To be specific, I was trying this code:
package hello;
public class Hello {
Clock clock = new Clock();
public static void main(String args[]) {
clock.sayTime();
}
}
But it gave the error
Cannot access non-static field in static method main
So I changed the declaration of clock to this:
static Clock clock = new Clock();
And it worked. What does it mean to put that keyword before the declaration? What exactly will it do and/or restrict in terms of what can be done to that object?
static members belong to the class instead of a specific instance.
It means that only one instance of a static field exists[1] even if you create a million instances of the class or you don't create any. It will be shared by all instances.
Since static methods also do not belong to a specific instance, they can't refer to instance members. In the example given, main does not know which instance of the Hello class (and therefore which instance of the Clock class) it should refer to. static members can only refer to static members. Instance members can, of course access static members.
Side note: Of course, static members can access instance members through an object reference.
Example:
public class Example {
private static boolean staticField;
private boolean instanceField;
public static void main(String[] args) {
// a static method can access static fields
staticField = true;
// a static method can access instance fields through an object reference
Example instance = new Example();
instance.instanceField = true;
}
[1]: Depending on the runtime characteristics, it can be one per ClassLoader or AppDomain or thread, but that is beside the point.
It means that there is only one instance of "clock" in Hello, not one per each separate instance of the "Hello" class, or more-so, it means that there will be one commonly shared "clock" reference among all instances of the "Hello" class.
So if you were to do a "new Hello" anywhere in your code:
A- in the first scenario (before the change, without using "static"), it would make a new clock every time a "new Hello" is called, but
B- in the second scenario (after the change, using "static"), every "new Hello" instance would still share and use the initial and same "clock" reference first created.
Unless you needed "clock" somewhere outside of main, this would work just as well:
package hello;
public class Hello
{
public static void main(String args[])
{
Clock clock=new Clock();
clock.sayTime();
}
}
The static keyword means that something (a field, method or nested class) is related to the type rather than any particular instance of the type. So for example, one calls Math.sin(...) without any instance of the Math class, and indeed you can't create an instance of the Math class.
For more information, see the relevant bit of Oracle's Java Tutorial.
Sidenote
Java unfortunately allows you to access static members as if they were instance members, e.g.
// Bad code!
Thread.currentThread().sleep(5000);
someOtherThread.sleep(5000);
That makes it look as if sleep is an instance method, but it's actually a static method - it always makes the current thread sleep. It's better practice to make this clear in the calling code:
// Clearer
Thread.sleep(5000);
The static keyword in Java means that the variable or function is shared between all instances of that class as it belongs to the type, not the actual objects themselves.
So if you have a variable: private static int i = 0; and you increment it (i++) in one instance, the change will be reflected in all instances. i will now be 1 in all instances.
Static methods can be used without instantiating an object.
Basic usage of static members...
public class Hello
{
// value / method
public static String staticValue;
public String nonStaticValue;
}
class A
{
Hello hello = new Hello();
hello.staticValue = "abc";
hello.nonStaticValue = "xyz";
}
class B
{
Hello hello2 = new Hello(); // here staticValue = "abc"
hello2.staticValue; // will have value of "abc"
hello2.nonStaticValue; // will have value of null
}
That's how you can have values shared in all class members without sending class instance Hello to other class. And whit static you don't need to create class instance.
Hello hello = new Hello();
hello.staticValue = "abc";
You can just call static values or methods by class name:
Hello.staticValue = "abc";
Static means that you don't have to create an instance of the class to use the methods or variables associated with the class. In your example, you could call:
Hello.main(new String[]()) //main(...) is declared as a static function in the Hello class
directly, instead of:
Hello h = new Hello();
h.main(new String[]()); //main(...) is a non-static function linked with the "h" variable
From inside a static method (which belongs to a class) you cannot access any members which are not static, since their values depend on your instantiation of the class. A non-static Clock object, which is an instance member, would have a different value/reference for each instance of your Hello class, and therefore you could not access it from the static portion of the class.
Static in Java:
Static is a Non Access Modifier.
The static keyword belongs to the class than instance of the class.
can be used to attach a Variable or Method to a Class.
Static keyword CAN be used with:
Method
Variable
Class nested within another Class
Initialization Block
CAN'T be used with:
Class (Not Nested)
Constructor
Interfaces
Method Local Inner Class(Difference then nested class)
Inner Class methods
Instance Variables
Local Variables
Example:
Imagine the following example which has an instance variable named count which in incremented in the constructor:
package pkg;
class StaticExample {
int count = 0;// will get memory when instance is created
StaticExample() {
count++;
System.out.println(count);
}
public static void main(String args[]) {
StaticExample c1 = new StaticExample();
StaticExample c2 = new StaticExample();
StaticExample c3 = new StaticExample();
}
}
Output:
1 1 1
Since instance variable gets the memory at the time of object creation, each object will have the copy of the instance variable, if it is incremented, it won't reflect to other objects.
Now if we change the instance variable count to a static one then the program will produce different output:
package pkg;
class StaticExample {
static int count = 0;// will get memory when instance is created
StaticExample() {
count++;
System.out.println(count);
}
public static void main(String args[]) {
StaticExample c1 = new StaticExample();
StaticExample c2 = new StaticExample();
StaticExample c3 = new StaticExample();
}
}
Output:
1 2 3
In this case static variable will get the memory only once, if any object changes the value of the static variable, it will retain its value.
Static with Final:
The global variable which is declared as final and static remains unchanged for the whole execution. Because, Static members are stored in the class memory and they are loaded only once in the whole execution. They are common to all objects of the class. If you declare static variables as final, any of the objects can’t change their value as it is final. Therefore, variables declared as final and static are sometimes referred to as Constants. All fields of interfaces are referred as constants, because they are final and static by default.
Picture Resource : Final Static
To add to existing answers, let me try with a picture:
An interest rate of 2% is applied to ALL savings accounts. Hence it is static.
A balance should be individual, so it is not static.
This discussion has so far ignored classloader considerations. Strictly speaking, Java static fields are shared between all instances of a class for a given classloader.
A field can be assigned to either the class or an instance of a class. By default fields are instance variables. By using static the field becomes a class variable, thus there is one and only one clock. If you make a changes in one place, it's visible everywhere. Instance varables are changed independently of one another.
In Java, the static keyword can be simply regarded as indicating the following:
"without regard or relationship to any particular instance"
If you think of static in this way, it becomes easier to understand its use in the various contexts in which it is encountered:
A static field is a field that belongs to the class rather than to any particular instance
A static method is a method that has no notion of this; it is defined on the class and doesn't know about any particular instance of that class unless a reference is passed to it
A static member class is a nested class without any notion or knowledge of an instance of its enclosing class (unless a reference to an enclosing class instance is passed to it)
The keyword static is used to denote a field or a method as belonging to the class itself and not to any particular instance. Using your code, if the object Clock is static, all of the instances of the Hello class will share this Clock data member (field) in common. If you make it non-static, each individual instance of Hello will have a unique Clock.
You added a main method to your class Hello so that you could run the code. The problem with that is that the main method is static and as such, it cannot refer to non-static fields or methods inside of it. You can resolve this in two ways:
Make all fields and methods of the Hello class static so that they could be referred to inside the main method. This is not a good thing to do (or the wrong reason to make a field and/or a method static)
Create an instance of your Hello class inside the main method and access all its fields and methods the way they were intended to be accessed and used in the first place.
For you, this means the following change to your code:
package hello;
public class Hello {
private Clock clock = new Clock();
public Clock getClock() {
return clock;
}
public static void main(String args[]) {
Hello hello = new Hello();
hello.getClock().sayTime();
}
}
static methods don't use any instance variables of the class they are defined in. A very good explanation of the difference can be found on this page
I have developed a liking for static methods (only, if possible) in "helper" classes.
The calling class need not create another member (instance) variable of the helper class. You just call the methods of the helper class. Also the helper class is improved because you no longer need a constructor, and you need no member (instance) variables.
There are probably other advantages.
Static makes the clock member a class member instead of an instance member. Without the static keyword you would need to create an instance of the Hello class (which has a clock member variable) - e.g.
Hello hello = new Hello();
hello.clock.sayTime();
//Here is an example
public class StaticClass
{
static int version;
public void printVersion() {
System.out.println(version);
}
}
public class MainClass
{
public static void main(String args[]) {
StaticClass staticVar1 = new StaticClass();
staticVar1.version = 10;
staticVar1.printVersion() // Output 10
StaticClass staticVar2 = new StaticClass();
staticVar2.printVersion() // Output 10
staticVar2.version = 20;
staticVar2.printVersion() // Output 20
staticVar1.printVersion() // Output 20
}
}
Can also think of static members not having a "this" pointer. They are shared among all instances.
Understanding Static concepts
public class StaticPractise1 {
public static void main(String[] args) {
StaticPractise2 staticPractise2 = new StaticPractise2();
staticPractise2.printUddhav(); //true
StaticPractise2.printUddhav(); /* false, because printUddhav() is although inside StaticPractise2, but it is where exactly depends on PC program counter on runtime. */
StaticPractise2.printUddhavsStatic1(); //true
staticPractise2.printUddhavsStatic1(); /*false, because, when staticPractise2 is blueprinted, it tracks everything other than static things and it organizes in its own heap. So, class static methods, object can't reference */
}
}
Second Class
public class StaticPractise2 {
public static void printUddhavsStatic1() {
System.out.println("Uddhav");
}
public void printUddhav() {
System.out.println("Uddhav");
}
}
main() is a static method which has two fundamental restrictions:
The static method cannot use a non-static data member or directly call non-static method.
this() and super() cannot be used in static context.
class A {
int a = 40; //non static
public static void main(String args[]) {
System.out.println(a);
}
}
Output: Compile Time Error
A question was asked here about the choice of the word 'static' for this concept. It was dup'd to this question, but I don't think the etymology has been clearly addressed. So...
It's due to keyword reuse, starting with C.
Consider data declarations in C (inside a function body):
void f() {
int foo = 1;
static int bar = 2;
:
}
The variable foo is created on the stack when the function is entered (and destroyed when the function terminates). By contrast, bar is always there, so it's 'static' in the sense of common English - it's not going anywhere.
Java, and similar languages, have the same concept for data. Data can either be allocated per instance of the class (per object) or once for the entire class. Since Java aims to have familiar syntax for C/C++ programmers, the 'static' keyword is appropriate here.
class C {
int foo = 1;
static int bar = 2;
:
}
Lastly, we come to methods.
class C {
int foo() { ... }
static int bar() { ... }
:
}
There is, conceptually speaking, an instance of foo() for every instance of class C. There is only one instance of bar() for the entire class C. This is parallel to the case we discussed for data, and therefore using 'static' is again a sensible choice, especially if you don't want to add more reserved keywords to your language.
Static Variables Can only be accessed only in static methods, so when we declare the static variables those getter and setter methods will be static methods
static methods is a class level we can access using class name
The following is example for Static Variables Getters And Setters:
public class Static
{
private static String owner;
private static int rent;
private String car;
public String getCar() {
return car;
}
public void setCar(String car) {
this.car = car;
}
public static int getRent() {
return rent;
}
public static void setRent(int rent) {
Static.rent = rent;
}
public static String getOwner() {
return owner;
}
public static void setOwner(String owner) {
Static.owner = owner;
}
}
A member in a Java program can be declared as static using the keyword “static” preceding its declaration/definition. When a member is declared static, then it essentially means that the member is shared by all the instances of a class without making copies of per instance.
Thus static is a non-class modifier used in Java and can be applied to the following members:
Variables
Methods
Blocks
Classes (more specifically, nested classes)
When a member is declared static, then it can be accessed without using an object. This means that before a class is instantiated, the static member is active and accessible. Unlike other non-static class members that cease to exist when the object of the class goes out of scope, the static member is still obviously active.
Static Variable in Java
A member variable of a class that is declared as static is called the Static Variable. It is also called as the “Class variable”. Once the variable is declared as static, memory is allocated only once and not every time when a class is instantiated. Hence you can access the static variable without a reference to an object.
The following Java program depicts the usage of Static variables:
class Main
{
// static variables a and b
static int a = 10;
static int b;
static void printStatic()
{
a = a /2;
b = a;
System.out.println("printStatic::Value of a : "+a + " Value of b :
"+b);
}
public static void main(String[] args)
{
printStatic();
b = a*5;
a++;
System.out.println("main::Value of a : "+a + " Value of b : "+b);
}
}
output::
printStatic::Value of a : Value of b : 5
main::Value of a : 6 Value of b : 25
In the above program, we have two static variables i.e. a and b. We modify these variables in a function “printStatic” as well as in “main”. Note that the values of these static variables are preserved across the functions even when the scope of the function ends. The output shows the values of variables in two functions.
Static Method
A method in Java is static when it is preceded by the keyword “static”.
Some points that you need to remember about the static method include:
A static method belongs to the class as against other non-static
methods that are invoked using the instance of a class.
To invoke a static method, you don’t need a class object.
The static data members of the class are accessible to the static
method. The static method can even change the values of the static
data member.
A static method cannot have a reference to ‘this’ or ‘super’ members.
Even if a static method tries to refer them, it will be a compiler
error.
Just like static data, the static method can also call other static
methods. A static method cannot refer to non-static data members or
variables and cannot call non-static methods too.
The following program shows the implementation of the static method in Java:
class Main
{
// static method
static void static_method()
{
System.out.println("Static method in Java...called without any
object");
}
public static void main(String[] args)
{
static_method();
}
}
output:
Static method in Java...called without any object
Static Block In Java
Just as you have function blocks in programming languages like C++, C#, etc. in Java also, there is a special block called “static” block that usually includes a block of code related to static data.
This static block is executed at the moment when the first object of the class is created (precisely at the time of classloading) or when the static member inside the block is used.
The following program shows the usage of a static block.
class Main
{
static int sum = 0;
static int val1 = 5;
static int val2;
// static block
static {
sum = val1 + val2;
System.out.println("In static block, val1: " + val1 + " val2: "+
val2 + " sum:" + sum);
val2 = val1 * 3;
sum = val1 + val2;
}
public static void main(String[] args)
{
System.out.println("In main function, val1: " + val1 + " val2: "+ val2 + " sum:" + sum);
}
}
output:
In static block, val1: 5 val2: 0 sum:5
In main function, val1: val2: 15 sum:20
Static Class
In Java, you have static blocks, static methods, and even static variables. Hence it’s obvious that you can also have static classes. In Java, it is possible to have a class inside another class and this is called a Nested class. The class that encloses the nested class is called the Outer class.
In Java, although you can declare a nested class as Static it is not possible to have the outer class as Static.
Let’s now explore the static nested classes in Java.
Static Nested Class
As already mentioned, you can have a nested class in Java declared as static. The static nested class differs from the non-static nested class(inner class) in certain aspects as listed below.
Unlike the non-static nested class, the nested static class doesn’t need an outer class reference.
A static nested class can access only static members of the outer class as against the non-static classes that can access static as well as non-static members of the outer class.
An example of a static nested class is given below.
class Main{
private static String str = "SoftwareTestingHelp";
//Static nested class
static class NestedClass{
//non-static method
public void display() {
System.out.println("Static string in OuterClass: " + str);
}
}
public static void main(String args[])
{
Main.NestedClassobj = new Main.NestedClass();
obj.display();
}
}
output
Static string in OuterClass: SoftwareTestingHelp
I think this is how static keyword works in java.
I have this code in java I write it by netbeans
class sample
{
public static int x;
public int y;
sample()
{
x=0;
}
}
public class JavaApplication1 {
/**
* #param args the command line arguments
*/
sample cchild=new sample();
public static void main(String[] args) {
// TODO code application logic here
sample.x=0;
cchild.x=9; // here error
}
explain the sample :
I make composition to class sample , class sample contain static variable x , But when I try to access to static variable x from the instance cchild in static method the compiler make error,
so in java I can not access to object in static methode even the instance contain static member ???
The first thing you need to understand is that static members belong to a class and not an instance and can therefore be accessed directly without the need to create a reference to an instance of the class. The following statement accesses the static member x in class sample where sample is the class name and x is the static member in sample :
sample.x=0;
The following statement on the other hand does not work because
the reference cchild is not static and is thus an instance field while main is a static method. An instance field cannot be accessed in a static method without a reference to an instance of the class.
cchild.x=9
For the above statement to work, you either declare cchild as static in JavaApplication1 or create an instance of JavaApplication1 in main as shown below :
JavaApplication1 instanceOfJApp = new JavaApplication1();
instanceOfJApp.cchild.x=9;
This should work. You need to declare the variable cchild to be a static member of the JavaApplicaiton1 class to be able to access it statically.
class sample
{
public static int x;
public int y;
sample()
{
x=0;
}
}
public class JavaApplication1 {
// NEW BIT - by making this variable static we can now access it without needing an instance of the object.
static sample cchild=new sample();
public static void main(String[] args) {
sample.x=0;
cchild.x=9;
}
}
Static in Java means it is a property of the class itself and not a property of an instance object of that class's type. When using non-static properties you need to have created an object of that class's type by calling a constructor and then you can use that object's reference to call non-static methods and access non-static variables. If you don't have a copy of an object of that type then you can only call the static methods and access the static variables.
The original didn't work because although you were trying to access the static variable from a static context (inside the main method which is static) you were creating the variable you used to access the static variable (cchild) in a non-static context (in the class definition). By not labelling the cchild variable 'static' it becomes an instance variable of the JavaApplication1 class and so can only be used if you create an instance of the JavaApplication1 class by calling a constructor, and not in the main method which is created statically.
I have suggested here that you change the variable to be static so that you can access it. I think this is the easiest way for you to make progress. However, in general, if you get stuck needing to make a change like this it probably shows that you need to think more about which members need to be static and which need to be on the instance object and so just making the variable static might not always be the best thing to do.
There are a few other things you might do differently in this code example. The first is that I would suggest that you use the Java naming convention of starting the name of your classes with a capital letter (Sample instead of sample in this case) otherwise they do not look like class names to Java people.
There are 2 things you can do to fix your problem:
Make cchild static
Move the declaration of cchild to your main method
It's because invoking
static void main(String[] args){
}
doesn't make the JavaApplication1 instance.
This question already has answers here:
Why can I access a private variable from main method?
(7 answers)
Closed 8 years ago.
I recently see a question that what is the execution result of this code below.
public class Sandys {
private int court;
public static void main(String argv[]){
Sandys s = new Sandys(99);
System.out.println(s.court);
}
Sandys(int ballcount){
court = ballcount;
}
}
I think it can't be executed because in main i try to access a private variable.
However, this code can be perfectly executed, and the result is 99. So I am confused, why the private variable can be accessed in this code? Though the main is in Sandys class, however i create a new Sandys. Can I still access the private variable of the new Sandys object in main?
You can access private members from inside the same class, even in static methods.
main() is a special method because it is used as a starting point for java application. However, it is still a normal static method and it follows all access restrictions of static methods. Is this main() a class method? Yes, it is. Hence, it can access private members of the same class.
Consider this: if there was no access to private fields factory methods would have to be written differently.
class A {
private int a=0;
private int b=0;
private A() {}
//getters
public static A getNewInstance(int a, int b) {
A a = new A();
a.a = a;
a.b = b;
return A;
}
It does make sense, doesn't it?
You can access ALL private, protected, public and not modifier variables inside class, but you can not access private variable from another class.
P.S. It is truth also for C#, C++ and many other programming languages.
A class is defined to some particular job and in that way it may use some variables that it don't want others to access it("others" means outsider classes) and some you want other classes to access it.It all depends on your requirement.
"Private instance variables are defined to be used in only with in the class".and since in your code you are trying to access the private instance variable inside the class itself then it is perfectly legal.
Note: A static method cannot access the instance variables if u want specify the instance i mean create the instance(object) inside of the static method and access it through the created reference.
I'm learning Java and I just wonder why public and private is used when a method or members is static? When static is used they are class methods and class members and could be used from other classes without creating an object, so is public and private necessary? Some help is preciated to understand. Sorry if this question is too simple for some.
The accessibility of a field or method is orthogonal to the fact that it's static or not.
You could have a static method accessible from the outside, and a static method that must only be used from inside the class itself (by other static or non-static methods). The same goes for fields.
For example:
// not visible from the outside
private static final long MILLISECONDS_IN_A_MINUTE = 1000L * 60 * 60;
public static Date addMinutes(Date d, int amount) {
return addMillis(d, MILLISECONDS_IN_A_MINUTE * amount);
}
// not visible from the outside
private static Date addMillis(Date d, long amount) {
return new Date(d.getTime() + amount);
}
It's not necessary, but there can be static methods and data members for internal use only.
An example for this is if you want an unique id for every instance of the class:
class Foo
{
private static int nextId = 0;
private static int generateId() { return ++nextId; }
private int id;
public Foo()
{
id = generateId();
}
}
As you can see, nextId and generateId() are not needed outside the class, nor should they be used outside the class. The class itself is responsible for generating id's. But you need them to be static (well, you need nextId to be static, but you can also make generateId() static since it doesn't access non-static members).
Whenever an object Foo is created, the static counter is incremented, thus you get different ids for each instance of the class. (this example is not thread-safe)
Suppose you have a static public method and this method must access to a private attribute. This private attribute must be static too. There's one reason why private static exists.
Example :
public class Test {
private static int myattr = 0;
public static void foo() {
myattr = 2;
}
}
Above, myattr must be a static attribute in order to use it in the foo() method.
Yes it is needed.
If you have a Static Method and want to use a private variables in that method, then you need to declare it static too.
Or you want the static variables not be visible to other packages, then don't declare it public.
From what I remember, it's not really needed. But public means, basically in any programming language, that it can be used by outside files. With private it can only be used within that file, and static means you cannot change the value of said reference. Whether these be functions, or variables, the same rules apply. I might be off. Haven't done Java in about a year and a half.
The ways you can incorporate these types is up to you. After all, a program is only as diverse as it's user. ^_^
Public and private keywoards have to do with visibility: which members do you want to accessible to other classes and which should be hidden or encapsulated.
Static members relate to the class as a whole, while non-static members operate on object instances.
I'm learning Java and I just wonder why public and private is used when a method or members is static?
I believe your question is due to a common misconception that the access modifiers are for instances, but they're not!
Two different instances can access each others private members if they are of the same class.
In other words, the access modifiers works on class level. Since also static members belong to some class, it makes sense to have access modifiers also on them.
A static method (or variable) that should only be used by code in the same class (as in the example by JB Nizet) should be private, while a static method or variable that may be used by code in any class should be public.
When the static is used with methods it doesn't only mean that it should be used by the members of other classes.
The case when we access the static methods of a class is one when
the class (which contains the method) cannot be instantiated i.e. no objects can be created of that class.
There may be situations when two different classes may have static methods with same name. In that case you want to use the method of the same class not the method of other class.
I heard that static methods should use only static variables in java.
But, main method is also static, right?
Your question: is the statement " static methods should use only static variables" correct?
No. The statement is not correct.
The correct statement will be "static methods can only use those instance variables that are defined static"
Take a look at following code and read the comments:
Class A{
int i;
static int j;
public static void methodA(){
i = 5; //cannot use since i is not static
j = 2; //can use.
int k = 3; //k is local variable and can be used no problem
**EDIT:**//if you want to access i
A a = new A();
//A.i = 5; //can use.
a.i = 5; // it should be non-capital "a" right?
}
}
First of all, a technicality: it's NOT true that "main method is also static". You can define a non-static main method, with whatever signature you choose; it just won't be a valid Java application entry point.
With regards to "static methods should use only static variables", this is also NOT true. The key concept here is that static methods and fields are class-specific, not instance-specific. You simply can't access an instance variable/method if you don't actually have an instance of that class; it's a compilation error.
So to be precise, without an instance, you can't access instance fields/methods. You can access static fields/methods without an instance. If you need to access instance fields/methods from a static method, you have to get an instance of that class one way or another, either by simply instantiating it, or by getting a reference to it from a static field or method parameter.
Let's take a look at this simple example:
public static void main(String args[]) {
System.out.println(args.length);
}
length is NOT a static field; it's an instance field of array instances, which args is. The static method main is able to get this instance (and thus access its instance methods and fields) because it's passed in as an argument.
Also, println is NOT a static method; it's an instance method of PrintStream instances. The static method main is able to get this instance by accessing the static field out of the class System.
To summarize:
A Java application entry point is a method that:
is named main
is public and static
returns void and takes a String[] argument as parameter
A method named main doesn't have to be a Java application entry point
(but it's best to reserve this name for that purpose)
Furthermore,
Instance fields/methods can only be accessed through an instance
Static fields/methods can be accessed without an instance
Static methods can get an instance of a class by one of the following ways:
creating a new instance
having it passed as an argument
accessing it through a static field of a class
accepting it as the return value of a static method of a class
catching it as a thrown Throwable
Maybe this piece of code will enlighten you:
public class Main {
private String instanceField = "Joe";
private void instanceMethod() {
System.out.println("Instance name=" + instanceField);
}
public static void main(String[] args) {
// cannot change instance field without an instance
instanceField = "Indy"; // compilation error
// cannot call an instance method without an instance
instanceMethod(); // compilation error
// create an instance
Main instance = new Main();
// change instance field
instance.instanceField = "Sydney";
// call instance method
instance.instanceMethod();
}
}
So you cannot access instance members without an instance. Within the context of a static method you don't have a reference to an instance unless you receive or create one.
Hope this helps.
To access non-static fields (instance variables) you need to have an instance.
Inside a non-static method, this is used as default instance:
class AnyClass {
private String nonStaticField = "Non static";
void nonStaticMethod() {
this.nonStaticField = "a text"; // accessing field of this
nonStaticField = "a text"; // same as before
}
}
Inside a static method there is no this to use as default instance, but you can1 still access instance variables if you provide the instance:
class AnyClass {
private String nonStaticField = "Non static";
static void staticMethod() {
AnyClass example = new AnyClass();
example.nonStaticField = "new value for non-static field";
}
}
1 - the field must also be accessible by Java's access control (declared public or in the same class ...)
A static method is called on a class instance and not to an object of a class. This means, that a static method is not able to access instance variables, because they are only instantiated in an object.
If you want to access an instance variable with a static method, you have to declare that variable as static.
public class Test {
private static int value = 0;
public static void main(String[] args) {
value++;
}
}
But to be honest, it's not the best idea to write everything in static methods. It's better to use the main method to instantiate new objects.
One important thing is that unless u create an instance of an class the instance variables don't exist practically it means JVM doesn't that there os an variable like int i unless u create an instance for that class. so, using an instance variable in a static method is an compilation error.
class A{
int i;
static int j;
static int b(){
i=10; // cannot be found
A a= new A();
a.i=10;//can be found in a's instance
}
}
But, we can use instance variables in instance methods because, instance methods are only called when object created so there is no problem in using instance variables inside it.
Hope ur clear about these things!
Yes static method cannot call non-static methods or variables of the class directly.
EDIT : One can create any local variables.
The static method can't access non-static variables outsides because you can use static method without class initialization, but it doesn't work for non-static outside variable. But you can define and use non-static variable in the static method.