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Declaring a static variable in a non-static class in Java

I am new to java. I am currently reading some articles about static variables. When I am trying to implement my learnings, I encountered a problem about static variables. Here is the first code sample.
public class Human {
// in Human.java
public static int population = 0;
public static void main(String[] argv) {
System.out.println(population);
}
}
This code works fine and the outcome is 0.
But for the following code, I wasn't allow to compile it.
public class Human {
// in Human.java
public class Charlie extends Human {
public static int number = 0;
}
public static void main(String[] argv) {
System.out.println(new Human().new Charlie().number);
}
}
An error occurred: The field number cannot be declared static in a non-static inner type, unless initialized with a constant expression
I am confused with this situation. For the first code sample, my Human class is non-static and I was allowed to declare a static variable inside it. How come I can't do the same for my second code sample.
Any help would be appreciated. Thanks. :)

Try with public static final int number = 0; because Java does not let you define non-final static fields inside function-local inner classes. Only top-level classes and static nested classes are allowed to have non-final static fields.
From the JLS section 8.1.3:
Inner classes may not declare static members, unless they are constant variables (§4.12.4), or a compile-time error occurs.
Other way to make inner class static and access it
public class Human {
// in Human.java
public static class Charlie extends Human {
public static int number = 0;
}
public static void main(String[] argv) {
System.out.println(new Human.Charlie().number);
}
}

Why are we allowed to have a final main method in java?

Can anyone tell me the use of making main method as final in java.
while this is allowed in java
public static final void main(String[] args) {
}
I dont see any use of making it final. anyways it is static so we can not override it.

Adding final to a static method can actually make a difference. Consider the following code:
class A {
public static void main(String[] args) {
System.out.println("A");
}
}
class B extends A {
public static void main(String[] args) {
System.out.println("B");
}
}
class C extends B {
}
public class Test {
public static void main(String[] args) {
C.main(args); // Will invoke B.main
}
}
Adding final to A.main would prevent accidental hiding of A.main. In other words, adding final to A.main guarantees that B.main is not allowed, and that C.main therefore prints "A" as opposed to for instance "B".
Why are we allowed to have a final main method in java?
Beside the above corner case, adding final to a static method doesn't make much difference, so I don't see a big point in adding a rule for disallowing it.
More information available here: Behaviour of final static method

why static final int MAX_VALUE=100; gives compile time error

public class A{
public static void main(String[] args){
static final int MAX_VALUE = 100; //COMPILE TIME ERROR
System.out.println("MAX_VALUE");
}
}
Why static final int MAX_VALUE=100; gives compile time error, it gives the error as "illegal modifier for parameter MAX_VALUE;only final is permitted "

You cannot declare static variable inside methods.
Static variables belong to the class; variables declared inside a method are local variables and belong to that method.

Static variables belong to the class.Not methods
The variables declared inside a method are local variables and belong to that method.
So it becomes
final int MAX_VALUE = 100;
Prefer to read : Docs on Understanding Instance and Class Members

The keyword static cannot be used inside methods. This would be valid code:
public class A{
static final int MAX_VALUE = 100; // This line can't go in a method.
public static void main(String[] args){
System.out.println("MAX_VALUE: "+MAX_VALUE);
}
}

A local variable can not be static. You can either create a final local variable, or a final static class variable (which are actually constants, btw.), but not a local static variable:
public class A{
static final int CLASS_CONST = 42;
public static void main(String[] args){
final int LOCAL_CONST = 43;
...
}
}

You can't delcare static things inside a method. Move it up a line.

Change
public class A{
public static void main(String[] args){
static final int MAX_VALUE = 100; //COMPILE TIME ERROR
System.out.println("MAX_VALUE");
}
}
To
public class A{
static final int MAX_VALUE = 100; //NO ERROR YAY!!
public static void main(String[] args){
System.out.println("MAX_VALUE");
}
}

you can't create static final in a method, you must create it outside the method:
public class A {
static final int MAX_VALUE = 100;
public static void main(String[] args){
System.out.println("MAX_VALUE");
}
}

static variables are class level variables ,you can't declare them at method.
According Docs
Sometimes, you want to have variables that are common to all objects.
This is accomplished with the static modifier. Fields that have the static
modifier in their declaration are called static fields or class variables.
They are associated with the class, rather than with any object

Others already pointed out that static members belong to the class instead of a specific instance. So you don't have to create a class instance to use a static member, rather you can call SomeClass.SOME_STATIC_MEMBER directly.
You cannot call any other member of a class that is not static without instantiating that class. Meaning if you have-
public class SomeClass {
public int add (int x, int y){
return x + y;
}
}
For you to use add method of SomeClass above, you have to instantiate first-
SomeClass someClass = new SomeClass();
int sum = someClass.add(5, 10);
So, there is no point of allowing us to declare static members in a non-static method as for us to use that method, we have to instatiate the class where that method belongs to.

you should declare this constant in the class A, not in the main() method
public class A{
static final int MAX_VALUE = 100; //COMPILE TIME ERROR
public static void main(String[] args){
System.out.println("MAX_VALUE");
}
}

public static void main () access non static variable

It is said that non-static variables cannot be used in a static method. But public static void main does. How is that?

No, it doesn't.
public class A {
int a = 2;
public static void main(String[] args) {
System.out.println(a); // won't compile!!
}
}
but
public class A {
static int a = 2;
public static void main(String[] args) {
System.out.println(a); // this works!
}
}
or if you instantiate A
public class A {
int a = 2;
public static void main(String[] args) {
A myA = new A();
System.out.println(myA.a); // this works too!
}
}
Also
public class A {
public static void main(String[] args) {
int a = 2;
System.out.println(a); // this works too!
}
}
will work, since a is a local variable here, and not an instance variable. A method local variable is always reachable during the execution of the method, regardless of if the method is static or not.

Yes, the main method may access non-static variables, but only indirectly through actual instances.
Example:
public class Main {
public static void main(String[] args) {
Example ex = new Example();
ex.variable = 5;
}
}
class Example {
public int variable;
}
What people mean when they say "non-static variables cannot be used in a static method" is that non-static members of the same class can't be directly accessed (as shown in Keppils answer for instance).
Related question:
Accessing non-static members through the main method in Java
Update:
When talking about non-static variables one implicitly means member variables. (Since local variables can't possible have a static modifier anyway.)
In the code
public class A {
public static void main(String[] args) {
int a = 2;
System.out.println(a); // this works!
}
}
you're declaring a local variable (which typically is not referred to as non-static even though it doesn't have a static modifier).

The main method does not have access to non-static members either.
final public class Demo
{
private String instanceVariable;
private static String staticVariable;
public String instanceMethod()
{
return "instance";
}
public static String staticMethod()
{
return "static";
}
public static void main(String[] args)
{
System.out.println(staticVariable); // ok
System.out.println(Demo.staticMethod()); // ok
System.out.println(new Demo().instanceMethod()); // ok
System.out.println(new Demo().instanceVariable); // ok
System.out.println(Demo.instanceMethod()); // wrong
System.out.println(instanceVariable); // wrong
}
}
This is because by default when you call a method or variable it is really accessing the this.method() or this.variable. But in the main() method or any other static method(), no "this" objects has yet been created.
In this sense, the static method is not a part of the object instance of the class that contains it. This is the idea behind utility classes.
To call any non-static method or variable in a static context, you need to first construct the object with a constructor or a factory like your would anywhere outside of the class.
More depth:
Basically it's a flaw in the design of Java IMO which allows static members (methods and fields) to be referenced as if they were instance members. This can be very confusing in code like this:
Thread newThread = new Thread(runnable);
newThread.start();
newThread.sleep(1000);
That looks like it's sending the new thread to sleep, but it actually compiles down into code like this:
Thread newThread = new Thread(runnable);
newThread.start();
Thread.sleep(1000);
because sleep is a static method which only ever makes the current thread sleep.
Indeed, the variable isn't even checked for non-nullity (any more; it used to be, I believe):
Thread t = null;
t.sleep(1000);
Some IDEs can be configured to issue a warning or error for code like this - you shouldn't do it, as it hurts readability. (This is one of the flaws which was corrected by C#...)

**Here you can see table that clear the access of static and non-static data members in static and non-static methods. **

You can create non-static references in static methods like :
static void method() {
A a = new A();
}
Same thing we do in case of public static void main(String[] args) method

public class XYZ
{
int i=0;
public static void increament()
{
i++;
}
}
public class M
{
public static void main(String[] args)
{
XYZ o1=new XYZ();
XYZ o2=new XYZ();
o1.increament();
XYZ.increament(); //system wont be able to know i belongs to which object
//as its increament method(static method)can be called using class name system
//will be confused changes belongs to which object.
}
}

What does this 'static' mean and why is it like that

public class tt {
static{
System.out.println("class tt");
}
}
It the first time ive come across it and im wondering what it is and what it's used for

It is the static initialiser of the class. When the class is loaded, the static initialiser is run. It is like the constructor, but for the class rather than for individual objects.
Multiple static initialisers can appear in a class, as well as direct initialisers for static variables. These will be combined into one initialiser in the order in which they are declared. For example, the following will print "foo" to stdout whenever the class is loaded (usually once per application).
public class Foo {
static String a;
static {
a = "foo";
}
static String b = a;
static {
System.println(b);
}
}

Its initilizer block
A static initialization block is a normal block of code enclosed in braces, { }, and preceded by the static keyword. Here is an example:
static {
// whatever code is needed for initialization goes here
}
A class can have any number of static initialization blocks, and they can appear anywhere in the class body. The runtime system guarantees that static initialization blocks are called in the order that they appear in the source code.
There is an alternative to static blocks —you can write a private static method:
class Whatever {
public static varType myVar = initializeClassVariable();
private static varType initializeClassVariable() {
//initialization code goes here
}
}
The advantage of private static methods is that they can be reused later if you need to reinitialize the class variable.
Resource

here is the static initializer tutorial http://download.oracle.com/javase/tutorial/java/javaOO/initial.html
It runs when the class is loaded before the initialization.
public class A {
static {
System.out.println("A from static initializer"); // first
}
public A(){
System.out.println("A"); // second
}
public static void main(String[] args){
new A();
}
}

It is a static initializer. The code inside that block runs when the JVM loads the class, which is immediately before the first time the program needs to do anything with that class (e.g. look up a static field, call a static method, instantiate an object,...).

It's a static initializer block. It will be executed once when the class is first loaded, along with static field initializers like this:
private static int staticField = someMethod();
The difference is that an initializer block can contain control flow structures like try/catch blocks.