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what is the name of such invocation "variable.methodName(data);" ? [duplicate]

See the code snippets below:
Code 1
public class A {
static int add(int i, int j) {
return(i + j);
}
}
public class B extends A {
public static void main(String args[]) {
short s = 9;
System.out.println(add(s, 6));
}
}
Code 2
public class A {
int add(int i, int j) {
return(i + j);
}
}
public class B extends A {
public static void main(String args[]) {
A a = new A();
short s = 9;
System.out.println(a.add(s, 6));
}
}
What is the difference between these code snippets? Both output 15 as an answer.

A static method belongs to the class itself and a non-static (aka instance) method belongs to each object that is generated from that class. If your method does something that doesn't depend on the individual characteristics of its class, make it static (it will make the program's footprint smaller). Otherwise, it should be non-static.
Example:
class Foo {
int i;
public Foo(int i) {
this.i = i;
}
public static String method1() {
return "An example string that doesn't depend on i (an instance variable)";
}
public int method2() {
return this.i + 1; // Depends on i
}
}
You can call static methods like this: Foo.method1(). If you try that with method2, it will fail. But this will work: Foo bar = new Foo(1); bar.method2();

Static methods are useful if you have only one instance (situation, circumstance) where you're going to use the method, and you don't need multiple copies (objects). For example, if you're writing a method that logs onto one and only one web site, downloads the weather data, and then returns the values, you could write it as static because you can hard code all the necessary data within the method and you're not going to have multiple instances or copies. You can then access the method statically using one of the following:
MyClass.myMethod();
this.myMethod();
myMethod();
Non-static methods are used if you're going to use your method to create multiple copies. For example, if you want to download the weather data from Boston, Miami, and Los Angeles, and if you can do so from within your method without having to individually customize the code for each separate location, you then access the method non-statically:
MyClass boston = new MyClassConstructor();
boston.myMethod("bostonURL");
MyClass miami = new MyClassConstructor();
miami.myMethod("miamiURL");
MyClass losAngeles = new MyClassConstructor();
losAngeles.myMethod("losAngelesURL");
In the above example, Java creates three separate objects and memory locations from the same method that you can individually access with the "boston", "miami", or "losAngeles" reference. You can't access any of the above statically, because MyClass.myMethod(); is a generic reference to the method, not to the individual objects that the non-static reference created.
If you run into a situation where the way you access each location, or the way the data is returned, is sufficiently different that you can't write a "one size fits all" method without jumping through a lot of hoops, you can better accomplish your goal by writing three separate static methods, one for each location.

Generally
static: no need to create object we can directly call using
ClassName.methodname()
Non Static: we need to create a object like
ClassName obj=new ClassName()
obj.methodname();

A static method belongs to the class
and a non-static method belongs to an
object of a class. That is, a
non-static method can only be called
on an object of a class that it
belongs to. A static method can
however be called both on the class as
well as an object of the class. A
static method can access only static
members. A non-static method can
access both static and non-static
members because at the time when the
static method is called, the class
might not be instantiated (if it is
called on the class itself). In the
other case, a non-static method can
only be called when the class has
already been instantiated. A static
method is shared by all instances of
the class. These are some of the basic
differences. I would also like to
point out an often ignored difference
in this context. Whenever a method is
called in C++/Java/C#, an implicit
argument (the 'this' reference) is
passed along with/without the other
parameters. In case of a static method
call, the 'this' reference is not
passed as static methods belong to a
class and hence do not have the 'this'
reference.
Reference:Static Vs Non-Static methods

Well, more technically speaking, the difference between a static method and a virtual method is the way the are linked.
A traditional "static" method like in most non OO languages gets linked/wired "statically" to its implementation at compile time. That is, if you call method Y() in program A, and link your program A with library X that implements Y(), the address of X.Y() is hardcoded to A, and you can not change that.
In OO languages like JAVA, "virtual" methods are resolved "late", at run-time, and you need to provide an instance of a class. So in, program A, to call virtual method Y(), you need to provide an instance, B.Y() for example. At runtime, every time A calls B.Y() the implementation called will depend on the instance used, so B.Y() , C.Y() etc... could all potential provide different implementations of Y() at runtime.
Why will you ever need that? Because that way you can decouple your code from the dependencies. For example, say program A is doing "draw()". With a static language, thats it, but with OO you will do B.draw() and the actual drawing will depend on the type of object B, which, at runtime, can change to square a circle etc. That way your code can draw multiple things with no need to change, even if new types of B are provided AFTER the code was written. Nifty -

A static method belongs to the class and a non-static method belongs to an object of a class.
I am giving one example how it creates difference between outputs.
public class DifferenceBetweenStaticAndNonStatic {
static int count = 0;
private int count1 = 0;
public DifferenceBetweenStaticAndNonStatic(){
count1 = count1+1;
}
public int getCount1() {
return count1;
}
public void setCount1(int count1) {
this.count1 = count1;
}
public static int countStaticPosition() {
count = count+1;
return count;
/*
* one can not use non static variables in static method.so if we will
* return count1 it will give compilation error. return count1;
*/
}
}
public class StaticNonStaticCheck {
public static void main(String[] args){
for(int i=0;i<4;i++) {
DifferenceBetweenStaticAndNonStatic p =new DifferenceBetweenStaticAndNonStatic();
System.out.println("static count position is " +DifferenceBetweenStaticAndNonStatic.count);
System.out.println("static count position is " +p.getCount1());
System.out.println("static count position is " +DifferenceBetweenStaticAndNonStatic.countStaticPosition());
System.out.println("next case: ");
System.out.println(" ");
}
}
}
Now output will be:::
static count position is 0
static count position is 1
static count position is 1
next case:
static count position is 1
static count position is 1
static count position is 2
next case:
static count position is 2
static count position is 1
static count position is 3
next case:

If your method is related to the object's characteristics, you should define it as non-static method. Otherwise, you can define your method as static, and you can use it independently from object.

Static method example
class StaticDemo
{
public static void copyArg(String str1, String str2)
{
str2 = str1;
System.out.println("First String arg is: "+str1);
System.out.println("Second String arg is: "+str2);
}
public static void main(String agrs[])
{
//StaticDemo.copyArg("XYZ", "ABC");
copyArg("XYZ", "ABC");
}
}
Output:
First String arg is: XYZ
Second String arg is: XYZ
As you can see in the above example that for calling static method, I didn’t even use an object. It can be directly called in a program or by using class name.
Non-static method example
class Test
{
public void display()
{
System.out.println("I'm non-static method");
}
public static void main(String agrs[])
{
Test obj=new Test();
obj.display();
}
}
Output:
I'm non-static method
A non-static method is always be called by using the object of class as shown in the above example.
Key Points:
How to call static methods: direct or using class name:
StaticDemo.copyArg(s1, s2);
or
copyArg(s1, s2);
How to call a non-static method: using object of the class:
Test obj = new Test();

Basic difference is non static members are declared with out using the keyword 'static'
All the static members (both variables and methods) are referred with the help of class name.
Hence the static members of class are also called as class reference members or class members..
In order to access the non static members of a class we should create reference variable .
reference variable store an object..

Simply put, from the point of view of the user, a static method either uses no variables at all or all of the variables it uses are local to the method or they are static fields. Defining a method as static gives a slight performance benefit.

Another scenario for Static method.
Yes, Static method is of the class not of the object. And when you don't want anyone to initialize the object of the class or you don't want more than one object, you need to use Private constructor and so the static method.
Here, we have private constructor and using static method we are creating a object.
Ex::
public class Demo {
private static Demo obj = null;
private Demo() {
}
public static Demo createObj() {
if(obj == null) {
obj = new Demo();
}
return obj;
}
}
Demo obj1 = Demo.createObj();
Here, Only 1 instance will be alive at a time.

- First we must know that the diff bet static and non static methods
is differ from static and non static variables :
- this code explain static method - non static method and what is the diff
public class MyClass {
static {
System.out.println("this is static routine ... ");
}
public static void foo(){
System.out.println("this is static method ");
}
public void blabla(){
System.out.println("this is non static method ");
}
public static void main(String[] args) {
/* ***************************************************************************
* 1- in static method you can implement the method inside its class like : *
* you don't have to make an object of this class to implement this method *
* MyClass.foo(); // this is correct *
* MyClass.blabla(); // this is not correct because any non static *
* method you must make an object from the class to access it like this : *
* MyClass m = new MyClass(); *
* m.blabla(); *
* ***************************************************************************/
// access static method without make an object
MyClass.foo();
MyClass m = new MyClass();
// access non static method via make object
m.blabla();
/*
access static method make a warning but the code run ok
because you don't have to make an object from MyClass
you can easily call it MyClass.foo();
*/
m.foo();
}
}
/* output of the code */
/*
this is static routine ...
this is static method
this is non static method
this is static method
*/
- this code explain static method - non static Variables and what is the diff
public class Myclass2 {
// you can declare static variable here :
// or you can write int callCount = 0;
// make the same thing
//static int callCount = 0; = int callCount = 0;
static int callCount = 0;
public void method() {
/*********************************************************************
Can i declare a static variable inside static member function in Java?
- no you can't
static int callCount = 0; // error
***********************************************************************/
/* static variable */
callCount++;
System.out.println("Calls in method (1) : " + callCount);
}
public void method2() {
int callCount2 = 0 ;
/* non static variable */
callCount2++;
System.out.println("Calls in method (2) : " + callCount2);
}
public static void main(String[] args) {
Myclass2 m = new Myclass2();
/* method (1) calls */
m.method();
m.method();
m.method();
/* method (2) calls */
m.method2();
m.method2();
m.method2();
}
}
// output
// Calls in method (1) : 1
// Calls in method (1) : 2
// Calls in method (1) : 3
// Calls in method (2) : 1
// Calls in method (2) : 1
// Calls in method (2) : 1

Sometimes, you want to have variables that are common to all objects. This is accomplished with the static modifier.
i.e. class human - number of heads (1) is static, same for all humans, however human - haircolor is variable for each human.
Notice that static vars can also be used to share information across all instances

Objects of multiple class in java

I was revising some of the old school concepts of Java in order to solve one problem . I have written the following code where i am trying the create objects of multiple class in that same classes and calling the methods with those objects from the main.
class a {
public void display() {
System.out.println("inside class a");
a a1= new a();
}
}
class b {
public void display() {
System.out.println("inside class b");
b b1= new b();
}
}
public class one {
void display() {
System.out.println("inside class one");
}
public static void main(String[] args) {
one o = new one();
a1.display();
b1.display();
o.display();
}
}
I am getting object cannot be resolved error. My question is what i need to change to let the above code work. And, do i need to always declare objects inside the main().
Any help will be highly appreciated

I'm not really sure why you would want to do that, but assuming you're just wondering about the possibility to implement such a thing - yes, it can be done.
You can create an instance of a class inside that same class, like so:
public class A {
public static A instance = new A();
public void display() {
System.out.println("inside class A");
}
}
Pay attention to the static modifier in the above code; it allows you now to access instance from another place (class, method, main) like so:
A.instance.display();
If you want to know whether you can declare a variable inside a method, and not a class, and make it accessible from another method, then the answer is - no, you cannot.

Yes you need to declare objects inside the main()
class a {
public void display() {
System.out.println("inside class a");
}
}
class b {
public void display() {
System.out.println("inside class b");
}
}
public class one {
void display() {
System.out.println("inside class one");
}
public static void main(String[] args) {
a a1= new a();
b b1= new b();
one o = new one();
a1.display();
b1.display();
o.display();
}
}

Don't know what you want to achieve and yes you should create object of class a and class b inside main functions to use instance methods of these classes.
package com.stack.overflow;
class a
{
public void display()
{
System.out.println("inside class a");
//a a1= new a(); ---> No need of this line as you can
// directly access instance variables and methods directly without
// creating any object or you can also use **this** keyword for the same
}
}
class b
{
public void display()
{
System.out.println("inside class b");
//b b1= new b(); ---> No need of this line as you can
// directly access instance variables and methods directly without
// creating any object or you can also use **this** keyword for the same
}
}
public class one
{
void display()
{
System.out.println("inside class one");
}
public static void main(String[] args) {
one o = new one();
a a1=new a();
b b1=new b();
a1.display();
b1.display();
o.display();
}
}

You may find the answer to your confusion easily - #ratul-sharker : a1 & b1 must be declared and instantiated inside the main. as well as other answers here correcting your code.
The real question is your notion of scoping and lifetime of variables - Not only a1 and b1 lie inside the classes a and b but they have been instantiated inside methods so they are local. So, try to understand the difference between field variables and local variables - their lifetimes and scopes are vastly different.
Accessing a local variable directly like that(which will be instantiated when the method is called) is like asking asking for a result from future in the present. Note that field variables will remain as long as object is alive but the local variables will remain only for the duration of the method call.
Hope it is clear to you now.
Also, your question:
My question was is it possible to create an object of an class in the
same class and call it from main?
Yes. Because main is a static method so it is not bound to an object like non-static method does. static methods are class level while non-static methods are object level. You can also create an instance in a non-static method for that matter.

Can not make static reference to non static method? [duplicate]

This question already has answers here:
Static Classes In Java
(14 answers)
Closed 3 years ago.
So, in short. I have two classes.
package rpg;
public class Engine {
public void main(String args[]) {
Start.gameStart();
System.out.println(menuResult);
}
}
and
package rpg;
public class Start {
int menuResult = 3;
public int gameStart()
{
return menuResult;
}
public int getMenuResult()
{
return Start.menuResult;
}
}
It keeps throwing up the error 'Cannot make static reference to non-static method gameStart()'.
I'm sure I'm missing something simple, but can't find it.
Thanks!

You need to create instance of Start class and call gameStart() method on that instance because gameStart() is instance method not static method.
public void main(String args[]) {
new Start().gameStart();
..................
}
Only static methods can be accessed by using class name as perfix.

public int gameStart() <--- Instance method not static method
call it on instance
Start start = new Start();
start.gameStart();
So finally your classes should look like below
public static void main(String args[]) {
Start start = new Start();
start.gameStart();
System.out.println(start.getMenuResult());
}
public class Start {
private int menuResult = 3;
public int gameStart() {
return this.menuResult;//Don't know why there are two methods
}
public int getMenuResult() {
return this.menuResult;
}
}

first of all the main method should be
public static void main(String args[]) {
}
I assume you can have multiple games, and hence you should be able to start multiple instances so you should create a non static class that can be created and then actions performed against.
to answer your original question, you need to have a static variable that have static getters and setters..
public class Start {
private static int menuResult = 3;
public static int gameStart()
{
return menuResult;
}
public static int getMenuResult()
{
return Start.menuResult;
}

If you need the method to be static, just add the keyword static in the function definition. That would get rid of the error. But if you want to keep the class Start the way it is, then you should create an instance of Start in the main function and then call the method. Hope that helps!

you are trying to invoke a method on its class name. you should be creating a new object and invoke its method
public void main(String args[]) {
new Start().gameStart();
System.out.println(menuResult);
}

Start.gameStart() refers to a method which would be public static int gameStart() because Start is the class and not an instance of the object.
If you declare a method on an object, you need to apply it to instance of the object and not its class.
When to use static or instanciated methods ?
instanciated : whenever you need to apply the method to the object you're in. example : mycake.cook();
static : when the actions you do inside your method have nothing to do with an object in particular. example : Cake.throwThemAll();
mycake is an instance of a Cake, declared this way : Cake mycake = new Cake();
Cake is the class representing the object.
You should, i guess, have a read at some object oriented programmation course if you still have a doubt about objects, classes and instances.

While Other answers are Correct , that remains the Question that Why you Can't access Instance
method Directly from Class name , In Java all static (methods , fields) bind with Class Name and when Class Is Loading to the Memory (Stack) all static members are Loading to the Stack , and this time Instance Method is not visible to Class. instance Method will Load into Heap portion in the memory and can only be access by Object references .

calling methods without having reference

if i want to call multiple methods of a one class from another class can i call them by using only 'new classname()' without catching it in class reference?
public class Example {
/**
* #param args the command line arguments
*/
public static void main(String[] args) {
new pqr().a=5;
new pqr().b=10;
new pqr().display();
}
}
class pqr
{
int a,b;
public void display()
{
System.out.println(a+" "+b);
}
}

This creates three new objects. Not just one.
new pqr().a=5;
new pqr().b=10;
new pqr().display();
One object with a = 5 and another with b = 10.
Remember you are not working with one object.
Whenever you use new keyword. JVM creates a new object.

if i want to call multiple methods of a one class from another class can i call them by using only 'new classname()' without catching it in class reference?
It's not clear what you mean by "catching it" but you are using a reference... you're just not assigning it to a variable.
In your example, you're creating three different objects - the calculation on your final line just prints 0, because you've only set a and b in other objects. If you want to use a single object for multiple operations, you'll either need to store the reference in a variable, or those operations will have to return "this", allowing you to chain method calls together:
class Sample {
private int a,b;
public void display() {
System.out.println(a+" "+b);
}
public Sample setA(int a) {
this.a = a;
return this;
}
public Sample setB(int b) {
this.b = b;
return this;
}
}
...
new Sample().setA(5).setB(10).display();
This sort of chaining for setters is common in the builder pattern.

You can use the Builder pattern if you want something like that:
NutritionFacts cocaCola = new NutritionFacts.Builder(240, 8).
calories(100).sodium(35).carbohydrate(27).build();

The builder pattern is one possibility. The another one is to have static properties so all object will share it's values.
static int a,b;

using methods of other classes to "overwrite" variables

i'm relativly new to java and experimantating a bit with javafx
i want to change a variable from class A while using a method from class B
Main: thats the main class, it contains all the needed stuff(shows the primaryStage etc) it does have an constructor, so its not creating an actual "main-object"
public class Main extends Application {
Sub sub = new Sub();
int a;
// stuff
public void aMethod() {
sub.subMethod();
}
}
Sub: this class solely surpose is to change the variable a, it does not contain a constructor to create a "sub-object"
public class Sub {
//stuff
subMethod(){
int a = 5;
}
if i put the line Main main; in the Sub class, the program will give me a nullpointer exception, if i'm calling the subMethod().
ok...i guess cause i didnt actually create the main object... so far so good.
BUT... if i put in the line Main main = new Main(); the program wont even start giving me an "exception while running application" error
the strange thing though is, if i put the line Main main = new Main(); in the subMethod...
subMethod(){
Main main = new Main();
int a = 5;
}
...the damn thing actually works...(well its slow, guess because with every calling of the method its creating a new object)
why is that so?
and how is it done correctly? :)
(using methods of other classes to "overwrite" variables)
regards
Red

You should not create more than one instance of Main in your program. Probably Main is not the best place to store mutable state (class members), but if you want that, you need to pass the instance of Main to subMethod (and make a public, or provide a public setter method):
public class Main extends Application {
Sub sub = new Sub();
public int a;
// stuff
public void aMethod() {
sub.subMethod(this);
}
}
public class Sub {
//stuff
subMethod(Main main){
main.a = 5;
}

So you want a method to change the value of another class's fields. There are a few ways to do this. If you have this class
public Class A {
private int a;
...
public void setA(int a) {
this.a = a;
}
}
You can do something like this
public Class B {
private static A instance;
....
public static void setA(int a) {
instance.setA(a);
}
}
Or you can take the A in as a parameter to the set method
public Class B {
...
public static void setA(A a, int val) {
a.setA(val);
}
}
If you want direct access to the fields on A you have to make them public (this is usually not what you want to do as it gives complete access rather than just giving just the access the other classes require)
Public Class A {
public int a;
...
}
Then you can do
Public Class B {
...
public static void setVal(A a, int val) {
a.a = val;
}
}
Also if you don't have the method setA in B as static you'll have to call it on an instance of B like
B b = new B();
b.setA(a, val);
Where as if it's static you call it on the class B
B.setA(a, val);