I have the following situation.
package A;
class SampleClass
{
static interface sampleInterface
{
....
}
}
Now when I try to import the sampleInterface from another package , jDev says 'access not allowed'. What could be the problem?
Currently, the interface is seen as package-private (there's no visibility modifier, so that's the default). Place public on the outer class and the interface, and it will become visible to other classes.
Just be careful - if you get caught in a situation where you have to do this:
public class Alpha extends Alpha.IAlpha {
public void doNothing();
public static interface IAlpha {
public void doNothing();
}
}
...you'll have an issue with cyclic inheritance, and your class won't compile. In fact, you won't be able to use the interface at all.
Keep these rules in mind for exposing interfaces, classes, or enums:
If you only need an inner class, interface, or enum for that particular object, then it's fine to declare it as static.
If you need a class, interface, or enum accessible from anywhere but that object, then it's best to move it out of the inner class, and into its own file.
In general, interfaces are seen as APIs to conform by - there's really no benefit in having them as nested unless the scope of them is extremely narrow.
Change visibility of the class and the interface to public. It will work for sure.
When you declare a class without a access specifier it is by package-default. This means you can access that class in that package only.
If you want to access class from another package, make class public, i.e.
public class SampleClass
Similarly, in your case, as you want to access the Interface as well, you have to make that interface public as well.
This will solve your problem.
Related
While reading oracles Java documentation(chapter 8) I came across this interesting piece of line:
"If two or more (distinct) class modifiers appear in a class declaration, then it is customary, though not required, that they appear in the order consistent with that shown above in the production for ClassModifier."
and here is the Class Modifiers
ClassModifiers:
ClassModifier
ClassModifiers ClassModifier
ClassModifier: one of
Annotation public protected private
abstract static final strictfp
My question is what is the need for multiple class modifiers? when we can make use of multiple class modifiers?
Well, you can have a public abstract static strictfp inner class; each one of those is orthogonal to the rest. In general, you can group them in the following way:
public/protected/private: access modifiers - determining who has access to the class literal.
abstract/final: inheritance modifiers
static: only for inner classes
strictfp: it's orthogonal to everything else.
Two examples:
public final class MyClass { }
protected abstract class MyClass {}
Simply, to create a class that has the features dictated by several modifiers. Not all the modifiers are mutually exclusive. For example, abstract disallows making instances of the class, and public makes the class accessible even outside its package. Hence, to make an abstract class that's accessible outside its own package, you'd need to make it public abstract class.
Why does the JAVA compiler not give error when we have public methods inside default class ?
When we have non public classes (lets say with default access level) and if we have public methods within that class as follows ,
class Main {
public void doStuff() {
}
}
then we cannot access above doStuff() method from outside of the same package. But it is legal to have public access modifier for above method deceleration. So what is the purpose of it ?
Because it's not an error, and because it could be necessary to avoid an error: for example, if it's an implementation of an interface method.
There is no harm in allowing public access to members of classes with package visibility: there is no contradiction in defining it this way, so the compiler allows it.
However, you are certainly right that making a member of a package-visible class public does not expand its visibility: if the owning class has package visibility, making all its public members package-visible as well is not going to change anything.
Somebody in the same package could instantiate a Main, and pass it to another class as a Object or an Interface. In the former case methods like toString() and equals() must be public, in the latter methods of that Interface must be.
I am wondering what does it mean to have a nested abstract class ? for example,
abstract class A{
abstract class B{
}
}
Are there any use cases or scenario that we might need such as design ? or is there something useful in such pattern ? and why Java allows us to do it ?
In design, you want the base class class A to present only an interface for its derived classes. This means, you don’t want anyone to actually instantiate an object of the base class. You only want to upcast to it (implicit upcasting, which gives you polymorphic behavior), so that its interface can be used. This is accomplished by making that class abstract using the abstract keyword. In other hand you want to use only part of functionality of class A so you create class B (as child) to reduce the coupling or implementation dependencies between systems and prevent duplicates.
But bear in mind when you define an inner class, code without inner classes is more maintainable and readable. When you access private data members of the outer class, the JDK compiler creates package-access member functions in the outer class for the inner class to access the private members. This leaves a security hole. In general we should avoid using inner classes. Use inner class only when an inner class is only relevant in the context of the outer class and/or inner class can be made private so that only outer class can access it. Inner classes are used primarily to implement helper classes like Iterators, Comparators etc which are used in the context of an outer class. About abstract class, it should be abstract to helpers, suppose your helpers should be too complicated to write abstract form for them.
In your case, I don't remember extensive usage of nested abstract classes, maybe in Swing world.
abstract classes are used to provide a partial implementation of a class for inheritance. it allows you to define the scheme of a class without providing the full definiton, so that it can be specified in a child class. it works somewhat like a Interface in that you can perform any operation specified in the abstract class upon an instance of any classes derived from it. Nested abstracted classes are designed to be inherited by other inner classes (even anonymous ones I think) but not by classes defined outside the outermost class.
public class HelloEveryone{
abstract class Hello{
void SayHello(){
System.out.println("Hello!");
}
abstract void SayHelloAlt();
}
public class HelloWorld extends Hello{
public void SayHelloAlt(){
System.out.println("HelloWorld!");
}
}
public class HelloUniverse extends Hello{
public void SayHelloAlt(){
System.out.println("HelloUniverse!");
}
}
void Go(){
ArrayList<Hello> hellos = new ArrayList<Hello>();
hellos.add(new HelloWorld());
hellos.add(new HelloUniverse());
for (Hello h : hellos){
h.SayHello();
h.SayHelloAlt();
}
}
}
static void main(){
HelloEveryone hello = new HelloEveryone();
hello.Go();
}
I have this interface:
public interface ISectionListItem {
public int getLayoutId();
public void setProps(View view, int position);
}
But i want all the classes who implements this interface be forces to have a static class inside them. I thought of:
public interface ISectionListItem {
public int getLayoutId();
public void setProps(View view, int position);
static class ViewHolder {};
}
But that dosn't force the classes to "add unimplemented inner classes". Anyway to accomplish this? Is it even possible?
Thanks :)
The point of an interface is that you're defining a contract, not an implementation. If you really need to have an instance then you'd want to do something like
public ViewHolder getViewHolder();
and define a viewholder interface. Again, the idea is that you're not creating an implementation, you're expressing a contract.
This is not possible by the nature of interfaces. An interface defines the public behavior of objects; if a class implements an interface, that means that all instances of that class obey the contract defined by the interface. For this reason interfaces can't contain static members. (There's an exception to this rule, but at this point you shouldn't care about that.)
Be careful of your terminology: static member classes are not inner classes. Static member classes are like normal classes, except that they live in a different namespace. Static member classes are not members of the instances of the containing class, just as any static class member is not part of the "instance template". An inner class is by definition not static.
So, why can't we declare a genuine (i.e. non-static) inner class in an interface? That's because in an interface you can only define the behavior of objects, not how they are composed. The whole point (and beauty) of interfaces is that they separate the behavior ("what does it do") from the implementation ("how is it done"). For that reason you can't declare inner classes in an interface, just as you can't declare fields in an interface.
I don't know what you're trying to do, but you might want to try the following. Let's call your original interface MyInterface.
Define an extra interface SomeType
Have your inner class implement SomeType
Declare a method like public SomeType getInnerClassInstance() (a terrible method name) in MyInterface
Note that MyInterface instances aren't forced to actually return an instance of an inner class as a result of getInnerClassInstance(). This is a nice thing, because you're not bound to a specific implementation.
You can't force someone to create an inner class.
Depending on what you want to do your best best is probably adding a method getViewHolder() to the interface and create another interface which specifies the behaviour of a ViewHolder.
I have just found a static nested interface in our code-base.
class Foo {
public static interface Bar {
/* snip */
}
/* snip */
}
I have never seen this before. The original developer is out of reach. Therefore I have to ask SO:
What are the semantics behind a static interface? What would change, if I remove the static? Why would anyone do this?
The static keyword in the above example is redundant (a nested interface is automatically "static") and can be removed with no effect on semantics; I would recommend it be removed. The same goes for "public" on interface methods and "public final" on interface fields - the modifiers are redundant and just add clutter to the source code.
Either way, the developer is simply declaring an interface named Foo.Bar. There is no further association with the enclosing class, except that code which cannot access Foo will not be able to access Foo.Bar either. (From source code - bytecode or reflection can access Foo.Bar even if Foo is package-private!)
It is acceptable style to create a nested interface this way if you expect it to be used only from the outer class, so that you do not create a new top-level name. For example:
public class Foo {
public interface Bar {
void callback();
}
public static void registerCallback(Bar bar) {...}
}
// ...elsewhere...
Foo.registerCallback(new Foo.Bar() {
public void callback() {...}
});
The question has been answered, but one good reason to use a nested interface is if its function is directly related to the class it is in. A good example of this is a Listener. If you had a class Foo and you wanted other classes to be able to listen for events on it, you could declare an interface named FooListener, which is ok, but it would probably be more clear to declare a nested interface and have those other classes implement Foo.Listener (a nested class Foo.Event isn't bad along with this).
Member interfaces are implicitly static. The static modifier in your example can be removed without changing the semantics of the code. See also the the Java Language Specification 8.5.1. Static Member Type Declarations
An inner interface has to be static in order to be accessed. The interface isn't associated with instances of the class, but with the class itself, so it would be accessed with Foo.Bar, like so:
public class Baz implements Foo.Bar {
...
}
In most ways, this isn't different from a static inner class.
Jesse's answer is close, but I think that there is a better code to demonstrate why an inner interface may be useful. Look at the code below before you read on. Can you find why the inner interface is useful? The answer is that class DoSomethingAlready can be instantiated with any class that implements A and C; not just the concrete class Zoo. Of course, this can be achieved even if AC is not inner, but imagine concatenating longer names (not just A and C), and doing this for other combinations (say, A and B, C and B, etc.) and you easily see how things go out of control. Not to mention that people reviewing your source tree will be overwhelmed by interfaces that are meaningful only in one class.So to summarize, an inner interface enables the construction of custom types and improves their encapsulation.
class ConcreteA implements A {
:
}
class ConcreteB implements B {
:
}
class ConcreteC implements C {
:
}
class Zoo implements A, C {
:
}
class DoSomethingAlready {
interface AC extends A, C { }
private final AC ac;
DoSomethingAlready(AC ac) {
this.ac = ac;
}
}
To answer your question very directly, look at Map.Entry.
Map.Entry
also this may be useful
Static Nested Inerfaces blog Entry
Typically I see static inner classes. Static inner classes cannot reference the containing classes wherease non-static classes can. Unless you're running into some package collisions (there already is an interface called Bar in the same package as Foo) I think I'd make it it's own file. It could also be a design decision to enforce the logical connection between Foo and Bar. Perhaps the author intended Bar to only be used with Foo (though a static inner interface won't enforce this, just a logical connection)
If you will change class Foo into interface Foo the "public" keyword in the above example will be also redundant as well because
interface defined inside another interface will implicitly public
static.
In 1998, Philip Wadler suggested a difference between static interfaces and non-static interfaces.
So far as I can see, the only difference in making an
interface non-static is that it can now include non-static inner
classes; so the change would not render invalid any existing Java
programs.
For example, he proposed a solution to the Expression Problem, which is the mismatch between expression as "how much can your language express" on the one hand and expression as "the terms you are trying to represent in your language" on the other hand.
An example of the difference between static and non-static nested interfaces can be seen in his sample code:
// This code does NOT compile
class LangF<This extends LangF<This>> {
interface Visitor<R> {
public R forNum(int n);
}
interface Exp {
// since Exp is non-static, it can refer to the type bound to This
public <R> R visit(This.Visitor<R> v);
}
}
His suggestion never made it in Java 1.5.0. Hence, all other answers are correct: there is no difference to static and non-static nested interfaces.
In Java, the static interface/class allows the interface/class to be used like a top-level class, that is, it can be declared by other classes. So, you can do:
class Bob
{
void FuncA ()
{
Foo.Bar foobar;
}
}
Without the static, the above would fail to compile. The advantage to this is that you don't need a new source file just to declare the interface. It also visually associates the interface Bar to the class Foo since you have to write Foo.Bar and implies that the Foo class does something with instances of Foo.Bar.
A description of class types in Java.
Static means that any class part of the package(project) can acces it without using a pointer. This can be usefull or hindering depending on the situation.
The perfect example of the usefullnes of "static" methods is the Math class. All methods in Math are static. This means you don't have to go out of your way, make a new instance, declare variables and store them in even more variables, you can just enter your data and get a result.
Static isn't always that usefull. If you're doing case-comparison for instance, you might want to store data in several different ways. You can't create three static methods with identical signatures. You need 3 different instances, non-static, and then you can and compare, caus if it's static, the data won't change along with the input.
Static methods are good for one-time returns and quick calculations or easy obtained data.