I've been wondering why it's allowed to do a code implementation in an interface, when interfaces are suppossed to contain no code implementation:
public interface someInterface{
String someString = "example";
}
I can make a class implement this interface, without getting an error:
public class someClass implements someInterface
How come?
You are allowed to declare constants in interfaces, which is what you have done. You have not implemented code.
Variables declared in interfaces are implicitly declared public static final.
The JLS, Section 9.3, covers this:
Every field declaration in the body of an interface is implicitly
public, static, and final. It is permitted to redundantly specify any
or all of these modifiers for such fields.
According to java docs
Interfaces form a contract between the class and the outside world, and this contract is enforced at build time by the compiler. If your class claims to implement an interface, all methods defined by that interface must appear in its source code before the class will successfully compile.
Here you are not defined any methods to implement.So you didn't get any error here.
There is no strict condition that an interface must have signatured methods.Remember there are Marker Interfaces too in java.
And secondly , You can declare variables inside interface.
And that variable someString assigned in a static context and shared across all the implemntations by that interface
Point is that the variables inside declared interface are implicitly static and final.You can use them.
Related
How can the method getSequencer in JavaSound API return an instance of the interface Sequencer?
Sequencer sequencer=MidiSystem.getSequencer();
I have read that we cant create an instance of an interface.
static is a very troublesome concept at best, its greatest akin for trouble at explaining is "volatile" keyword declaration.
You would have less trouble with "synchronized" keyword on a code block than the previous two for explaining their usage parameters and concept!
"static" is not constructed as "new" because it is not a "separate instance" it is already available when compiled in as a static object.
All interfaces in Java are abstract but have "static" fields(variables) only,
ONLY ONE of those loaded static class instruction version of AKA(A Kind of Alias) "instance" of a class(or alternately interface) will be present at that class hierarchy level on the process in the JVM runtime in that particular "user classes" hierarchy structure of call for any number of classes created that commit call of a static object or a static method (static code DOES NOT MAKE A NEW SEPARATE SET OF INSTRUCTION IF CALLED CONCURRENTLY FROM VARIOUS CLASS COPIES).
With anything "static" there is only one copy in use for all of the program at that calling class on the PID process level in the JVM during runtime.
You cannot instantiate MidiSystem because all its methods are "static"
So to use ANY static class to call one of its static methods from it (or the same on an interface)
you only use its class name followed by the dot operator on its method you wish to call.
Exactly as you have it in the code and syntax you posted. (NOTE "Sequencer" is actually static )
But if you need your variable non static to remove static you cast it if the class type to cast to is non static, only if the class you are casting is not itself an actual "static" compiled class !
e.g. DriverManager.getDriver() for JDBC database running more than one connection concurrently cannot use static driver copies or there would only be one copy available in use of during runtime with the instructions template (the class byte code for the static class) !
To remove "static" from an object, the object must be cast to non static into a variable of same object type that is not of static notation(declared).
// the getDriver() method is static inside class DriverManager ,
// Driver is an Interface not a class
Driver driver = (Driver) DriverManager.getDriver( configuration.jdbcUrl() );
// After casting, there is now a separate non static reference of Driver
// interface , so **note that neither MidiSystem or DriverManager class are**
// actually declared static and both have no constructor and not declared
// abstract but contain only static methods !
an "INSTANCE" is something you construct , so another one is a new instance !
Interfaces are not constructed, they operate much more alike "abstract" and "static" declarations.
Using the class name only is the syntax of calling EITHER abstract or static classes to obtain their methods.
A final point , to refer to an interface as a "data type" is to make a reference variable to represent it because an interface IS a data type (known as an object) the same as a class or abstract class.
So your above code has "Sequencer" interface as a data type.
When a class "implements" an interface the class itself can be cast to that interface because it is ALSO that object type.
e.g.
public class Example implements Extra{.....}
Extra example = (Extra)new Example();
// next below shorthand implicit cast is compiler dependent
public class Example implements Extra{.....}
Extra example = new Example();
If you do not implement an interface in a class the interface can be called into the code with assignment of a reference variable by using a class that has a method that obtains that interface data type.
There is a huge relationship between abstract classes and interfaces but they are not the same.
Abstract classes do not have global variables.
Interfaces do have global variables but all of them must be static and final. Abstract classes cannot have any global variables or it would be an "instance of a class" and would then require to be constructed as "new".
Abstract classes have less strict rules on method declaration than interfaces.
Abstract classes can have most class modifiers interfaces are all public
Interfaces have "default" modifier for methods that contain an implementation body of code or must be static method.
In short the variable for Sequencer interface is not an instance variable, it is a reference and (clause for static) you are referring to something defined as "static" so IT MUST be there when the class that calls it starts !
Why can't constructors be final, static, or abstract in Java?
For instance, can you explain to me why this is not valid?
public class K {
abstract public K() {
// ...
}
}
When you set a method as final it means: "I don't want any class override it." But according to the Java Language Specification:
JLS 8.8 - "Constructor declarations are not members. They are never inherited and therefore are not subject to hiding or overriding."
When you set a method as abstract it means: "This method doesn't have a body and it should be implemented in a child class." But the constructor is called implicitly when the new keyword is used so it can't lack a body.
When you set a method as static it means: "This method belongs to the class, not a particular object." But the constructor is implicitly called to initialize an object, so there is no purpose in having a static constructor.
The question really is why you want constructor to be static or abstract or final.
Constructors aren't inherited so can't be overridden so whats the use
to have final constructor
Constructor is called automatically when an instance of the class is
created, it has access to instance fields of the class. What will be
the use of a static constructor.
Constructor can't be overridden so what will you do with an abstract
constructor.
A Java constructor is implicitly final, the static / non-static aspects of its semantics are implicit1, and it is meaningless for a Java constructor to be abstract.
This means that the final and static modifiers would be redundant, and the abstract keyword would have no meaning at all.
Naturally, the Java designers didn't see in any point in allowing redundant and/or meaningless access modifiers on constructors ... so these are not allowed by the Java grammar.
Aside: It is a shame that they didn't make the same design call for interface methods where the public and abstract modifiers are also redundant, but allowed anyway. Perhaps there is some (ancient) historical reason for this. But either way, it cannot be fixed without rendering (probably) millions of existing Java programs uncompilable.
1 - Actually, constructors have a mixture of static and non-static semantics. You can't "call" a constructor on an instance, and it they are not inherited, or overridable. This is similar to the way static methods work. On the other hand, the body of a constructor can refer to this, and call instance methods ... like an instance method. And then there is constructor chaining, which is unique to constructors. But the real point is that these semantics are fixed, and there is no point allowing a redundant and probably confusing static modifier.
public constructor: Objects can be created anywhere.
default constructor: Objects can be created only in the same package.
protected constructor: Objects can be created by classes outside the package only if it's a subclass.
private constructor: Object can only be created inside the class (e.g., when implementing a singleton).
The static, final and abstract keywords are not meaningful for a constructor because:
static members belong to a class, but the constructor is needed to create an object.
An abstract class is a partially implemented class, which contains abstract methods to be implemented in child class.
final restricts modification: variables become constant, methods can't be overridden, and classes can't be inherited.
Final: Because you can't overwrite/extend a constructor anyway. You can extend a class (to prevent that you make it final) or overwrite a method (to prevent that you make it final), but there is nothing like this for constructors.
Static: If you look at the execution a constructor is not static (it can access instance fields), if you look at the caller side it is (kind of) static (you call it without having an instance. Its hard to imagine a constructor being completely static or not static and without having a semantic separation between those two things it doesn't make sense to distinguish them with a modifier.
Abstract: Abstract makes only sense in the presence of overwriting/extension, so the same argument as for 'final' applies
No Constructors can NEVER be declared as final. Your compiler will always give an error of the type "modifier final not allowed"
Final, when applied to methods, means that the method cannot be overridden in a subclass.
Constructors are NOT ordinary methods. (different rules apply)
Additionally, Constructors are NEVER inherited. So there is NO SENSE in declaring it final.
Constructors are NOT ordinary methods. (different rules apply)
Additionally, Constructors are NEVER inherited. So there is NO SENSE in declaring it final.
No Constructors can NEVER be declared final. YOur compiler will always give an error of the type "modifer final not allowed"
Check the JLS Section 8.8.3 (The JLS & API docs should be some of your primary sources of information).
JLS section 8 mentions this.
Constructors (§8.8) are similar to methods, but cannot be invoked
directly by a method call; they are used to initialize new class
instances. Like methods, they may be overloaded (§8.8.8).
But constructors per say are not regular methods. They can't be compared as such.
why constructor can not be static and final are well defined in above answers.
Abstract: "Abstract" means no implementation . and it can only be implemented via inheritance. So when we extends some class, all of parent class members are inherited in sub-class(child class) except "Constructor". So, lets suppose, you some how manage to declare constructor "Abstract", than how can you give its implementation in sub class, when constructor does not get inherit in child-class?
that's why constructor can't be
abstract .
lets see first
final public K(){
*above the modifier final is restrict 'cause if it final then some situation where in some other class or same class only we will override it so thats not gonna happen here proximately not final
eg:
we want public void(int i,String name){
//this code not allowed
let static,, static itz all about class level but we create the object based constructor by using 'new' keyword so,,,,,, thatsall
abstract itz worst about here not at 'cause not have any abstract method or any declared method
Unfortunately in PHP the compiler does not raise any issue for both abstract and final constructor.
<?php
abstract class AbstractClass
{
public abstract function __construct();
}
class NormalClass
{
public final function __construct() {
echo "Final constructor in a normal class!";
}
}
In PHP static constructor is not allowed and will raise fatal exception.
Here in AbstractClass obviously a constructor either can be declared as abstract plus not implemented or it can be declared as something among (final, public, private, protected) plus a function body.
Some other related facts on PHP:
In PHP having multiple constructor __construct() is not possible.
In PHP a constructor __construct() can be declared as abstract, final, public, private and protected!
This code was tested and stood true for in PHP versions from 5.6 up to 7.4!
I was reading this post Why would a static nested interface be used in Java? in particular the first answer. In that answer is written that use the words "public" or "public final" on interface fields are redundant. My question is: why?
Why should I remove them? If I have something like this:
public interface Int1 {
public void add();
void remove();
}
Doesn't it mean that I want add method to be implementated by whatever class while remove method to be implementated only by classes of my same package?
Are “public” and “public final” redundant for interface methods?
Yes.
All methods in an interface are implicitly public and abstract (but not final).
All fields in an interface are implicitly public, static and final.
The JLS states this. It also states that these modifiers can be left out.
Why? Well there are a variety of reasons:
Fields and methods are implicitly public because the point of an interface is to declare an ... interface that other classes can see. (If you want / need to restrict access, this is done via an access modifier on the interface itself.)
Fields are static because if they were not you would be declaring visible instance fields on an object ... and that's bad for encapsulation.
Fields are final because non-final fields would be another way of declaring public static fields ... which are terrible from an OO perspective.
Methods are abstract because allowing method bodies would effectively turn interfaces into abstract classes.
Another reason for making methods abstract and fields static in an interface is that if they didn't, diamond inheritance, and inheritance of a method from two distinct interfaces would both be problematic.
But either way, this is how Java is defined, so the questions are moot ... unless you are thinking of inventing your own programming language.
Note that in Java 8, you can declare methods in an interface, using the default modifier. And in Java 9, you can declare private methods, in some cases. But use of the public keyword is still redundant.
Why should I remove them?
You don't have to remove them. The Java compiler doesn't care.
You can remove them, but you don't have to remove them, unless you are trying to conform to some Java style guidelines that insist on this.
Your code will probably be more readable if you are consistent, but you could make it consistent by using the redundant modifiers everywhere; e.g. adding them rather than removing them.
Doesn't it mean that I want add method be implemented by whatever class while remove method implemented only by classes of my same package?
No it doesn't mean that. Or at least, it might mean that to you, but it won't mean that to the Java compiler, other Java tools ... or other people reading and maintaining your code. IMO, it would be ill-advised to place any meaning on the presence or absence of redundant keywords.
You cannot have a final method declared in an interface. Fields are always final but methods are always abstract (and never final). You cannot define an interface method that is to be implemented only by classes in the same package.* From section 9.3 of the Java Language Specification:
Every field declaration in the body of an interface is implicitly public, static, and final. It is permitted to redundantly specify any or all of these modifiers for such fields.
and from section 9.4:
Every method declaration in the body of an interface is implicitly public (§6.6).
Every method declaration in the body of an interface is implicitly abstract, so its body is always represented by a semicolon, not a block.
It is permitted, but discouraged as a matter of style, to redundantly specify the public and/or abstract modifier for a method declared in an interface.
* As Paul Bellora points out in a comment, you can make the interface itself package-private (or protected, or even private) if you want to restrict its visibility.
Interfaces by definition are abstract so the abstract modifier on the interface is redundant.
Variables in interfaces and annotations are automatically public, static and final, so these modifiers are redundant as well.
As annotations are a form of interface, their fields are also automatically public, static and final just as their annotation fields are automatically public and abstract.
Final classes by definition cannot be extended so the final modifier on the method of a final class is redundant.
reading this: http://checkstyle.sourceforge.net/config_modifier.html
Yes the public is redundant, because in an Interface all methods are implictly public and abstract.
I think its is a bad style to add public, or abstract, because both are implicitly applied.
public interface Int1 {
void add();
void remove();
}
This looks cleaner, and shows that you know, that they are implict public
from Java Language Specification (JLS)
9.4. Abstract Method Declarations
Every method declaration in the body of an interface is implicitly public (§6.6).
Every method declaration in the body of an interface is implicitly
abstract, so its body is always represented by a semicolon, not a
block.
It is permitted, but discouraged as a matter of style, to redundantly
specify the public and/or abstract modifier for a method declared in
an interface.
I write interfaces without the public keyword for methods. It's redundant.
As you know, in a java interface, all methods have to be defined as abstract. But when I define a method as not typing abstract, the compiler says it is okay. I know that an abstract method must not have a body. Does a method somewhere in an interface necessarily have a name abstract or not? : What i mean is, what is the difference between:
public interface blabla {
public void aMethod();
//or
public abstract void aMethod();
}
No, marking an interface method as abstract has no meaning and is never required.
All interface methods are implicitly abstract (and public too btw).
From the JLS:
Every method declaration in the body of an interface is implicitly abstract, so its body is always represented by a semicolon, not a block.
Every method declaration in the body of an interface is implicitly public.
For compatibility with older versions of the Java platform, it is permitted but discouraged, as a matter of style, to redundantly specify the abstract modifier for methods declared in interfaces.
It is permitted, but strongly discouraged as a matter of style, to redundantly specify the public modifier for interface methods.
Related question (+ answer with a historical reference to a statement saying that abstract was once required for interface methods):
Java abstract interface
See the sample example below
interface xyz
{
void methodA();
}
Save this to xyz.java
Now compile this using javac tool
and then use the command given belo
javap xyz
the output would be
Compiled from "xyz.java"
interface xyz {
public abstract void methodA();
}
That means when you compile an interface, compiler makes its signature to public and abstract by default.
So it is not necessary to use abstract keyword for any method of interface.
I don't know that they have to be defined as abstract. Probably because they don't. See Oracle's tutorial.
you don't need to specify abstract (default) because within an interface it does not make sense as all the method of the interface needs to be implemented
All methods in an interface are abstract by definition.
You can't create an object out of an interface (e.g., using Interface i = new Interface();) so there's no difference between marking a method as abstract or not.
Any class that implements the interface needs to decide whether to implement it or to let a subclass do it. So as far as the interface is concerned, all methods are abstract by default.
An abstract method provides no implementation. A class which has an abstract method is necessarily abstract, which means that you cannot create instances of this class. To create an instance of that class, you need to subclass and provide non-abstract overwrites for the abstract methods.
An interface never provides an implementation of its methods and it cannot be instantiated. Therefore every method of an interface is per definition abstract. You do not need to provide the keyword abstract when declaring a method in an interface. And by convention the keyword abstract is not used within an interface.
The methods of an interface don't have to be explicitly defined as abstract because they are implicitly abstract and public as defined in the Java Language Specification §9.4. A redundant declaration is perfectly legal though.
If you forgot to put abstract keyword before interface method, Java will implicitly put public abstract keyword before it. Because all interface methods must be abstract.
In java document, it is said :
Unlike interfaces, abstract classes
can contain fields that are not
static and final, and they can contain
implemented methods.
Is that a correct text? that not part confuses me because interfaces don't have static or final fields, right?
Source : http://download.oracle.com/javase/tutorial/java/IandI/abstract.html
Thanks.
Edit :
public interface GroupedInterface extends Interface1,
Interface2, Interface3 {
// constant declarations
double E = 2.718282; // base of natural logarithms
// method signatures
void doSomething (int i, double x);
int doSomethingElse(String s);
}
An interface can contain constant
declarations in addition to method
declarations. All constant values
defined in an interface are implicitly
public, static, and final. Once again,
these modifiers can be omitted.
Every field declaration in the body of
an interface is implicitly public,
static, and final. It is permitted to
redundantly specify any or all of
these modifiers for such fields.
from section 9.3 of the Java Language Specification (here)
Click on "Defining an Interface" on the link in your question:
An interface can contain constant
declarations in addition to method
declarations. All constant values
defined in an interface are implicitly
public, static, and final. Once again,
these modifiers can be omitted.
That is the correct text.
All fields in an interface are inferred to be public, static and final, whether or not explicitly so declared. Just as all methods are public and abstract, whether or not so declared.
the think is.. all fields inside an interface will be static and final, even if you didnt write the static and final!
The documentation is correct. Interfaces may contain static final fields to be used as constants. Abstract classes may contain instance variables to be inherited by extending classes. Those variables are then available in instances of the extending classes.
The quote is correct. Interfaces can have static final fields, but cannot have any other combination (non-static or non-final).
Fields on an interface are static and final by default, adding the modifiers is not necessary because there's no alternative.
For an abstract class it can make sense to give it mutable state, see java.util.AbstractList. Interfaces are not allowed to have any member that would confer mutable state on a class implementing it.