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Why Must this method be static(Java)?

For some background, I'm currently on chapter 8 in my book, we finished talking about arraylists, arrays, if statements, loops etc. Now this part of the book talks about call by reference,value and some other pretty neat things that seem odd to me at first.I've read What situation to use static and some other SO questions, and learned quite a bit as well.
Consider the following example my book gave (among many examples)
There is another reason why static methods are sometimes necessary. If
a method manipulates a class that you do not own, you cannot add it to
that class. Consider a method that computes the area of a rectangle.
The Rectangle class in the standard library has no such feature, and
we cannot modify that class. A static method solves this problem:
public class Geometry
{
public static double area(Rectangle rect)
{
return rect.getWidth() * rect.getHeight();
}
// More geometry methods can be added here.
}
Now we can tell you why the main method is static. When the program
starts, there aren’t any objects. Therefore, the first method in the
program must be a static method.
Ok, thats pretty cool, up until now I've just been really blindly putting public in front of all my methods, so this is great to know. But the review small problem on the next page caught my attention
The following method computes the average of an array list of numbers:
public static double average(ArrayList<Double> values)
Why must it be a static method?
Here I was like wait a sec. I'm pretty sure I did this without using static before. So I tried doing this again and pretty easily came up with the following
import java.util.ArrayList;
class ArrList
{
private double sum;
public ArrList()
{
sum = 0;
}
public double average(ArrayList <Double> values)
{
for(int i = 0; i < values.size() ; i++)
{
sum+=values.get(i);
}
return sum / values.size();
}
}
public class Average
{
public static void main(String [] args)
{
ArrList arrListObj = new ArrList();
ArrayList<Double> testArrList = new ArrayList<Double>();
testArrList.add(10.0);
testArrList.add(50.0);
testArrList.add(20.0);
testArrList.add(20.0);
System.out.println(arrListObj.average(testArrList));
}
}
TLDR
Why does my book say that public static double average(ArrayList<Double> values) needs to be static?
ATTEMPT AT USING STATIC
public class Average
{
public static void main(String [] args)
{
ArrayList<Double> testArrList = new ArrayList<Double>();
ArrayList<Double> testArrListTwo = new ArrayList<Double>();
testArrList.add(10.0);
testArrList.add(50.0);
testArrList.add(20.0);
testArrList.add(20.0);
testArrListTwo.add(20.0);
testArrListTwo.add(20.0);
testArrListTwo.add(20.0);
System.out.println(ArrList.average(testArrList));
System.out.println(ArrList.average(testArrListTwo)); // we don't get 20, we get 53.3333!
}
}

It doesn't.
The only method which needs to be static is the initial main() method. Anything and everything else is up to you as the programmer to decide what makes sense in your design.
static has nothing to do with public accessors (as you allude to), and it has nothing to do with the technical operation being performed. It has everything to do with the semantics of the operation and the class which holds it.
An instance (non-static) method exists on a particular instance of a class. Semantically it should perform operations related to that specific instance. A static method exists on a class in general and is more conceptual. It doesn't do anything to a particular instance (unless it's provided an instance of something as a method argument of course).
So you really just need to ask yourself about the semantics of the operation. Should you need new instance of an object to perform an operation? Or should the operation be available without an instance? That depends on the operation, on what the objects represent, etc.

If it is not static, then any other class that wants to use this method must first create an instance of this object.
From some other class:
Average.average(new ArrayList<Double>()); // legal only if static
new Average().average(new ArrayList<Double>()); // necessary if not static
// and only makes sense if Average can be instantiated in the first place
It's legal to make it an instance (i.e. not static) variable, but the method is actually harder to understand. If it is static then whoever reads the code knows it does not use any member variables of the class.
// In the class body
int x = 0; // member variable
public static double average() {
x = x + 1; // illegal
}
The less something can do, the easier to understand what it does do.

Static methods like the area, average are usually utility functions. You don't need any object to use an utility function. For example consider Math.pow you don't need to instantiate any object to use the power function, just use Math.pow(10.0, 2.0) to get (10.0)^2
In short :
Static method means class method, that is no instance of that object is needed to invoke.
whereas your average method is an instance method, you need an object to invoke that method.

Why "reinitialise the class variable" when there is only one copy of class variable that is shared between all instance of this particular class

I'm new to Java and is trying to learn the concept of static initialisation blocks. I saw the code and statements below from Java tutorial oracle. My question is, why did the tutorial states:
"the advantage of private static methods is that they can be reused later if you need to reinitialise the class variable"
when there is only one copy of class variable that is shared between all instance of this particular class?
class Whatever {
public static varType myVar = initializeClassVariable();
private static varType initializeClassVariable() {
// initialization code goes here
}
}

Sometimes you want to reset the static variable to its initial value.
One example could be a global counter that you want to reset from time to time
class Whatever {
private static int counter = getCountInit();
public static resetCounter() {
counter = getCountInit();
}
private static getCountInit() {
return 0; // or some fancy computation
}
}
Another example is testing: Imagine you have a test-case A that changes some static variable of a class and a test-case B that also uses the static variable. Without setting the static variable back to the initial value, the outcome of the tests would be different depending on the order in that they run.
(side note: That is one major reason why having global state (and a static variable is global state) is often a bad idea - at least in larger software projects.)

static for variable word means it is shared among all of the instances of the class so for example if you have
class SpaceShip and have static variable color = "blue";
and create alot of spaceships I mean instances of that class
and you then change color to "red"
then all spaceships will have red color ...

Static objects can be accessible only by static methods. So in case if you want to reset a static object value we should use static method for that. Generally this will not be exposed to API users so its better to keep them private here.

Does Java support static variables inside a function to keep values between invocations?

https://stackoverflow.com/a/572550/1165790
I want to use this feature in Java because the function that I'm designing is called rarely (but when it is called, it starts a recursive chain) and, therefore, I do not want to make the variable an instance field to waste memory each time the class is instantiated.
I also do not want to create an additional parameter, as I do not want to burden external calls to the function with implementation details.
I tried the static keyword, but Java says it's an illegal modifier. Is there a direct alternative? If not, what workaround is recommended?
I want it to have function scope, not class scope.

I want it to have function scope, not class scope.
Then you are out of luck. Java provides static (class scoped), instance and local variables. There is no Java equivalent to C's function-scoped static variables.
If the variable really needs to be static, then your only choice is to make it class scoped. That's all you've got.
On the other hand, if this is a working variable used in some recursive method call, then making it static is going to mean that your algorithm is not reentrant. For instance, if you try to run it on multiple threads it will fall apart because the threads will all try to use the same static ... and interfere with each other. In my opinion, the correct solution would be either to pass this state using a method parameter. (You could also use a so-called "thread local" variable, but they have some significant down-sides ... if you are worrying about overheads that are of the order of 200 bytes of storage!)

How are you going to keep a value between calls without "wasting memory"? And the memory consumed would be negligible.
If you need to store state, store state: Just use a static field.
Caution is advised when using static variables in multi-threaded applications: Make sure that you synchronise access to the static field, to cater for the method being called simultaneously from different threads. The simplest way is to add the synchronized keyword to a static method and have that method as the only code that uses the field. Given the method would be called infrequently, this approach would be perfectly acceptable.

Static variables are class level variables. If you define it outside of the method, it will behave exactly as you want it to.
See the documentation:
Understanding instance and Class Members
The code from that answer in Java...
public class MyClass {
static int sa = 10;
public static void foo() {
int a = 10;
a += 5;
sa += 5;
System.out.println("a = " + a + " sa = " + sa);
}
public static void main(String[] args) {
for (int i = 0; i < 10; i++) {
foo();
}
}
}
Output:
$ java MyClass
a = 15 sa = 15
a = 15 sa = 20
a = 15 sa = 25
a = 15 sa = 30
a = 15 sa = 35
a = 15 sa = 40
a = 15 sa = 45
a = 15 sa = 50
a = 15 sa = 55
a = 15 sa = 60
sa Only exists once in memory, all the instances of the class have access to it.

Probably you got your problem solved, but here is a little more details on static in Java. There can be static class, function or variable.
class myLoader{
static int x;
void foo(){
// do stuff
}
}
versus
class myLoader{
static void foo(){
int x;
// do stuff
}
}
In the first case, it is acting as a class variable. You do not have to "waste memory" this way. You can access it through myLoader.x
However, in the second case, the method itself is static and hence this itself belongs to the class. One cannot use any non-static members within this method.
Singleton design pattern would use a static keyword for instantiating the class only once.
In case you are using multi-threaded programming, be sure to not generate a race condition if your static variable is being accessed concurrently.

I agree with Bohemian it is unlikely memory will be an issue. Also, duplicate question: How do I create a static local variable in Java?
In response to your concern about adding an additional parameter to the method and exposing implementation details, would like to add that there is a way to achieve this without exposing the additional parameter. Add a separate private function, and have the public function encapsulate the recursive signature. I've seen this several times in functional languages, but it's certainly an option in Java as well.
You can do:
public int getResult(int parameter){
return recursiveImplementation(parameter, <initialState>)
}
private int recursiveImplementation(int parameter, State state){
//implement recursive logic
}
Though that probably won't deal with your concern about memory, since I don't think the java compiler considers tail-recursive optimizations.

The variables set up on the stack in the recursive call will be function (frame) local:
public class foo {
public void visiblefunc(int a, String b) {
set up other things;
return internalFunc(a, b, other things you don't want to expose);
}
private void internalFunc(int a, String b, other things you don't want to expose) {
int x; // a different instance in each call to internalFunc()
String bar; // a different instance in each call to internalFunc()
if(condition) {
internalFunc(a, b, other things);
}
}
}

Sometimes state can be preserved by simply passing it around. If required only internally for recursions, delegate to a private method that has the additional state parameter:
public void f() { // public API is clean
fIntern(0); // delegate to private method
}
private void fIntern(int state) {
...
// here, you can preserve state between
// recursive calls by passing it as argument
fIntern(state);
...
}

How about a small function-like class?
static final class FunctionClass {
private int state1; // whichever state(s) you want.
public void call() {
// do_works...
// modify state
}
public int getState1() {
return state1;
}
}
// usage:
FunctionClass functionObject = new FunctionClass();
functionObject.call(); // call1
int state1AfterCall1 = functionObject.getState1();
functionObject.call(); // call2
int state1AfterCall2 = functionObject.getState1();

Is static bad? How to remove static variables?

I have some code that I am working on. It's basically takes in user input and creates a directed graph. One person can travel one way, the other person the opposite. The output is the overlap of where they can visit.
I have most everything working the way that I want it to, but I am concerned with the use of static that I have. I don't seem to fully understand it and no matter where I look, I can't find out its exact use OR how to get rid of it.
Could someone please help me to understand what static is and why it would be helpful?
Also, would it be better to move most the code from MAIN to helper methods? If I do this I have to move all my variables from main to the top of the class and then they all have to be declared as static?!

The reason everything has to be static is because you aren't creating any objects. If you were to create an object by calling new in your main method, you could use non-static variables on that object. This isn't really a good place to give you a tutorial on why you might want to use object-oriented design; you can find one of those online to read (a commenter above gave a possible reference). But the reason everything has to be static is because it's all just running from the main method, which is always static in java. If you were to call new somewhere, you could use non-static variables.

Static makes a method or a variable accessible to all the instances of a class. It's like a constant, but for classes. To make it more easy to understand some code will do the work:
public class Example {
public static int numero;
}
public class Implementation {
public static void main (String args[]) {
Example ex1 = new Example();
Example ex2 = new Example();
Example.numero=10;
System.out.println("Value for instance 1 is: " + ex1.numero);
System.out.println("Value for instance 2 is: " + ex2.numero);
}
}
Running the follwing code will output:
Value for instance 1 is: 10
Value for instance 2 is: 10
Because you set the static variable numero (number in italian) to 10.
Got it?

It looks like a lot of your static methods (findNodeInList, etc) all take the ArrayList (which represents a map) as their first argument. So instead of having it static, you could have a class Map, which stores a list of nodes and has methods on them. Then the main method would read the input, but not have to manage any nodes directly. e.g:
class Map {
ArrayList<Node> nodes;
public void addNode(Node n) { nodes.add(n); }
public int findNodeInList(String s) { ... }
...
public static void main(String[] args) {
Map peggyMap = new Map();
Map samMap = new Map();
// Read the data
samMap.add(new Node(...));
}
}
This keeps all the stuff to do with nodes/maps well encapsulated and not mixed in with stuff to do with reading the data.

Static is useful if you going to be using the class/method throught out your program and you don't what to create a instance every time you need to use that method.
For ex
public class StaticExample {
public static void reusable() {
//code here
}
}
It means you can use it like this
StaticExample.reusable();
and you don't have to create an instance like this
StaticExample staticExample = new StaticExample();
staticExample.reuseable();
I hope this help you decide whether to use static or not.

Is using static private methods really faster/better than instance private methods?

What I'm asking is whether there is a difference between doing this:
public Something importantBlMethod(SomethingElse arg) {
if (convenienceCheckMethod(arg)) {
// do important BL stuff
}
}
private boolean convenienceCheckMethod(SomethingElse arg) {
// validate something
}
And this:
public Something importantBlMethod(SomethingElse arg) {
if (convenienceCheckMethod(arg)) {
// do important BL stuff
}
}
private static boolean convenienceCheckMethod(SomethingElse arg) {
// validate something
}
I actually use option 1 as it seems more natural to me.
So is there a style/convention/performance difference between the first and the second way ?
Thanks,
As suggested in the comments I tested it, in my benchmarks the dynamic method is faster.
This is the test code:
public class Tests {
private final static int ITERATIONS = 100000;
public static void main(String[] args) {
final long start = new Date().getTime();
final Service service = new Service();
for (int i = 0; i < ITERATIONS; i++) {
service.doImportantBlStuff(new SomeDto());
}
final long end = new Date().getTime();
System.out.println("diff: " + (end - start) + " millis");
}
}
This is the service code:
public class Service {
public void doImportantBlStuff(SomeDto dto) {
if (checkStuffStatic(dto)) {
}
// if (checkStuff(dto)) {
// }
}
private boolean checkStuff(SomeDto dto) {
System.out.println("dynamic");
return true;
}
private static boolean checkStuffStatic(SomeDto dto) {
System.out.println("static");
return true;
}
}
For 100000 iterations the dynamic method passes for 577ms, the static 615ms.
This however is inconclusive for me since I don't know what and when the compiler decides to optimize.
This is what I'm trying to find out.

Performance wise: The difference, if any, is negligible.
The rule of thumb is to declare your method static if it doesn't interact with any members of its class.

If the result of the function does not depend on anything but the arguments, it should be static. If it depends on an instance, make it an instance member.
It's not about performance; it's about semantics. Unless you're calling this function a million times a second, you will not notice a performance difference, and even then the difference won't be significant.

It all depends on the context. Generally static methods/variables are declared in a class so that an external class can make use of them.
If you are making a call to a local method then you should generally use instance methods rather than making static calls.
FYI, your syntax for calling the static method from an instance method is wrong. You have to supply the class name.

If your method requires instance data or calls to other instance methods, it must be an instance method.
If the function only depends on its arguments, and no other static data, then it might as well be an instance method too - you'll avoid the need to repeat the class name when you invoke the static function.
IMHO, there's no particular need to make the function static unless:
it's callable from other classes (i.e. not private), and
it doesn't refer to instance variables, and
it refers to other static class data

It might, it might not. It might be different between different executions of your code.
Here's the only thing that you can know without digging into the Hotsport code (or the code of your non-Hotspot JVM):
The static method is invoked with invokestatic, which does not require an object reference.
The instance private method is invoked with invokespecial, which does require an object reference.
Both of those opcodes have a process for resolving the actual method to invoke, and those processes are relatively similar (you can read the specs). Without counting the instructions of an actual implementation, it would be impossible to say which is faster.
The invokespecial pushes an extra value onto the stack. The time to do this is counted in fractions of a nanosecond.
And making all of this moot, Hotspot has a wide range of optimizations that it can perform. It probably doesn't have to do the actual method resolution more than once during your program's run. It might choose to inline the method (or it might not), but that cost would again be roughly equivalent.

According to me NO binding of static method is same as non-static private i.e early binding.
.
Compiler actually adds code of method (static or non-static private) to your code while creating it's byte code.
Update : Just came through this article. It says instance methods binding is dynamic so if method is not non-static private then. Your static method is faster.

I checked, I hope it does what you wanted to know, the code won't be beautiful:
public class main {
#SuppressWarnings("all")
public static void main(String[] args) {
main ma = new main();
int count = Integer.MAX_VALUE;
long beg = (new Date()).getTime();
for (int i = 0; i < count; i++) {
ma.doNothing();
}
System.out.println("priv : " + new Long((new Date()).getTime() - beg).toString());
beg = (new Date()).getTime();
for (int i = 0; i < count; i++) {
doNothingStatic();
}
System.out.println("privstat : " + new Long((new Date()).getTime() - beg).toString());
}
private void doNothing() {
int i = 0;
}
private static void doNothingStatic() {
int i = 0;
}
}
results:
priv : 1774
privstat : 1736
priv : 1906
privstat : 1783
priv : 1963
privstat : 1751
priv : 1782
privstat : 1929
priv : 1876
privstat : 1867
It doesn't look like dependent on static - nonstatic private method. I am sure the differences are coming from the current burden of the machine.

I take part in coding competitions and I have observed, that non-static methods are faster(however minimal) than the static methods. Of course, it depends on your use and the what the situation demands, but the static methods gives poorer performance as compared to non-static ones. By convention, you can use static methods for the ease of code, but creating an instance of the class and calling the method will give better performance.