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Is Default keyword in default methods in Java an optional specifier? [duplicate]

An interface in Java is similar to a class, but the body of an
interface can include only abstract methods and final fields
(constants).
Recently, I saw a question, which looks like this
interface AnInterface {
public default void myMethod() {
System.out.println("D");
}
}
According to the interface definition, only abstract methods are allowed. Why does it allow me to compile the above code? What is the default keyword?
On the other hand, when I was trying to write below code, then it says modifier default not allowed here
default class MyClass{
}
instead of
class MyClass {
}
Can anyone tell me the purpose of the default keyword? Is it only allowed inside an interface? How does it differ from default (no access modifier)?

It's a new feature in Java 8 which allows an interface to provide an implementation. Described in Java 8 JLS-13.5.6. Interface Method Declarations which reads (in part)
Adding a default method, or changing a method from abstract to default, does not break compatibility with pre-existing binaries, but may cause an IncompatibleClassChangeError if a pre-existing binary attempts to invoke the method. This error occurs if the qualifying type, T, is a subtype of two interfaces, I and J, where both I and J declare a default method with the same signature and result, and neither I nor J is a subinterface of the other.
What's New in JDK 8 says (in part)
Default methods enable new functionality to be added to the interfaces of libraries and ensure binary compatibility with code written for older versions of those interfaces.

Default methods were added to Java 8 primarily to support lambda expressions. The designers (cleverly, in my view) decided to make lambdas syntax for creating anonymous implementations of an interface. But given lambdas can only implement a single method they would be limited to interfaces with a single method which would be a pretty severe restriction. Instead, default methods were added to allow more complex interfaces to be used.
If you need some convincing of the claim that default was introduced due to lambdas, note that the straw man proposal of Project Lambda, by Mark Reinhold, in 2009, mentions 'Extension methods' as a mandatory feature to be added to support lambdas.
Here's an example demonstrating the concept:
interface Operator {
int operate(int n);
default int inverse(int n) {
return -operate(n);
}
}
public int applyInverse(int n, Operator operator) {
return operator.inverse(n);
}
applyInverse(3, n -> n * n + 7);
Very contrived I realise but should illustrate how default supports lambdas. Because inverse is a default it can easily be overriden by a implementing class if required.

A new concept is introduced in Java 8 called default methods. Default methods are those methods which have some default implementation and helps in evolving the interfaces without breaking the existing code. Lets look at an example:
public interface SimpleInterface {
public void doSomeWork();
//A default method in the interface created using "default" keyword
default public void doSomeOtherWork() {
System.out.println("DoSomeOtherWork implementation in the interface");
}
}
class SimpleInterfaceImpl implements SimpleInterface {
#Override
public void doSomeWork() {
System.out.println("Do Some Work implementation in the class");
}
/*
* Not required to override to provide an implementation
* for doSomeOtherWork.
*/
public static void main(String[] args) {
SimpleInterfaceImpl simpObj = new SimpleInterfaceImpl();
simpObj.doSomeWork();
simpObj.doSomeOtherWork();
}
}
and the output is:
Do Some Work implementation in the class
DoSomeOtherWork implementation in the interface

Something that was overlooked in the other answers was its role in annotations. As far back as Java 1.5, the default keyword came about as a means to provide a default value for an annotation field.
#Retention(RetentionPolicy.RUNTIME)
#Target(ElementType.TYPE)
public #interface Processor {
String value() default "AMD";
}
It usage was overloaded with the introduction of Java 8 to allow one to define a default method in interfaces.
Something else that was overlooked: the reason that the declaration default class MyClass {} is invalid is due to the way that classes are declared at all. There's no provision in the language that allows for that keyword to appear there. It does appear for interface method declarations, though.

Default methods in an interface allow us to add new functionality without breaking old code.
Before Java 8, if a new method was added to an interface, then all the implementation classes of that interface were bound to override that new method, even if they did not use the new functionality.
With Java 8, we can add the default implementation for the new method by using the default keyword before the method implementation.
Even with anonymous classes or functional interfaces, if we see that some code is reusable and we don’t want to define the same logic everywhere in the code, we can write default implementations of those and reuse them.
Example
public interface YourInterface {
public void doSomeWork();
//A default method in the interface created using "default" keyword
default public void doSomeOtherWork(){
System.out.println("DoSomeOtherWork implementation in the interface");
}
}
class SimpleInterfaceImpl implements YourInterface{
/*
* Not required to override to provide an implementation
* for doSomeOtherWork.
*/
#Override
public void doSomeWork() {
System.out.println("Do Some Work implementation in the class");
}
/*
* Main method
*/
public static void main(String[] args) {
SimpleInterfaceImpl simpObj = new SimpleInterfaceImpl();
simpObj.doSomeWork();
simpObj.doSomeOtherWork();
}
}

The new Java 8 feature (Default Methods) allows an interface to provide an implementation when its labeled with the default keyword.
For Example:
interface Test {
default double getAvg(int avg) {
return avg;
}
}
class Tester implements Test{
//compiles just fine
}
Interface Test uses the default keyword which allows the interface to provide a default implementation of the method without the need for implementing those methods in the classes that uses the interface.
Backward compatibility:
Imagine that your interface is implemented by hundreds of classes, modifying that interface will force all the users to implement the newly added method, even though its not essential for many other classes that implements your interface.
Facts & Restrictions:
1-May only be declared within an interface and not within a class or
abstract class.
2-Must provide a body
3-It is not assumed to be public or abstract as other normal methods used in an interface.

A very good explanation is found in The Java™ Tutorials, part of the explanation is as follows:
Consider an example that involves manufacturers of computer-controlled cars who publish industry-standard interfaces that describe which methods can be invoked to operate their cars. What if those computer-controlled car manufacturers add new functionality, such as flight, to their cars? These manufacturers would need to specify new methods to enable other companies (such as electronic guidance instrument manufacturers) to adapt their software to flying cars. Where would these car manufacturers declare these new flight-related methods? If they add them to their original interfaces, then programmers who have implemented those interfaces would have to rewrite their implementations. If they add them as static methods, then programmers would regard them as utility methods, not as essential, core methods.
Default methods enable you to add new functionality to the interfaces of your libraries and ensure binary compatibility with code written for older versions of those interfaces.

Default methods enable you to add new functionality to the interfaces of your apps. It can also be used to have a multi inheritance.
In addition to default methods, you can define static methods in interfaces. This makes it easier for you to organize helper methods

Why does Java not allow multiple inheritance but does allow conforming to multiple interfaces with default implementations

I am not asking this -> Why is there no multiple inheritance in Java, but implementing multiple interfaces is allowed?
In Java, multiple inheritance isn't allowed, but, after Java 8, Interfaces can have default methods (can implement methods itself), just like abstract classes. Within this context, it multiple inheritance should also be allowed.
interface TestInterface
{
// abstract method
public void square(int a);
// default method
default void show()
{
System.out.println("Default Method Executed");
}
}

Things are not so simple.
If a class implements multiple interfaces that defines default methods with the same signature the compiler will force you to override this method for the class.
For example with these two interfaces :
public interface Foo {
default void doThat() {
// ...
}
}
public interface Bar {
default void doThat() {
// ...
}
}
It will not compile :
public class FooBar implements Foo, Bar{
}
You should define/override the method to remove the ambiguity.
You could for example delegate to the Bar implementation such as :
public class FooBar implements Foo, Bar{
#Override
public void doThat() {
Bar.super.doThat();
}
}
or delegate to the Foo implementation such as : :
public class FooBar implements Foo, Bar {
#Override
public void doThat() {
Foo.super.doThat();
}
}
or still define another behavior :
public class FooBar implements Foo, Bar {
#Override
public void doThat() {
// ...
}
}
That constraint shows that Java doesn't allow multiple inheritancy even for interface default methods.
I think that we cannot apply the same logic for multiple inheritances because multiples issues could occur which the main are :
overriding/removing the ambiguity for a method in both inherited classes could introduce side effects and change the overall behavior of the inherited classes if they rely on this method internally. With default interfaces this risk is also around but it should be much less rare since default methods are not designed to introduce complex processings such as multiple internal invocations inside the class or to be stateful (indeed interfaces cannot host instance field).
how to inherit multiple fields ? And even if the language allowed it you would have exactly the same issue as this previously quoted : side effect in the behavior of the inherited class : a int foo field defined in a A and B class that you want to subclass doesn't have the same meaning and intention.

The language designers already thought about that, so these things are enforced by the compiler. So if you define:
interface First {
default void go() {
}
}
interface Second {
default void go() {
}
}
And you implement a class for both interfaces:
static class Impl implements First, Second {
}
you will get a compilation error; and you would need to override go to not create the ambiguity around it.
But you could be thinking that you can trick the compiler here, by doing:
interface First {
public default void go() {
}
}
static abstract class Second {
abstract void go();
}
static class Impl extends Second implements First {
}
You could think that First::go already provides an implementation for Second::go and it should be fine. This is too taken care of, thus this does not compile either.
JLS 9.4.1.3 : Similarly, when an abstract and a default method with matching signatures are inherited, we produce an error. In this case, it would be possible to give priority to one or the other - perhaps we would assume that the default method provides a reasonable implementation for the abstract method, too. But this is risky, since other than the coincidental name and signature, we have no reason to believe that the default method behaves consistently with the abstract method's contract - the default method may not have even existed when the subinterface was originally developed. It is safer in this situation to ask the user to actively assert that the default implementation is appropriate (via an overriding declaration).
The last point I would bring in, to solidify that multiple inheritance is not allowed even with new additions in java, is that static methods from interfaces are not inherited. static methods are inherited by default:
static class Bug {
static void printIt() {
System.out.println("Bug...");
}
}
static class Spectre extends Bug {
static void test() {
printIt(); // this will work just fine
}
}
But if we change that for an interface (and you can implement multiple interfaces, unlike classes):
interface Bug {
static void printIt() {
System.out.println("Bug...");
}
}
static class Spectre implements Bug {
static void test() {
printIt(); // this will not compile
}
}
Now, this is prohibited by the compiler and JLS too:
JLS 8.4.8 : A class does not inherit static methods from its superinterfaces.

Java doesn't allow multiple inheritance for fields. This would be difficult to support in the JVM as you can only have references to the start of an object where the header is, not arbitrary memory locations.
In Oracle/Openjdk, objects have a header followed by the fields of the most super class, then the next most super class, etc. It would be a significant change to allow the fields of a class to appear at different offsets relative to the header of an object for different subclasses. Most likely object references would have to become a reference to the object header and a reference to the fields to support this.

default methods in interfaces pose a problem that :
If both of the implemented interfaces define a default method with
same method signature, then the implementation class does not know
which default method to use.
The implementation class should define explicitly specify which default method to use or define it's own one.
Thus default methods in Java-8 do not facilitate multiple inheritance. The main motivation behind default methods is that if at some point we need to add a method to an existing interface, we can add a method without changing the existing implementation classes. In this way, the interface is still compatible with older versions. However, we should remember the motivation of using Default Methods and should keep the separation of interface and implementation.

The main issues with multiple inheritance are ordering (for overriding and calls to super), fields and constructors; interfaces don't have fields or constructors, so they don't cause problems.
If you look at other languages they usually fall in two broad categories:
Languages with multiple inheritance plus a few features to disambiguate special cases: virtual inheritance [C++], direct calls to all superconstructors in the most-derived class [C++], linearization of superclasses [Python], complex rules for super [Python], etc.
Languages with a differente concept, usually called interfaces, traits, mixins, modules, etc. that impose some limitations such as: no constructors [Java] or no constructors with parameters [Scala until very recently], no mutable fields [Java], specific rules for overriding (e.g. mixins take precedence over base classes [Ruby] so you can include them when you need a bunch of utility methods), etc. Java has become a language like these.
Why just by disallowing fields and constructors you solve many issues related to multiple inheritance?
You can't have duplicated fields in duplicated base classes.
The main class hierarchy is still linear.
You can't construct your base objects the wrong way.
Imagine if Object had public/protected fields and all subclasses had constructors setting those fields. When you inherit from more than one class (all of them derived from Object), which one gets to set the fields? The last class? They become siblings in the hierarchy, so they know nothing about each other. Should you have multiple copies of Object to avoid this? Would all classes interoperate correctly?
Remember that fields in Java are not virtual (overridable), they are simply data storage.
You could make a language where fields behave like methods and could be overridden (the actual storage would be always private), but that would be a much bigger change and problably wouldn't be called Java anymore.
Interfaces can't be instantiated by themselves.
You should always combine them with a concrete class. That eliminates the need for constructors and makes the programmer's intent clearer too (that is, what is meant to be a concrete class and what's an accessory interface/mixin). This also provides a well-defined place to solve all ambiguities: the concrete class.

That is mostly related to "diamonds problem" i think. Right now if you implement multiple interfaces with the same method, compiler forces you to override method the one you want to implement, because it don't know which on to use. I guess Java creators wanted to remove this problem back when interfaces couldn't use default methods. Now they came up with idea, that is good to be able to have methods with implementation in interfaces, as you can still use those as functional interfaces in streams / lambda expressions and utilize their default methods in processing. You cannot do that with classes but diamond problem still exist there. That is my guess :)

class A{
void m1(){
System.out.println("m1-A");
}
}
class B{
void m1(){
System.out.println("m1-B");
}
}
class C extends A, B{ // this will give an error
// inheritance means making all variables and/or methods available to the child class, here child class will get confused as which m1() method to inherit, hence an error
}

JAVA DOES SUPPORT MULTIPLE INHERITANCE.
If you make a OVERALL COMPARISON OF THE PROGRAMMING LANGUAGE,JAVA,THEN YOU COME TO KNOW THAT I AM TRUE.
Java's topclass or the root class in the Ancestor Hierarchy is the Object class.
This class is a Superclass of all other classes. Hence, each class in Java that we declare or is predefined in the API itself inherits this Object class.
Moreover, Java provides us to inherit one more class of our choice.
Hence, we can say that we are performing INTERLOCKED BUT MULTIPLE INHERITANCE.
2ND Way
Java supports Multiple Inheritance of Interfaces. So you can use as many interface implementations you want. But note, implementing an interface does not define IS A relationship as in case of Inheritance of Classes is possible.

Significance of inheriting method from superclass instead of default method from implementing interface in java 8

I came across following paragraph while reading about java 8 default methods from here:
If any class in the hierarchy has a method with same signature, then default methods become irrelevant. A default method cannot override a method from java.lang.Object. The reasoning is very simple, it’s because Object is the base class for all the java classes. So even if we have Object class methods defined as default methods in interfaces, it will be useless because Object class method will always be used. That’s why to avoid confusion, we can’t have default methods that are overriding Object class methods.
I quickly tried following code to confirm the behavior
public class DefaultMethodClass {
public void defaultMethod()
{
System.out.println("DefaultMethodClass.defaultMethod()");
}
}
public interface DefaultMethodInterface {
public default void defaultMethod()
{
System.out.println("DefaultMethodInterface.defaultMethod()");
}
}
public class DefaultMethodClassInterfaceChild extends DefaultMethodClass implements DefaultMethodInterface
{
public static void main(String[] args) {
(new DefaultMethodClassInterfaceChild()).defaultMethod();
}
}
This prints
DefaultMethodClass.defaultMethod()
I am not able to see any specific reason behind why this particular behavior is chosen by language designer. Is their any such significant reason that I am missing? Or its just that interface default method logically bears lesser importance than concrete implementation provided by the super class?

I am not able to see any specific reason behind why this particular behavior is chosen by language designer.
Method definitions were in Java 1.0, Interface default methods where added in Java 8. Code written for Java 1.0 still needs to work.
Or its just that interface default method logically bears lesser importance than concrete implementation provided by the super class?
Effectively they have lesser importance being added after 20+ years of existing code being written.

Answer is simple: you are allowed to implement multiple interface but you can just extend one class. Indeed you can't have multiple default methods with the same name by implementing multiple interfaces.
If in any circumstance a default method has higher priority than a method implemented inside a real class in the hierarchy you end up having the same exact problem for which Java designers didn't want multiple inheritance: ambiguity.
As a personal opinion: a default method is seen as a "implementation of a functionality when none is defined", if the method is defined in a real class it is expected to be a more specific implementation.

What is the purpose of the default keyword in Java?

An interface in Java is similar to a class, but the body of an
interface can include only abstract methods and final fields
(constants).
Recently, I saw a question, which looks like this
interface AnInterface {
public default void myMethod() {
System.out.println("D");
}
}
According to the interface definition, only abstract methods are allowed. Why does it allow me to compile the above code? What is the default keyword?
On the other hand, when I was trying to write below code, then it says modifier default not allowed here
default class MyClass{
}
instead of
class MyClass {
}
Can anyone tell me the purpose of the default keyword? Is it only allowed inside an interface? How does it differ from default (no access modifier)?

It's a new feature in Java 8 which allows an interface to provide an implementation. Described in Java 8 JLS-13.5.6. Interface Method Declarations which reads (in part)
Adding a default method, or changing a method from abstract to default, does not break compatibility with pre-existing binaries, but may cause an IncompatibleClassChangeError if a pre-existing binary attempts to invoke the method. This error occurs if the qualifying type, T, is a subtype of two interfaces, I and J, where both I and J declare a default method with the same signature and result, and neither I nor J is a subinterface of the other.
What's New in JDK 8 says (in part)
Default methods enable new functionality to be added to the interfaces of libraries and ensure binary compatibility with code written for older versions of those interfaces.

Default methods were added to Java 8 primarily to support lambda expressions. The designers (cleverly, in my view) decided to make lambdas syntax for creating anonymous implementations of an interface. But given lambdas can only implement a single method they would be limited to interfaces with a single method which would be a pretty severe restriction. Instead, default methods were added to allow more complex interfaces to be used.
If you need some convincing of the claim that default was introduced due to lambdas, note that the straw man proposal of Project Lambda, by Mark Reinhold, in 2009, mentions 'Extension methods' as a mandatory feature to be added to support lambdas.
Here's an example demonstrating the concept:
interface Operator {
int operate(int n);
default int inverse(int n) {
return -operate(n);
}
}
public int applyInverse(int n, Operator operator) {
return operator.inverse(n);
}
applyInverse(3, n -> n * n + 7);
Very contrived I realise but should illustrate how default supports lambdas. Because inverse is a default it can easily be overriden by a implementing class if required.

A new concept is introduced in Java 8 called default methods. Default methods are those methods which have some default implementation and helps in evolving the interfaces without breaking the existing code. Lets look at an example:
public interface SimpleInterface {
public void doSomeWork();
//A default method in the interface created using "default" keyword
default public void doSomeOtherWork() {
System.out.println("DoSomeOtherWork implementation in the interface");
}
}
class SimpleInterfaceImpl implements SimpleInterface {
#Override
public void doSomeWork() {
System.out.println("Do Some Work implementation in the class");
}
/*
* Not required to override to provide an implementation
* for doSomeOtherWork.
*/
public static void main(String[] args) {
SimpleInterfaceImpl simpObj = new SimpleInterfaceImpl();
simpObj.doSomeWork();
simpObj.doSomeOtherWork();
}
}
and the output is:
Do Some Work implementation in the class
DoSomeOtherWork implementation in the interface

Something that was overlooked in the other answers was its role in annotations. As far back as Java 1.5, the default keyword came about as a means to provide a default value for an annotation field.
#Retention(RetentionPolicy.RUNTIME)
#Target(ElementType.TYPE)
public #interface Processor {
String value() default "AMD";
}
It usage was overloaded with the introduction of Java 8 to allow one to define a default method in interfaces.
Something else that was overlooked: the reason that the declaration default class MyClass {} is invalid is due to the way that classes are declared at all. There's no provision in the language that allows for that keyword to appear there. It does appear for interface method declarations, though.

Default methods in an interface allow us to add new functionality without breaking old code.
Before Java 8, if a new method was added to an interface, then all the implementation classes of that interface were bound to override that new method, even if they did not use the new functionality.
With Java 8, we can add the default implementation for the new method by using the default keyword before the method implementation.
Even with anonymous classes or functional interfaces, if we see that some code is reusable and we don’t want to define the same logic everywhere in the code, we can write default implementations of those and reuse them.
Example
public interface YourInterface {
public void doSomeWork();
//A default method in the interface created using "default" keyword
default public void doSomeOtherWork(){
System.out.println("DoSomeOtherWork implementation in the interface");
}
}
class SimpleInterfaceImpl implements YourInterface{
/*
* Not required to override to provide an implementation
* for doSomeOtherWork.
*/
#Override
public void doSomeWork() {
System.out.println("Do Some Work implementation in the class");
}
/*
* Main method
*/
public static void main(String[] args) {
SimpleInterfaceImpl simpObj = new SimpleInterfaceImpl();
simpObj.doSomeWork();
simpObj.doSomeOtherWork();
}
}

The new Java 8 feature (Default Methods) allows an interface to provide an implementation when its labeled with the default keyword.
For Example:
interface Test {
default double getAvg(int avg) {
return avg;
}
}
class Tester implements Test{
//compiles just fine
}
Interface Test uses the default keyword which allows the interface to provide a default implementation of the method without the need for implementing those methods in the classes that uses the interface.
Backward compatibility:
Imagine that your interface is implemented by hundreds of classes, modifying that interface will force all the users to implement the newly added method, even though its not essential for many other classes that implements your interface.
Facts & Restrictions:
1-May only be declared within an interface and not within a class or
abstract class.
2-Must provide a body
3-It is not assumed to be public or abstract as other normal methods used in an interface.

A very good explanation is found in The Java™ Tutorials, part of the explanation is as follows:
Consider an example that involves manufacturers of computer-controlled cars who publish industry-standard interfaces that describe which methods can be invoked to operate their cars. What if those computer-controlled car manufacturers add new functionality, such as flight, to their cars? These manufacturers would need to specify new methods to enable other companies (such as electronic guidance instrument manufacturers) to adapt their software to flying cars. Where would these car manufacturers declare these new flight-related methods? If they add them to their original interfaces, then programmers who have implemented those interfaces would have to rewrite their implementations. If they add them as static methods, then programmers would regard them as utility methods, not as essential, core methods.
Default methods enable you to add new functionality to the interfaces of your libraries and ensure binary compatibility with code written for older versions of those interfaces.

Default methods enable you to add new functionality to the interfaces of your apps. It can also be used to have a multi inheritance.
In addition to default methods, you can define static methods in interfaces. This makes it easier for you to organize helper methods

When you implement an interface in Java is it explicit or implicit?

I just started figuring out the difference between implicit and explicit interface implmentation in .Net. Since I come from a Java background the idea is still a bit confusing. I am hoping knowing which Java does will make it more obvious what the difference is. I am assuming the Java is explicit???

Nope Java is implicit. Explicit is where you are implementing multiple interfaces that have the same method signatures in them and you explicitly state which interface the implementation is for.
An example from MSDN:
public class SampleClass : IControl, ISurface
{
void IControl.Paint()
{
System.Console.WriteLine("IControl.Paint");
}
void ISurface.Paint()
{
System.Console.WriteLine("ISurface.Paint");
}
}
Here we have two Paint() methods, one from each interface. In Java you can only have one Paint() implementation. In C# you have the option of implementing versions for each interface so you get different behaviour depending how the class is called.
So if I called:
SampleClass c = new SampleClass();
((IControl)c).Paint();
((ISurface)c).Paint();
I'd get "IControl.Paint" printed out followed by "ISurface.Paint" printed out.

In Java there is no distinction between explicit and implicit. If you have two interfaces that both declare a method with the same signature, and a class that implements both interface, then a method in the class with the correct signature implements the method in BOTH interfaces.