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.
There are to separate classes class One and class Two. Both of classes are in same package. I want to access one class data into other class how can i access variable data. My program is very lengthy ,I just want the logic of this.Thanking you in advance.
Class A.java
public class A
{
public static void main(String ar[])
{
int a=100;
}
}
Class B.java
public class B extends A
{
public static void main(String m[])
{
A obj=new A();
System.out.println("Variable of class A is :"+ obj.a);
}
}
I have done this thing to get access like i declared variable a as Static so that i can directly get access but it's not working. and when i am compiling B.java It giving me error
cannot find symbol at := System.out.println("Variable of class A is :"+ obj.a);
And
Illegal start of expression (when i am delaring variable a as public)
:-(error)public int a=100; [in class A].
Why are you using the static main method? Besides that the field a is local and not accessible outside the scope. Use this instead.
public class A
{
public int a;
public A()
{
a=100;
}
}
You don't have two true object-oriented classes above, but rather little more than two receptacles for static main methods. To combine code from two classes well, you will want to scrap that code and make OOP-compliant classes, complete with instance fields and methods. For more on this, check out the OOP section of the Java tutorials: link to OOP tutorial.
First, get rid of main() in A. You only want one main() in your application, and it's in B (since the one in A doesn't actually do anything):
public class A {
}
Now, you want A to have a class-level int value:
public class A {
private int a;
}
And you want it to have a default value of 100, yes? A constructor is a good place to do that:
public class A {
private int a;
public A() {
this.a = 100;
}
}
Now any time you do this:
A obj = new A();
you will have an object with a value. In order to access that value from outside that object, you need a "getter":
public class A {
private int a;
public A() {
this.a = 100;
}
public int get_a() {
return this.a;
}
}
Now in B (or anywhere, really), you can create an instance of A and access that value by using the "getter":
A obj=new A();
System.out.println("Variable of class A is :"+ obj.get_a());
Semantically, don't think of it as "accessing a variable from another class". Instead, think of what your objects are and what they represent. If it were a physical, real-world object which internally contained some kind of value.
When you create an instance of that object, the instance would internally have that value somewhere. From the outside of that object, it doesn't really matter how that value is internally maintained. There just needs to be some kind of interface to see the value. Which is what the "getter" method does.
One-liner answer: To access a variable outside a class, make it class-level. You have written a method-level variable that's accessible only inside that scope (method).
To elaborate:
There are to separate classes class One and class Two. Both of classes are in same package. I want to access one class data into other class how can i access variable data.
So basically you know that to by extending, you should be able to access parent class data into your subclass. For that, simply make the data in your parent class as class level.
class A {
int var = 10; //class level, but non-static, so to access you need A object
void method() {
int var = 20; //this is method local and can not be accessed outside
}
}
public class B extends A {
public static void main(String[] args) {
A aObj = new A();
System.out.println(aObj.var);
}
}
Illegal start of expression (when i am delaring variable a as public)
Its illegal. Because access modifiers like public, private etc. are applicable to class-level stuff like the first var or the main method in class B you see.
Said that:
You need to immediately go here: https://docs.oracle.com/javase/tutorial/
rather than just trying to run some classes when you lack language basics.
I am new to Java. I know static is class level and this is object level but please let me know if a static method can refer to the this pointer in java?
No it does not make sense to refer this in static methods/blocks. Static methods can be called without creating an instance hence this cannot be used to refer instance fields.
If you try to put this in a static method, compiler will throw an error saying
cannot use this in static context
No, this cannot be accessed in a static context. June Ahsan said everything that you probably need to know but I would like to add a little background.
The only difference between an object method and a static method on byte code level is an extra first parameter for object methods. This parameter can be accessed through this.
Since this parameter is missing for static methods, there is no this.
Example:
class MyClass {
private int b;
public void myMethod(int a){
System.out.println(this.b + a);
}
public static void myStaticMethod(int a){
System.out.println(a*a); // no access to b
}
}
on byte code level becomes (roughly speaking, because byte code does not look like this)
class MyClass {
int b;
}
void myMethod(MyClass this, int a){
System.out.println(this.b + a);
}
static void myStaticMethod(int a){
System.out.println(a*a); // no access to b
}
No but you can create a static object of you class in your class like this
private static ClassName instance; and then you can set it with instance = this; in the constructor. this will then be available for you to use in a static method.
At present I have a class that is calling the static method of a different class. What I am trying to do however is have the static method change a variable of the calling class, is that possible?
Example code:
public class exClass {
private int aVariable;
public exClass() {
othClass.aMethod();
}
}
public class othClass {
static void aMethod() {
// stuff happens, preferably stuff that
// allows me to change exClass.aVariable
}
}
So what I would like to know is, if there is a way to access aVariable of the instance of exClass that is calling othClass. Other than using a return statement, obviously.
Not if aClass doesn't expose that variable. This is what encapsulation and information hiding are about: if the designer of the class makes a variable private, then only the component that owns it can modify or access it.
Of course, the dirty little secret in Java is that reflection can get you around any private restriction.
But you should not resort to that. You should design your classes appropriately and respect the designs of others.
You can pass this as a parameter to the second function.
public class exClass {
public int aVariable;
public exClass()
{
othClass.aMethod(this);
}
}
public class othClass{
static void aMethod(exClass x)
{
x.aVariable = 0; //or call a setter if you want to keep the member private
}
}
you should gave the static method in othClass the instance of exClass like othClass.aMethod(this), then you can change the variable of that instance, or make the variable static if you dont need an instance
Can we declare Static Variables inside Main method? Because I am getting an error message:
Illegal Start of Expression
Obviously, no, we can't.
In Java, static means that it's a variable/method of a class, it belongs to the whole class but not to one of its certain objects.
This means that static keyword can be used only in a 'class scope' i.e. it doesn't have any sense inside methods.
You can use static variables inside your main method (or any other method), but you need to declare them in the class:
This is totally fine:
public Class YourClass {
static int someNumber = 5;
public static void main(String[] args) {
System.out.println(someNumber);
}
}
This is fine too, but in this case, someNumber is a local variable, not a static one.
public Class YourClass {
public static void main(String[] args) {
int someNumber = 5;
System.out.println(someNumber);
}
}
Because the static variables are allocated memory at the class loading time,and the memory is allocated only once.Now if you have a static variable inside a method,then that variable comes under the method's scope,not class's scope,and JVM is unable to allocate memory to it,because a method is called by the help of class's object,and that is at runtime,not at class loading time.
As static variables are available for the entire class so conceptually it can only be declared after the class whose scope is global where as static block or methods all have their own scope.
You cannot, why would you want to do that? You can always declare it on the class level where it belongs.
In C, you can have statically allocated locally scoped variables. Unfortunately this is not directly supported in Java. But you can achieve the same effect by using nested classes.
For example, the following is allowed but it is bad engineering, because the scope of x is much larger than it needs to be. Also there is a non-obvious dependency between two members (x and getNextValue).
static int x = 42;
public static int getNextValue() {
return ++x;
}
One would really like to do the following, but it is not legal:
public static int getNextValue() {
static int x = 42; // not legal :-(
return ++x;
}
However you could do this instead,
public static class getNext {
static int x = 42;
public static int value() {
return ++x;
}
}
It is better engineering at the expense of some ugliness.