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error: Illegal static declaration in inner class asignacion1.sumar [duplicate]

I have an inner class that stores the info of the controls I'm using for a game, now I want to store a static ArrayList in it that holds all the names of the controls. But I am getting this error: "Modifier static is only allowed in constant variable declarations"
private class Control{
public static ArrayList<String> keys = new ArrayList<String>();
public final String key;
public final Trigger trigger;
Control(String k, Trigger t){
key = k;
trigger = t;
keys.add(key);
}
}
Now I know this can easily be solved by taking the ArrayList out of the class and storing it in the main class. But I'd prefer to keep all the information in one class where I can access everything.
"Control.key, Control.trigger, Control.keys"
is just more elegant/readable than
"key, trigger, keys"
Or maybe I just have Obsessive–compulsive disorder, still I'd like to do it my way.

You can make the Control class static.
private static class Control {
^^^^^^
// Ok to have static members:
public static ArrayList<String> keys = new ArrayList<String>();
...
This is described in the Java Language Specification Section §8.1.3
8.1.3 Inner Classes and Enclosing Instances
An inner class is a nested class that is not explicitly or implicitly declared static. Inner classes may not declare static initializers (§8.7) or member interfaces. Inner classes may not declare static members, unless they are compile-time constant fields (§15.28).

Make your inner class static and it will work:
private static class Control { ...

Why we are restricted to declare static member variable in inner Class in java?

Consider the below example
Why we are restricted to declare static member variable in inner Class when there isn't any restriction on inheriting static variables in Inner classes?
public class Outer {
public class Inner {
public static String notAllowed;
/* Above line give following compilation error
The field notAllowed cannot be declared static in a non-static inner type, unless initialized with a constant expression
*/
}
}
But now if my inner class extends other class which contains static variable than this works fine.
Consider below code:
public class Outer {
public class Inner extends InnerBase {
/* Since it extends InnerBase so we can access Outer.Inner.allowed */
public Inner(){
Outer.Inner.allowed = null; // Valid statement
}
}
}
public class InnerBase {
public static String allowed;
}
So what is the reason for restricting static variable in inner class as it is achievable through inheritance?
Am I missing something very basic?

From oracle website:
As with instance methods and variables, an inner class is associated with an instance of its enclosing class and has direct access to that object's methods and fields.
Because an inner class is associated with an instance, it cannot define any static members itself.
I understand it this way:
If inner class have their own static field,and static field have to initialize before class instantiate;
But a innner class only exist with an instance of outterclass ,so it can not initialize its static member before instantiate,then in Contradiction.

Because in order to access the static field, you will need an instance of the Outer class, from which you will have to create an instance of the non-static Inner class.
static fields are not supposed to be bound to instances and therefore you receive a compilation error.
The JLS 8.1.3 specifies:
Inner classes may not declare static initializers or member
interfaces, or a compile-time error occurs.
Inner classes may not declare static members, unless they are constant
variables, or a compile-time error occurs.

It is very much possible that the purpose of declaring a static variable opposes the purpose of declaring ANY variable in an inner class. Static variables are meant to be used statically - in any other static methods and classes, while inner classes imply that those classes serve their outer classes ONLY.
I guess the Java creators just wanted it this way.

What's the advantage of making an inner class as static with Java?

I have an inner class in my Java class.
When I run find bugs, it recommends(warns) to make it as static.
What's the point of this warning? What's the advantage of making a inner class as static?

If the nested class does not access any of the variables of the enclosing class, it can be made static. The advantage of this is that you do not need an enclosing instance of the outer class to use the nested class.

An inner class, by default, has an implicit reference to an object of the outer class. If you instantiate an object of this from the code of the outer class, this is all done for you. If you do otherwise you need to provide the object yourself.
A static inner class does not have this.
That means it can be instantiated outside the scope of an outer class object. It also means that if you 'export' an instance of the inner class, it will not prevent the current object to be collected.
As a basic rule, if the inner class has no reason to access the outer one, you should make it static by default.

A static inner class is a semantically simpler thing. It's just like a top-level class except you have more options for visibility (e.g. you can make it private).
An important reason to avoid non-static inner classes is that they are more complex. There is the hidden reference to the outer class (maybe even more than one). And a simple name in a method of the inner class may now be one of three things: a local, a field, or a field of an outer class.
An artifact of that complexity is that the hidden reference to the outer class can lead to memory leaks. Say the inner class is a listener and could be a static inner class. As long as the listener is registered, it holds a reference to the instance of the outer class, which may in turn hold on to large amounts of memory. Making the listener static may allow the outer instance to be garbage collected.

A Non-static inner class has an implicit reference to outer class. If you make the class as static, you could save some memory and code.

Benefits of static inner classes:
Instantiation of static inner class does not rely on external class guidance, and the memory overhead of instantiation.
Static inner class does not hold external class guidance, does not affect the collection of external class, to avoid the extension of the external class in memory survival time leading to memory leakage.

We already have good answers, here are my 5 cents:
Both static and non-static inner classes are used when we need to separate logical functionalities yet using the methods and variables of the outer class. Both of the inner classes have access to the private variables of the outer class.
Advantages of static inner class:
1) static classes can access the static variables from outer class
2) static classes can be treated like an independent class
Non-static inner class:
1) cannot use static members of the outer class
2) cannot be treated like an independent class
public class NestedClassDemo {
private int a = 100;
int b = 200;
private static int c = 500;
public NestedClassDemo() {
TestInnerStatic teststat = new TestInnerStatic();
System.out.println("const of NestedClassDemo, a is:"+a+", b is:"+b+".."+teststat.teststat_a);
}
public String getTask1(){
return new TestInnerClass().getTask1();
}
public String getTask2(){
return getTask1();
}
class TestInnerClass{
int test_a = 10;
TestInnerClass() {
System.out.println("const of testinner private member of outerlcass"+a+"..."+c);
}
String getTask1(){
return "task1 from inner:"+test_a+","+a;
}
}
static class TestInnerStatic{
int teststat_a = 20;
public TestInnerStatic() {
System.out.println("const of testinnerstat:"+teststat_a+" member of outer:"+c);
}
String getTask1stat(){
return "task1 from inner stat:"+teststat_a+","+c;
}
}
public static void main(String[] args){
TestInnerStatic teststat = new TestInnerStatic();
System.out.println(teststat.teststat_a);
NestedClassDemo nestdemo = new NestedClassDemo();
System.out.println(nestdemo.getTask1()+"...."+nestdemo.getTask2());
}
}
Accessing the static inner and non-static inner class from outside:
public class TestClass {
public static void main(String[] args){
NestedClassDemo.TestInnerClass a = new NestedClassDemo().new TestInnerClass();
NestedClassDemo.TestInnerStatic b = new NestedClassDemo.TestInnerStatic();
}
}
The official java doc for static inner class can be found at https://docs.oracle.com/javase/tutorial/java/javaOO/nested.html

Referencing non static variable from within static Inner Class

I need to reference a variable of a top level class from a method within a static class.
This method should act on unique instances of the top level class and so it feels like I shouldn't instantiate the top level class inside the static class.
Basically I want something like
public class TopLevel{
// private
int innerV
public static class Inner implements X {
for(i=0; i<innerV,i++){
doSomething
}
}
}
Is it possible to just say this.innerV or something similar in the for loop and similar places?

From a static inner class, you can't refer to (nonstatic) members of the outer class directly. If you remove the static qualifier, it will work, because instances of nonstatic inner classes are implicitly tied to an instance of the containing class, so they can refer to its members directly.
Declaring your inner class static removes this link, so you need to either pass an instance of the outer class to the inner class method (or its constructor) as a parameter, or create it inside the method.

You can't do that. Create a TopLevel instance and if you make an innerV accessor (getter/setter) or make it public, than you can.
public class TopLevel {
public int innerV
public static class Inner implements X {
for(i=0; i<innerV,i++){
TopLevel tl = new TopLevel()
tl.innerV = 12345678;
}
}
}

You can't do that because it doesn't make sense, any more than referring to a non-static member from a static function makes sense. There is no current instance of the outer class in the context of the static inner class to get the instance variable from.

Why are you not able to declare a class as static in Java?

Why are you not able to declare a class as static in Java?

Only nested classes can be static. By doing so you can use the nested class without having an instance of the outer class.
class OuterClass {
public static class StaticNestedClass {
}
public class InnerClass {
}
public InnerClass getAnInnerClass() {
return new InnerClass();
}
//This method doesn't work
public static InnerClass getAnInnerClassStatically() {
return new InnerClass();
}
}
class OtherClass {
//Use of a static nested class:
private OuterClass.StaticNestedClass staticNestedClass = new OuterClass.StaticNestedClass();
//Doesn't work
private OuterClass.InnerClass innerClass = new OuterClass.InnerClass();
//Use of an inner class:
private OuterClass outerclass= new OuterClass();
private OuterClass.InnerClass innerClass2 = outerclass.getAnInnerClass();
private OuterClass.InnerClass innerClass3 = outerclass.new InnerClass();
}
Sources :
Oracle tutorial on nested classes
On the same topic :
Java: Static vs non static inner class
Java inner class and static nested class

Top level classes are static by default. Inner classes are non-static by default. You can change the default for inner classes by explicitly marking them static. Top level classes, by virtue of being top-level, cannot have non-static semantics because there can be no parent class to refer to. Therefore, there is no way to change the default for top-level classes.

So, I'm coming late to the party, but here's my two cents - philosophically adding to Colin Hebert's answer.
At a high level your question deals with the difference between objects and types. While there are many cars (objects), there is only one Car class (type). Declaring something as static means that you are operating in the "type" space. There is only one. The top-level class keyword already defines a type in the "type" space. As a result "public static class Car" is redundant.

Class with private constructor is static.
Declare your class like this:
public class eOAuth {
private eOAuth(){}
public final static int ECodeOauthInvalidGrant = 0x1;
public final static int ECodeOauthUnknown = 0x10;
public static GetSomeStuff(){}
}
and you can used without initialization:
if (value == eOAuth.ECodeOauthInvalidGrant)
eOAuth.GetSomeStuff();
...

You can create a utility class (which cannot have instances created) by declaring an enum type with no instances. i.e. you are specificly declaring that there are no instances.
public enum MyUtilities {;
public static void myMethod();
}

Sure they can, but only inner nested classes. There, it means that instances of the nested class do not require an enclosing instance of the outer class.
But for top-level classes, the language designers couldn't think of anything useful to do with the keyword, so it's not allowed.

public class Outer {
public static class Inner {}
}
... it can be declared static - as long as it is a member class.
From the JLS:
Member classes may be static, in which case they have no access to the instance variables of the surrounding class; or they may be inner classes (§8.1.3).
and here:
The static keyword may modify the declaration of a member type C within the body of a non-inner class T. Its effect is to declare that C is not an inner class. Just as a static method of T has no current instance of T in its body, C also has no current instance of T, nor does it have any lexically enclosing instances.
A static keyword wouldn't make any sense for a top level class, just because a top level class has no enclosing type.

As explained above, a Class cannot be static unless it's a member of another Class.
If you're looking to design a class "of which there cannot be multiple instances", you may want to look into the "Singleton" design pattern.
Beginner Singleton info here.
Caveat:
If you are thinking of using the
singleton pattern, resist with all
your might. It is one of the easiest
DesignPatterns to understand, probably
the most popular, and definitely the
most abused.
(source: JavaRanch as linked above)

In addition to how Java defines static inner classes, there is another definition of static classes as per the C# world [1]. A static class is one that has only static methods (functions) and it is meant to support procedural programming. Such classes aren't really classes in that the user of the class is only interested in the helper functions and not in creating instances of the class. While static classes are supported in C#, no such direct support exists in Java. You can however use enums to mimic C# static classes in Java so that a user can never create instances of a given class (even using reflection) [2]:
public enum StaticClass2 {
// Empty enum trick to avoid instance creation
; // this semi-colon is important
public static boolean isEmpty(final String s) {
return s == null || s.isEmpty();
}
}

Everything we code in java goes into a class. Whenever we run a class JVM instantiates an object. JVM can create a number of objects, by definition Static means you have the same set of copy to all objects.
So, if Java would have allowed the top class to be static whenever you run a program it creates an Object and keeps overriding on to the same Memory Location.
If You are just replacing the object every time you run it whats the point of creating it?
So that is the reason Java got rid of the static for top-Level Class.
There might be more concrete reasons but this made much logical sense to me.

The only classes that can be static are inner classes. The following code works just fine:
public class whatever {
static class innerclass {
}
}
The point of static inner classes is that they don't have a reference to the outer class object.

I think this is possible as easy as drink a glass of coffee!.
Just take a look at this.
We do not use static keyword explicitly while defining class.
public class StaticClass {
static private int me = 3;
public static void printHelloWorld() {
System.out.println("Hello World");
}
public static void main(String[] args) {
StaticClass.printHelloWorld();
System.out.println(StaticClass.me);
}
}
Is not that a definition of static class?
We just use a function binded to just a class.
Be careful that in this case we can use another class in that nested.
Look at this:
class StaticClass1 {
public static int yum = 4;
static void printHowAreYou() {
System.out.println("How are you?");
}
}
public class StaticClass {
static int me = 3;
public static void printHelloWorld() {
System.out.println("Hello World");
StaticClass1.printHowAreYou();
System.out.println(StaticClass1.yum);
}
public static void main(String[] args) {
StaticClass.printHelloWorld();
System.out.println(StaticClass.me);
}
}

One can look at PlatformUI in Eclipse for a class with static methods and private constructor with itself being final.
public final class <class name>
{
//static constants
//static memebers
}

if the benefit of using a static-class was not to instantiate an object and using a method then just declare the class as public and this method as static.