This question already has answers here:
What does the 'static' keyword do in a class?
(22 answers)
Closed 2 years ago.
I've learned that I should use Object(instance) for using instance field in static method.
For example:
INSTANCE FIELD(==speed)
that is declared in public class Car, should be used through Object in static method (ex. 'public static void main(String[] args) )
like this.
Car myCar = new Car();
myCar.speed = 60;
So, the reason I should use object is because static method is located in CLASS and for being shared to objects, but on the other hand, instance field is just Frame that is not substance?
For using this instance field in static method, do I have to make the instance that is called 'object'?
In other words, is this process right?:
instance field -> OBJECT( substantialization) -> static method.
I'm wondering what I understood
static methods are used for 3 reasons:
"stateless" methods. A good example of this would be Math.sin
Global "singletons": singleton is in quotes because the singleton pattern is not used everywhere in java. An example of where it is could be Runtime.getRuntime() an example of where it isn't might be Thread.getUncaughtExceptionHandler (implicit singleton)
Program entry point (public static void main) : It makes sense for a program to start outside of an object context (rather than innside)
Related
This question already has answers here:
Difference between Static methods and Instance methods
(10 answers)
Closed 4 years ago.
Whenever I have to call a method from another class, I first create an object and then call it through the object. But while I was writing some code, I mistakenly wrote classname.methodname(); and it worked.
I would usually write,
classname obj = new classname();
obj.methodname();
Here is the actual code:
Class 1
public class Dataset {
public static List<ECCardData> getDataset() {
//Code
}
in Class 2
List<ECCardData> dataset = Dataset.getDataset();
I noticed that the methodname() was static. Was that the reason?
Yes, for static methods (with suitable access modifier) you can call directly with your class by
YourClass.yourMethod();
and this way, too
YourClass anObject = new YourClass();
anObject.yourMethod();
Happy coding.
I hate answering my question, but I found the correct answer.
When a method is declare static, only one instance of that method will exist. When you create an object a new instance of the method is created, which is not possible for a static method. Therefore you use the class name.
classname.methodname(); //only one instance
classname obj;
obj.methodname(); //instance with obj as Object(IDE gives warning, should not be allowed, ideally)
The basic paradigm in Java is that you write classes, and that those
classes are instantiated. Instantiated objects (an instance of a
class) have attributes associated with them (member variables) that
affect their behavior; when the instance has its method executed it
will refer to these variables.
However, all objects of a particular type might have behavior that is
not dependent at all on member variables; these methods are best made
static. By being static, no instance of the class is required to run
the method.
You can do this to execute a static method:
classname.staticMethod();//Simply refers to the class's static code But
> to execute a non-static method, you must do this:
>
> classname obj = new classname();//Create an instance
> obj.nonstaticMethod();//Refer to the instance's class's code
This question already has answers here:
Static method in a generic class?
(12 answers)
Closed 5 years ago.
Thank you all for reading, i'm trying to understand Generics, and i got this excersice where i create a singleton with a generic parameter.
public class Singleton<T> {
public static T getInstance() {
if (instance == null)
instance = new Singleton<T>();
return instance;
}
private static T instance = null;
}
But i got this error: Cannot make a static reference to the non-static type T
What can i use as a workaround? Or better yet, what causes the error?
Look at newaccts answer to this post:
You can't use a class's generic type parameters in static methods or static fields. The class's type parameters are only in scope for instance methods and instance fields. For static fields and static methods, they are shared among all instances of the class, even instances of different type parameters, so obviously they cannot depend on a particular type parameter.
This question already has answers here:
Non-static variable cannot be referenced from a static context
(15 answers)
Closed 7 years ago.
New to Java, trying to figure out how to resolve this issue.
boolean myBool = G(A,n,m,0);
For some reason it isn't like this line. Why won't it let me call this simple function? Both main() and G() are part of class C().
A non static method belongs to a specific instance of a class, while a static method belongs to the class itself. Inside main, which is a static method, you cannot reference non-static methods without having a specific object to run them. E.g.:
boolean myBool = new C().G(A,n,m,0);
However, if the class has no interesting state, or it's state does not effect the method G, you should define G as static.
It's likely because you didn't include static in the definition of the G() method.
Main() is a static method, and since static things run before non static things do, static things can only call/use static things.
Note that your Main() doesn't require you to make a C object. It's the entry point to the program, and it doesn't make sense if you have to first make an object in order to run your program - where would you make that object from?
If you want to make non-static calls, create objects of the corresponding class.
This question already has answers here:
How come invoking a (static) method on a null reference doesn't throw NullPointerException?
(5 answers)
Closed 8 years ago.
Recently I was going through a page on javarevisited and found a block of code which asked the readers to determine what would be the output for it...
Though I got the output I am not satisfied with the result (WHICH CAME OUT TO BE "Hello") since I don't know how a static member is accessed from a null reference. What's happening in the background?
public class StaticDEMO {
private static String GREET = "Hello";
public static void main(String[] args) {
StaticDEMO demo = null;
System.out.println(demo.GREET);
// TODO code application logic here
}
}
This works because the JVM knows you are trying to access a static member on a specific class. Because you had to declare demo as a specific class (in this case a StaticDEMO), it knows to use that to find GREET.
To be clear, you don't run into this often (I actually had to type this code in to verify it, I can't say I've ever seen this). Mainly, it's good practice to always refer to static fields by their class, not an object instance (which may be null, as we can see!).
Meaning, prefer this:
System.out.println(StaticDEMO.GREET);
EDIT
I found a reference to this in the Java Specification: Chapter 15, Section 11: Field Access Expressions.
Example 15.11.1-2. Receiver Variable Is Irrelevant For static Field Access
The following program demonstrates that a null reference may be used to access a class (static) variable without causing an exception
[example not shown here for brevity]
Any method which is being decorated as static in Java means, that method is a class level memeber. That means, you do not need an object to accss static members. The static method/variable is maintained by the Class itself, and not by any instance of the class. Here in your example, the compiler already knows that the member is static member and it doesn't need any instance to access that static method.
Static members are stored with the Class, not with any specific instance of it. So, it doesn't matter that the instance is null - the member from the Class is still be accessible.
The JVM simply ignores null, because GREET is a class field and demo is irrelevant reference to refer Class field.
Static method not needed object reference to call it so you can call it even reference to the object is null.
This question already has answers here:
What is the actual memory place for static variables?
(7 answers)
Closed 9 years ago.
Why static variables are directly called without any use of object in java?Are they stored in some different memory location?And why only static methods can called with the name of the class directly without creating its object ?for example
class First
{
public static void ss()
{
System.out.println("This genius will give answer");
}
}
class Second
{
public static void main(String ar[])
{
First.ss();
}
}
Yes static resources belong to the class and not the objects. And are stored in a separate location kind of global location. You can read more here.
As docs says
Every instance of the class shares a class variable, which is in one fixed location in memory.
The Java programming language supports static methods as well as static variables. Static methods, which have the static modifier in their declarations, should be invoked with the class name, without the need for creating an instance of the class, as in
ClassName.methodName(args)