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Why to move constants to class or enum from Interface? [duplicate]

I'm looking at some open source Java projects to get into Java and notice a lot of them have some sort of 'constants' interface.
For instance, processing.org has an interface called PConstants.java, and most other core classes implement this interface. The interface is riddled with static members. Is there a reason for this approach, or is this considered bad practice? Why not use enums where it makes sense, or a static class?
I find it strange to use an interface to allow for some sort of pseudo 'global variables'.
public interface PConstants {
// LOTS OF static fields...
static public final int SHINE = 31;
// emissive (by default kept black)
static public final int ER = 32;
static public final int EG = 33;
static public final int EB = 34;
// has this vertex been lit yet
static public final int BEEN_LIT = 35;
static public final int VERTEX_FIELD_COUNT = 36;
// renderers known to processing.core
static final String P2D = "processing.core.PGraphics2D";
static final String P3D = "processing.core.PGraphics3D";
static final String JAVA2D = "processing.core.PGraphicsJava2D";
static final String OPENGL = "processing.opengl.PGraphicsOpenGL";
static final String PDF = "processing.pdf.PGraphicsPDF";
static final String DXF = "processing.dxf.RawDXF";
// platform IDs for PApplet.platform
static final int OTHER = 0;
static final int WINDOWS = 1;
static final int MACOSX = 2;
static final int LINUX = 3;
static final String[] platformNames = {
"other", "windows", "macosx", "linux"
};
// and on and on
}

It's generally considered bad practice. The problem is that the constants are part of the public "interface" (for want of a better word) of the implementing class. This means that the implementing class is publishing all of these values to external classes even when they are only required internally. The constants proliferate throughout the code. An example is the SwingConstants interface in Swing, which is implemented by dozens of classes that all "re-export" all of its constants (even the ones that they don't use) as their own.
But don't just take my word for it, Josh Bloch also says it's bad:
The constant interface pattern is a poor use of interfaces. That a class uses some constants internally is an implementation detail. Implementing a constant interface causes this implementation detail to leak into the class's exported API. It is of no consequence to the users of a class that the class implements a constant interface. In fact, it may even confuse them. Worse, it represents a commitment: if in a future release the class is modified so that it no longer needs to use the constants, it still must implement the interface to ensure binary compatibility. If a nonfinal class implements a constant interface, all of its subclasses will have their namespaces polluted by the constants in the interface.
An enum may be a better approach. Or you could simply put the constants as public static fields in a class that cannot be instantiated. This allows another class to access them without polluting its own API.

Instead of implementing a "constants interface", in Java 1.5+, you can use static imports to import the constants/static methods from another class/interface:
import static com.kittens.kittenpolisher.KittenConstants.*;
This avoids the ugliness of making your classes implement interfaces that have no functionality.
As for the practice of having a class just to store constants, I think it's sometimes necessary. There are certain constants that just don't have a natural place in a class, so it's better to have them in a "neutral" place.
But instead of using an interface, use a final class with a private constructor. (Making it impossible to instantiate or subclass the class, sending a strong message that it doesn't contain non-static functionality/data.)
Eg:
/** Set of constants needed for Kitten Polisher. */
public final class KittenConstants
{
private KittenConstants() {}
public static final String KITTEN_SOUND = "meow";
public static final double KITTEN_CUTENESS_FACTOR = 1;
}

I do not pretend the right to be right, but lets see this small example:
public interface CarConstants {
static final String ENGINE = "mechanical";
static final String WHEEL = "round";
// ...
}
public interface ToyotaCar extends CarConstants //, ICar, ... {
void produce();
}
public interface FordCar extends CarConstants //, ICar, ... {
void produce();
}
// and this is implementation #1
public class CamryCar implements ToyotaCar {
public void produce() {
System.out.println("the engine is " + ENGINE );
System.out.println("the wheel is " + WHEEL);
}
}
// and this is implementation #2
public class MustangCar implements FordCar {
public void produce() {
System.out.println("the engine is " + ENGINE );
System.out.println("the wheel is " + WHEEL);
}
}
ToyotaCar doesnt know anything about FordCar, and FordCar doesnt know about ToyotaCar. principle CarConstants should be changed, but...
Constants should not be changed, because the wheel is round and egine is mechanical, but...
In the future Toyota's research engineers invented electronic engine and flat wheels! Lets see our new interface
public interface InnovativeCarConstants {
static final String ENGINE = "electronic";
static final String WHEEL = "flat";
// ...
}
and now we can change our abstraction:
public interface ToyotaCar extends CarConstants
to
public interface ToyotaCar extends InnovativeCarConstants
And now if we ever need to change the core value if the ENGINE or WHEEL we can change the ToyotaCar Interface on abstraction level, dont touching implementations
Its NOT SAFE, I know,
but I still want to know that do you think about this

There is a lot of hate for this pattern in Java. However, an interface of static constants does sometimes have value. You need to basically fulfill the following conditions:
The concepts are part of the public interface of several
classes.
Their values might change in future releases.
Its critical that all implementations use the same values.
For example, suppose that you are writing an extension to a hypothetical query language. In this extension you are going to expand the language syntax with some new operations, which are supported by an index. E.g. You are going to have a R-Tree supporting geospatial queries.
So you write a public interface with the static constant:
public interface SyntaxExtensions {
// query type
String NEAR_TO_QUERY = "nearTo";
// params for query
String POINT = "coordinate";
String DISTANCE_KM = "distanceInKm";
}
Now later, a new developer thinks he needs to build a better index, so he comes and builds an R* implementation. By implementing this interface in his new tree he guarantees that the different indexes will have identical syntax in the query language. Moreover, if you later decided that "nearTo" was a confusing name, you could change it to "withinDistanceInKm", and know that the new syntax would be respected by all your index implementations.
PS: The inspiration for this example is drawn from the Neo4j spatial code.

Given the advantage of hindsight, we can see that Java is broken in many ways. One major failing of Java is the restriction of interfaces to abstract methods and static final fields. Newer, more sophisticated OO languages like Scala subsume interfaces by traits which can (and typically do) include concrete methods, which may have arity zero (constants!). For an exposition on traits as units of composable behavior, see http://scg.unibe.ch/archive/papers/Scha03aTraits.pdf. For a short description of how traits in Scala compare with interfaces in Java, see http://www.codecommit.com/blog/scala/scala-for-java-refugees-part-5. In the context of teaching OO design, simplistic rules like asserting that interfaces should never include static fields are silly. Many traits naturally include constants and these constants are appropriately part of the public "interface" supported by the trait. In writing Java code, there is no clean, elegant way to represent traits, but using static final fields within interfaces is often part of a good workaround.

According to JVM specification, fields and methods in a Interface can have only Public, Static, Final and Abstract. Ref from Inside Java VM
By default, all the methods in interface is abstract even tough you didn't mention it explicitly.
Interfaces are meant to give only specification. It can not contain any implementations. So To avoid implementing classes to change the specification, it is made final. Since Interface cannot be instantiated, they are made static to access the field using interface name.

I do not have enough reputation to give a comment to Pleerock, therefor do I have to create an answer. I am sorry for that, but he put some good effort in it and I would like to answer him.
Pleerock, you created the perfect example to show why those constants should be independent from interfaces and independent from inheritance. For the client of the application is it not important that there is a technical difference between those implementation of cars. They are the same for the client, just cars. So, the client wants to look at them from that perspective, which is an interface like I_Somecar. Throughout the application will the client use only one perspective and not different ones for each different car brand.
If a client wants to compare cars prior to buying he can have a method like this:
public List<Decision> compareCars(List<I_Somecar> pCars);
An interface is a contract about behaviour and shows different objects from one perspective. The way you design it, will every car brand have its own line of inheritance. Although it is in reality quite correct, because cars can be that different that it can be like comparing completely different type of objects, in the end there is choice between different cars. And that is the perspective of the interface all brands have to share. The choice of constants should not make this impossible. Please, consider the answer of Zarkonnen.

This came from a time before Java 1.5 exists and bring enums to us. Prior to that, there was no good way to define a set of constants or constrained values.
This is still used, most of the time either for backward compatibility or due to the amount of refactoring needed to get rid off, in a lot of project.

Java, interface or composition for constants class

I have a question regarding the design of my program. I have a class A that stores public constant so that i can use these constants in another class.
public static final String error_code1 = "Fatal Error";
public static final String error_code2 = "XXXX";
...
...
Between Composition vs Interface, i dont know which 1 is more suitable. From what i think, since i only need the constants for value-comparing in my program, so i think composition is enough (low coupling).
But can you guys give me some advice/arguments from software deign point of view? (cohesion, coupling, difficulties of maintenance, etc )

First of all I'd recommend you to use an enum for this case.
public enum ErrorCode {
FATAL_ERROR("Fatal Error"),
X_ERROR("XXXX");
public final String msg;
private ErrorCode(String msg) {
this.msg = msg;
}
}
If this doesn't suit you for some reason, I'd go with a final utility class with private (unused) constructor.
Regardless, since the fields are static and final, I would not consider having a reference to A or implement A to get hold of the constants.

Adding constants to interfaces is considered an anti-pattern since the primary purpose of an interface is to define behavior contracts. Use either an enum or access them directly since they are public.

I wouldn't use interface to store constant as having static members into an interface (and implementing that interface) is a bad practice and there is even a name for it, the Constant Interface Antipattern, see [Effective Java][1], Item 17:
The constant interface pattern is a poor use of interfaces. That a class uses some constants internally is an implementation detail. Implementing a constant interface causes this implementation detail to leak into the class's exported API. It is of no consequence to the users of a class that the class implements a constant interface. In fact, it may even confuse them. Worse, it represents a commitment: if in a future release the class is modified so that it no longer needs to use the constants, it still must implement the interface to ensure binary compatibility. If a nonfinal class implements a constant interface, all of its subclasses will have their namespaces polluted by the constants in the interface.
I would personally go for enum and if needed i could even use it to have error code or add relevant field/method as well.

String/int/... constants in another class have one problem: they are copied into the using class' constant pool, and after that no import to the original class exists. Should you then alter a constant's value, the using class is not forced to be recompiled.
The solution would be to use an interface, and "implement" that interface; ugly maybe.
Better is to use an enum.
For open ended value domains one would not use an enumeration, but an object oriented approach:
abstract class ParseError extends RuntimeException
class ExpressionExpectedError extends ParseError
class DigitsMayNotFollowLeadingZeroError extends ParseError
..
In the javadoc one might see all child classes of ParseError. Here the classes themselves form the domain values, and an instantiation bears the actual context information. That is more OOP. Calling several methods on an object is better than having several switches on constants. An enum however may be used with categorical method too: boolean errorHandledBySkippingToNextExpr().

Header like Class for defining constants in Java [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
How do you define a class of constants in Java?
I would like to define a class in my package to contain only the constant vales like defines in C. I am a C programmer learning Java so perhaps that is why I still want to have some header like class :)
For this purpose here is the class I have:
package com.myclasses
public class defines{
public static byte final ID_1= 0x01;
public static final ID_2= 0x02;
public static String company_name="XYZ";
}
Then somewhere in a another class in the same package, I use these defines as follows:
byte idval = defines.ID_1;
... and so on.
My question is for such a "header" class what is the best way of defining it?
It has only static variables so should I define the class also static?
What about the access modifier? Since it has defines in it I thought it could be made "public".
Please advise.

Don't
There are hardly any constants that have value in their own. They only make sense in context. That context is a real class, i.e. a class that has instances (at least one). Declare the constants in that class.
As for the modifiers: reduce the scope as far as possible: Private if only used inside the class where they are declared, public if anybody using the class needs the constants as well.
If you declare more then one constant of same type in one class, think about if a enum makes mores sense.
And yes, constants should be static.

This pattern is called the "constant class" pattern (I think).
One way of using it is to make it an interface and implement it, then you get the references for "free":
public interface Defines {
static byte final ID_1= 0x01;
static final ID_2= 0x02;
// etc
}
public class MyClass implements Defines {
byte idval = ID_1; // Note: No need to refer to the class "Defines" here
}
but most people consider this an anti-pattern, because it isn't a real interface (it has no methods). Nevertheless, it is kind of cool, and may be a good way for you to ease into java.
The "standard" approach is to define a "utility class", which is one that has only static fields and methods, give it a private constructor to reinforce that you shouldn't create one of these. This is what you have done - keep doing it.
If you have a few constants that are different values of "the same thing", eg directions on a compass, etc, strongly consider using an enum. You should read up on them.

Use a final class
eg : public final class defines {
// private constructor
private defines() {
}
}
The constants should be defined as
public static final <type> constantName = <value>;
Wouldn't recommend enums in this scenario as Enums should be used when we are having constants which are having some relation between them.
Having a utility class like this, is the approach we use in our project to define constants that needs to be accessed across a project.
If you needs the constants only in that certain class then defining them in the class itself will be the best solution. eg:
private static final <type> constantName = <value>;

Just use an Interface in Java to define all your Constants..
public interface Const {
String GOVERNMENT = "Government";
String PUBLIC = "Public";
...
}
You can use class also.

Should a class implement a constants-only interface?

Today I looked at the ZipEntry class and found the following:
public class ZipEntry implements ZipConstants, Cloneable
ZipConstants does not define any methods - only constants (static final int LOCHDR = 30)
It then occurred to me that implementing the interface with constants lets you access those constants directly, as if they were defined in the class itself. For example:
public interface Constants {
static final int CONST = 2;
}
public class implements Constants {
int doSomething(int input) {
return CONST * input;
}
}
Is there another reason not to use this, apart from:
it is at first confusing where the constant is coming from
it is considered wrong to use interfaces for constants definition
I'm curious because it is definitely not a very common practice.

Another reasons not to use this:
Since Java 5, there is a "clean" language feature that achieves the same goal: static imports.
Implementing interfaces to use constants is basically a pre-Java-5 hack to simulate static imports.

It is not so rare as you might think, for instance in the static analysis of Parasofts JTest both the rule that constants should be declared in a class and the rule that constants should be declared in interfaces are present, and it's up to the project to choose between them.
That said, in all my projects I disallow the practice of defining constants in interfaces. Creating a meaningfull class and being explicit about the context of a constant makes code much more readable and thus maintainable than in the case where a developer has to check that constants used in one class are actually the same as those in another class (or not.)

I think that using an interface for shared constants is an example of confusing two different concepts:
Code reuse
Subtyping
In my experience using subclassing, or interface implementation simply to prevent the duplication of code leads to problems. Your code becomes more fragile. For example, someone might accidental redefine the constant - especially if your class hierarchy is several classes deep.
It is often better to use composition to keep your code DRY.
Another problem with using inheritance in this way is that generally this type of inheritance forms part of the API of your class. The hierarchy of the class is visible outside of the class. This breaks encapsulation. There is no need for you to expose your use of the constants outside of the class, they are to do with how you have chosen to implement your class and are not part of its API (in your example).
This can lead to horrible backwards compatibility problems. Someone else might come along and write code like this:
public interface Constants {
static final int CONST = 2;
}
public class MyClass implements Constants {
int doSomething(int input) {
return CONST * input;
}
}
public class ThirdPartyClass {
int doSomethingElse(int input) {
return MyClass.CONST + input;
}
}
Now, if you decide you no longer need to use CONST in MyClass you are stuck. Because ThirdPartyClass has create a dependency on CONST being available in MyClass.
You can end up with this. Where MyClass is not using any of the constants in the interface, but still has to implement it.
public interface Constants {
static final int CONST = 2;
}
public class MyClass implements Constants {
int doSomething(int input) {
return input;
}
}
public class ThirdPartyClass {
int doSomethingElse(int input) {
return MyClass.CONST + input;
}
}
In short; never do this!

... because it is considered wrong to use interfaces for constants definition
This is a bad reason not to do something. In fact, it is not a reason at all.
EDIT
Consider this.
The reason that XXX is bad style is YYY.
The reason you should do XXX is that it is bad style.
How many substantive reasons are there for not doing XXX? One or two?
If the answer is two, I can make it three, four, five and so on by adding extra tenuous chains of reasons. For example "The reason you should not do XXX is because it is a bad idea." "The reasons it is a bad idea is that it is bad style". And so on. That is plainly silly.
No the real reason for not doing XXX is YYY, and the "Bad style" reason is not a substantive reason. Rather, it is a short cut for saying don't do XXX because of YYY and ZZZ, and any other substantive reasons.
In fact, even the OP's "it is confusing" reason is incompletely stated. WHY is it confusing?
Because an interface is normally a type with classes that implement the interface are subtype. But a constant only interface is not a type in any useful sense, and classes that implement the interface are not subtypes in any useful sense. Ultimately, this is the real reason that implementing constant-only interfaces is called bad style and an "anti-pattern", and it is the main reason that the static imports were added in Java 5.

Nope, this is also known as the "Constant Interface Antipattern". An alternative is writing a concrete class which defines the constants and then use static import.
Class Constants
package util;
public class Constants {
public static final String CONSTANT_STRING = "abc";
private Constants() {
throw new AssertionError();
}
}
Class Test
import static util.Constants.CONSTANT_STRING;
public class Test {
System.out.println(CONSTANT_STRING);
}
See
Wikipedia
for further details.

One of the reasons for not putting your constants in your interface is is that if you expose your interface to a thirdparty they have access to your constants.
This may not seem like a bad idea to start off but imagine if you want to change the value of a constant but people are still using an old interface.
When you add something to an interface it has the potential to be set in stone so only add what you want others to see and use.

Why no static methods in Interfaces, but static fields and inner classes OK? [pre-Java8] [duplicate]

This question already has answers here:
Why can't I define a static method in a Java interface?
(24 answers)
Closed 3 years ago.
There have been a few questions asked here about why you can't define static methods within interfaces, but none of them address a basic inconsistency: why can you define static fields and static inner types within an interface, but not static methods?
Static inner types perhaps aren't a fair comparison, since that's just syntactic sugar that generates a new class, but why fields but not methods?
An argument against static methods within interfaces is that it breaks the virtual table resolution strategy used by the JVM, but shouldn't that apply equally to static fields, i.e. the compiler can just inline it?
Consistency is what I desire, and Java should have either supported no statics of any form within an interface, or it should be consistent and allow them.

An official proposal has been made to allow static methods in interfaces in Java 7. This proposal is being made under Project Coin.
My personal opinion is that it's a great idea. There is no technical difficulty in implementation, and it's a very logical, reasonable thing to do. There are several proposals in Project Coin that I hope will never become part of the Java language, but this is one that could clean up a lot of APIs. For example, the Collections class has static methods for manipulating any List implementation; those could be included in the List interface.
Update: In the Java Posse Podcast #234, Joe D'arcy mentioned the proposal briefly, saying that it was "complex" and probably would not make it in under Project Coin.
Update: While they didn't make it into Project Coin for Java 7, Java 8 does support static functions in interfaces.

I'm going to go with my pet theory with this one, which is that the lack of consistency in this case is a matter of convenience rather than design or necessity, since I've heard no convincing argument that it was either of those two.
Static fields are there (a) because they were there in JDK 1.0, and many dodgy decisions were made in JDK 1.0, and (b) static final fields in interfaces are the closest thing java had to constants at the time.
Static inner classes in interfaces were allowed because that's pure syntactic sugar - the inner class isn't actually anything to do with the parent class.
So static methods aren't allowed simply because there's no compelling reason to do so; consistency isn't sufficiently compelling to change the status quo.
Of course, this could be permitted in future JLS versions without breaking anything.

There is never a point to declaring a static method in an interface. They cannot be executed by the normal call MyInterface.staticMethod(). (EDIT:Since that last sentence confused some people, calling MyClass.staticMethod() executes precisely the implementation of staticMethod on MyClass, which if MyClass is an interface cannot exist!) If you call them by specifying the implementing class MyImplementor.staticMethod() then you must know the actual class, so it is irrelevant whether the interface contains it or not.
More importantly, static methods are never overridden, and if you try to do:
MyInterface var = new MyImplementingClass();
var.staticMethod();
the rules for static say that the method defined in the declared type of var must be executed. Since this is an interface, this is impossible.
You can of course always remove the static keyword from the method. Everything will work fine. You may have to suppress some warnings if it is called from an instance method.
To answer some of the comments below, the reason you can't execute "result=MyInterface.staticMethod()" is that it would have to execute the version of the method defined in MyInterface. But there can't be a version defined in MyInterface, because it's an interface. It doesn't have code by definition.

The purpose of interfaces is to define a contract without providing an implementation. Therefore, you can't have static methods, because they'd have to have an implementation already in the interface since you can't override static methods. As to fields, only static final fields are allowed, which are, essentially, constants (in 1.5+ you can also have enums in interfaces). The constants are there to help define the interface without magic numbers.
BTW, there's no need to explicitly specify static final modifiers for fields in interfaces, because only static final fields are allowed.

This is an old thread , but this is something very important question for all. Since i noticed this today only so i am trying to explain it in cleaner way:
The main purpose of interface is to provide something that is unimplementable, so if they provide
static methods to be allowed
then you can call that method using interfaceName.staticMethodName(), but this is unimplemented method and contains nothing. So it is useless to allow static methods. Therefore they do not provide this at all.
static fields are allowed
because fields are not implementable, by implementable i mean you can not perform any logical operation in field, you can do operation on field. So you are not changing behavior of field that is why they are allowed.
Inner classes are allowed
Inner classes are allowed because after compilation different class file of the Inner class is created say InterfaceName$InnerClassName.class , so basically you are providing implementation in different entity all together but not in interface. So implementation in Inner classes is provided.
I hope this would help.

Actually sometimes there are reasons someone can benefit from static methods. They can be used as factory methods for the classes that implement the interface. For example that's the reason we have Collection interface and the Collections class in openjdk now. So there are workarounds as always - provide another class with a private constructor which will serve as a "namespace" for the static methods.

Prior to Java 5, a common usage for static fields was:
interface HtmlConstants {
static String OPEN = "<";
static String SLASH_OPEN = "</";
static String CLOSE = ">";
static String SLASH_CLOSE = " />";
static String HTML = "html";
static String BODY = "body";
...
}
public class HtmlBuilder implements HtmlConstants { // implements ?!?
public String buildHtml() {
StringBuffer sb = new StringBuffer();
sb.append(OPEN).append(HTML).append(CLOSE);
sb.append(OPEN).append(BODY).append(CLOSE);
...
sb.append(SLASH_OPEN).append(BODY).append(CLOSE);
sb.append(SLASH_OPEN).append(HTML).append(CLOSE);
return sb.toString();
}
}
This meant HtmlBuilder would not have to qualify each constant, so it could use OPEN instead of HtmlConstants.OPEN
Using implements in this way is ultimately confusing.
Now with Java 5, we have the import static syntax to achieve the same effect:
private final class HtmlConstants {
...
private HtmlConstants() { /* empty */ }
}
import static HtmlConstants.*;
public class HtmlBuilder { // no longer uses implements
...
}

There is no real reason for not having static methods in interfaces except: the Java language designers did not want it like that.
From a technical standpoint it would make sense to allow them. After all an abstract class can have them as well. I assume but did not test it, that you can "hand craft" byte code where the interface has a static method and it should imho work with no problems to call the method and/or to use the interface as usually.

I often wonder why static methods at all? They do have their uses, but package/namespace level methods would probably cover 80 of what static methods are used for.

Two main reasons spring to mind:
Static methods in Java cannot be overridden by subclasses, and this is a much bigger deal for methods than static fields. In practice, I've never even wanted to override a field in a subclass, but I override methods all the time. So having static methods prevents a class implementing the interface from supplying its own implementation of that method, which largely defeats the purpose of using an interface.
Interfaces aren't supposed to have code; that's what abstract classes are for. The whole point of an interface is to let you talk about possibly-unrelated objects which happen to all have a certain set of methods. Actually providing an implementation of those methods is outside the bounds of what interfaces are intended to be.

Static methods are tied to a class. In Java, an interface is not technically a class, it is a type, but not a class (hence, the keyword implements, and interfaces do not extend Object). Because interfaces are not classes, they cannot have static methods, because there is no actual class to attach to.
You may call InterfaceName.class to get the Class Object corresponding to the interface, but the Class class specifically states that it represents classes and interfaces in a Java application. However, the interface itself is not treated as a class, and hence you cannot attach a static method.

Only static final fields may be declared in an interface (much like methods, which are public even if you don't include the "public" keyword, static fields are "final" with or without the keyword).
These are only values, and will be copied literally wherever they are used at compile time, so you never actually "call" static fields at runtime. Having a static method would not have the same semantics, since it would involve calling an interface without an implementation, which Java does not allow.

The reason is that all methods defined in an interface are abstract whether or not you explicitly declare that modifier. An abstract static method is not an allowable combination of modifiers since static methods are not able to be overridden.
As to why interfaces allow static fields. I have a feeling that should be considered a "feature". The only possibility I can think of would be to group constants that implementations of the interface would be interested in.
I agree that consistency would have been a better approach. No static members should be allowed in an interface.

I believe that static methods can be accessed without creating an object and the interface does not allow creating an object as to restrict the programmers from using the interface methods directly rather than from its implemented class.
But if you define a static method in an interface, you can access it directly without its implementation. Thus static methods are not allowed in interfaces.
I don't think that consistency should be a concern.

Java 1.8 interface static method is visible to interface methods only, if we remove the methodSta1() method from the InterfaceExample class,
we won’t be able to use it for the InterfaceExample object. However like other static methods, we can use interface static methods using class name.
For example, a valid statement will be:
exp1.methodSta1();
So after looking below example we can say :
1) Java interface static method is part of interface, we can’t use it for implementation class objects.
2) Java interface static methods are good for providing utility methods, for example null check, collection sorting ,log etc.
3) Java interface static method helps us in providing security by not allowing implementation classes (InterfaceExample) to override them.
4) We can’t define interface static method for Object class methods, we will get compiler error as “This static method cannot hide the instance method from Object”. This is because it’s not allowed in java, since Object is the base class for all the classes and we can’t have one class level static method and another instance method with same signature.
5) We can use java interface static methods to remove utility classes such as Collections and move all of it’s static methods to the corresponding interface,
that would be easy to find and use.
public class InterfaceExample implements exp1 {
#Override
public void method() {
System.out.println("From method()");
}
public static void main(String[] args) {
new InterfaceExample().method2();
InterfaceExample.methodSta2(); // <--------------------------- would not compile
// methodSta1(); // <--------------------------- would not compile
exp1.methodSta1();
}
static void methodSta2() { // <-- it compile successfully but it can't be overridden in child classes
System.out.println("========= InterfaceExample :: from methodSta2() ======");
}
}
interface exp1 {
void method();
//protected void method1(); // <-- error
//private void method2(); // <-- error
//static void methodSta1(); // <-- error it require body in java 1.8
static void methodSta1() { // <-- it compile successfully but it can't be overridden in child classes
System.out.println("========= exp1:: from methodSta1() ======");
}
static void methodSta2() { // <-- it compile successfully but it can't be overridden in child classes
System.out.println("========= exp1:: from methodSta2() ======");
}
default void method2() { System.out.println("--- exp1:: from method2() ---");}
//synchronized default void method3() { System.out.println("---");} // <-- Illegal modifier for the interface method method3; only public, abstract, default, static
// and strictfp are permitted
//final default void method3() { System.out.println("---");} // <-- error
}