word. It's redundant there.
Interfaces are implicitly abstract.
Language Spec JLS# 9.1.1.1 abstract Interfaces
Every interface is implicitly abstract.
This modifier is obsolete and should not be used in new programs.

refer to http://docs.oracle.com/javase/tutorial/java/IandI/createinterface.html for the same.
Also see https://stackoverflow.com/a/18778307/805378 to get the difference between abstract and interface.

to make a class 100% abstract without using 'abstract'.