This question already has answers here:
Why keyword 'this' cannot be used in a static method?
(13 answers)
Closed 9 years ago.
I knew that we can not use this keyword in a static method, But I got confused why we can not use this inside static blocks orstatic methods. Even the same case with super(). Could anyone shed light on this?
Thanks
this and super refer to the current instance and the parent instance respectively.
Within any static context, whether it is a static block or a static method, there is no instance to refer to, and therefore the keywords are not permitted.
super() is a call to the parent instance's no-arg constructor, and is only permitted as the first statement in a constructor, which disqualifies it from appearing in any static context.
According to the doc
Within an instance method or a constructor, this is a reference to the
current object — the object whose method or constructor is being
called. You can refer to any member of the current object from within
an instance method or a constructor by using this.
But the static methods are related to the Class not to the object. in static methods you dont have any instance.
Static methods and blocks are, by definition, linked to the class and not any instance of this class.
As this refers to the current object instance, it is perfectly normal that you cannot use it in any static initialization block or method.
Because this points to an instance of the class, in the static method/block you don't have an instance.
according to Oracle
Within an instance method or a constructor, this is a reference to the
current object — the object whose method or constructor is being
called.
So, in a static class you have NO instance variables-objects created. That's why you cannot use the this keyword
Static blocks can be used to initialize static variables for example.
Static merthods are not working within an instance scope either.
Both are related to only the class and have nothing to do with an instance of that class in any way.
To answer this we should consider what static means - it means that this bit is put in a 'static' place in memory, it exists only once.
So every class has static bits and non-static bits. The static bits exist once and the non-static bits can exist many times (i.e. lots of different instances). The word 'this' can then be thought of as shorthand for 'this version of the non-static bits'.
In a static context we have no version of the non-static bits to refer to (or to be more precise we don't know which version we should refer to!) so we have no this. The same argument can be made for super.
Related
This question already has answers here:
Non-static variable cannot be referenced from a static context
(15 answers)
Closed 8 years ago.
The community reviewed whether to reopen this question last year and left it closed:
Original close reason(s) were not resolved
The very common beginner mistake is when you try to use a class property "statically" without making an instance of that class. It leaves you with the mentioned error message:
You can either make the non static method static or make an instance of that class to use its properties.
What the reason behind this? Am not concern with the solution, rather the reason.
private java.util.List<String> someMethod(){
/* Some Code */
return someList;
}
public static void main(String[] strArgs){
// The following statement causes the error.
java.util.List<String> someList = someMethod();
}
You can't call something that doesn't exist. Since you haven't created an object, the non-static method doesn't exist yet. A static method (by definition) always exists.
The method you are trying to call is an instance-level method; you do not have an instance.
static methods belong to the class, non-static methods belong to instances of the class.
The essence of object oriented programming is encapsulating logic together with the data it operates on.
Instance methods are the logic, instance fields are the data. Together, they form an object.
public class Foo
{
private String foo;
public Foo(String foo){ this.foo = foo; }
public getFoo(){ return this.foo; }
public static void main(String[] args){
System.out.println( getFoo() );
}
}
What could possibly be the result of running the above program?
Without an object, there is no instance data, and while the instance methods exist as part of the class definition, they need an object instance to provide data for them.
In theory, an instance method that does not access any instance data could work in a static context, but then there isn't really any reason for it to be an instance method. It's a language design decision to allow it anyway rather than making up an extra rule to forbid it.
I just realized, I think people shouldn't be exposed to the concept of "static" very early.
Static methods should probably be the exception rather than the norm. Especially early on anyways if you want to learn OOP. (Why start with an exception to the rule?) That's very counter-pedagogical of Java, that the "first" thing you should learn is the public static void main thing. (Few real Java applications have their own main methods anyways.)
I think it is worth pointing out that by the rules of the Java language the Java compiler inserts the equivalent of "this." when it notices that you're accessing instance methods or instance fields without an explicit instance. Of course, the compiler knows that it can only do this from within an instance method, which has a "this" variable, as static methods don't.
Which means that when you're in an instance method the following are equivalent:
instanceMethod();
this.instanceMethod();
and these are also equivalent:
... = instanceField;
... = this.instanceField;
The compiler is effectively inserting the "this." when you don't supply a specific instance.
This (pun intended) bit of "magic help" by the compiler can confuse novices: it means that instance calls and static calls sometimes appear to have the same syntax while in reality are calls of different types and underlying mechanisms.
The instance method call is sometimes referred to as a method invocation or dispatch because of the behaviors of virtual methods supporting polymorphism; dispatching behavior happens regardless of whether you wrote an explicit object instance to use or the compiler inserted a "this.".
The static method call mechanism is simpler, like a function call in a non-OOP language.
Personally, I think the error message is misleading, it could read "non-static method cannot be referenced from a static context without specifying an explicit object instance".
What the compiler is complaining about is that it cannot simply insert the standard "this." as it does within instance methods, because this code is within a static method; however, maybe the author merely forgot to supply the instance of interest for this invocation — say, an instance possibly supplied to the static method as parameter, or created within this static method.
In short, you most certainly can call instance methods from within a static method, you just need to have and specify an explicit instance object for the invocation.
The answers so far describe why, but here is a something else you might want to consider:
You can can call a method from an instantiable class by appending a method call to its constructor,
Object instance = new Constuctor().methodCall();
or
primitive name = new Constuctor().methodCall();
This is useful it you only wish to use a method of an instantiable class once within a single scope. If you are calling multiple methods from an instantiable class within a single scope, definitely create a referable instance.
If we try to access an instance method from a static context , the compiler has no way to guess which instance method ( variable for which object ), you are referring to. Though, you can always access it using an object reference.
A static method relates an action to a type of object, whereas the non static method relates an action to an instance of that type of object. Typically it is a method that does something with relation to the instance.
Ex:
class Car might have a wash method, which would indicate washing a particular car, whereas a static method would apply to the type car.
if a method is not static, that "tells" the compiler that the method requires access to instance-level data in the class, (like a non-static field). This data would not be available unless an instance of the class has been created. So the compiler throws an error if you try to call the method from a static method.. If in fact the method does NOT reference any non-static member of the class, make the method static.
In Resharper, for example, just creating a non-static method that does NOT reference any static member of the class generates a warning message "This method can be made static"
The compiler actually adds an argument to non-static methods. It adds a this pointer/reference. This is also the reason why a static method can not use this, because there is no object.
So you are asking for a very core reason?
Well, since you are developing in Java, the compiler generates an object code that the Java Virtual Machine can interpret. The JVM anyway is a binary program that run in machine language (probably the JVM’s version specific for your operating system and hardware was previously compiled by another programming language like C in order to get a machine code that can run in your processor). At the end, any code is translated to machine code. So, create an object (an instance of a class) is equivalent to reserve a memory space (memory registers that will be processor registers when the CPU scheduler of the operating system put your program at the top of the queue in order to execute it) to have a data storage place that can be able to read and write data. If you don’t have an instance of a class (which happens on a static context), then you don’t have that memory space to read or write the data. In fact, like other people had said, the data don’t exist (because from the begin you never had written neither had reserved the memory space to store it).
Sorry for my english! I'm latin!
The simple reason behind this is that Static data members of parent class
can be accessed (only if they are not overridden) but for instance(non-static)
data members or methods we need their reference and so they can only be
called through an object.
A non-static method is dependent on the object. It is recognized by the program once the object is created.
Static methods can be called even before the creation of an object. Static methods are great for doing comparisons or operations that aren't dependent on the actual objects you plan to work with.
This question already has answers here:
Non-static variable cannot be referenced from a static context
(15 answers)
Closed 8 years ago.
The community reviewed whether to reopen this question last year and left it closed:
Original close reason(s) were not resolved
The very common beginner mistake is when you try to use a class property "statically" without making an instance of that class. It leaves you with the mentioned error message:
You can either make the non static method static or make an instance of that class to use its properties.
What the reason behind this? Am not concern with the solution, rather the reason.
private java.util.List<String> someMethod(){
/* Some Code */
return someList;
}
public static void main(String[] strArgs){
// The following statement causes the error.
java.util.List<String> someList = someMethod();
}
You can't call something that doesn't exist. Since you haven't created an object, the non-static method doesn't exist yet. A static method (by definition) always exists.
The method you are trying to call is an instance-level method; you do not have an instance.
static methods belong to the class, non-static methods belong to instances of the class.
The essence of object oriented programming is encapsulating logic together with the data it operates on.
Instance methods are the logic, instance fields are the data. Together, they form an object.
public class Foo
{
private String foo;
public Foo(String foo){ this.foo = foo; }
public getFoo(){ return this.foo; }
public static void main(String[] args){
System.out.println( getFoo() );
}
}
What could possibly be the result of running the above program?
Without an object, there is no instance data, and while the instance methods exist as part of the class definition, they need an object instance to provide data for them.
In theory, an instance method that does not access any instance data could work in a static context, but then there isn't really any reason for it to be an instance method. It's a language design decision to allow it anyway rather than making up an extra rule to forbid it.
I just realized, I think people shouldn't be exposed to the concept of "static" very early.
Static methods should probably be the exception rather than the norm. Especially early on anyways if you want to learn OOP. (Why start with an exception to the rule?) That's very counter-pedagogical of Java, that the "first" thing you should learn is the public static void main thing. (Few real Java applications have their own main methods anyways.)
I think it is worth pointing out that by the rules of the Java language the Java compiler inserts the equivalent of "this." when it notices that you're accessing instance methods or instance fields without an explicit instance. Of course, the compiler knows that it can only do this from within an instance method, which has a "this" variable, as static methods don't.
Which means that when you're in an instance method the following are equivalent:
instanceMethod();
this.instanceMethod();
and these are also equivalent:
... = instanceField;
... = this.instanceField;
The compiler is effectively inserting the "this." when you don't supply a specific instance.
This (pun intended) bit of "magic help" by the compiler can confuse novices: it means that instance calls and static calls sometimes appear to have the same syntax while in reality are calls of different types and underlying mechanisms.
The instance method call is sometimes referred to as a method invocation or dispatch because of the behaviors of virtual methods supporting polymorphism; dispatching behavior happens regardless of whether you wrote an explicit object instance to use or the compiler inserted a "this.".
The static method call mechanism is simpler, like a function call in a non-OOP language.
Personally, I think the error message is misleading, it could read "non-static method cannot be referenced from a static context without specifying an explicit object instance".
What the compiler is complaining about is that it cannot simply insert the standard "this." as it does within instance methods, because this code is within a static method; however, maybe the author merely forgot to supply the instance of interest for this invocation — say, an instance possibly supplied to the static method as parameter, or created within this static method.
In short, you most certainly can call instance methods from within a static method, you just need to have and specify an explicit instance object for the invocation.
The answers so far describe why, but here is a something else you might want to consider:
You can can call a method from an instantiable class by appending a method call to its constructor,
Object instance = new Constuctor().methodCall();
or
primitive name = new Constuctor().methodCall();
This is useful it you only wish to use a method of an instantiable class once within a single scope. If you are calling multiple methods from an instantiable class within a single scope, definitely create a referable instance.
If we try to access an instance method from a static context , the compiler has no way to guess which instance method ( variable for which object ), you are referring to. Though, you can always access it using an object reference.
A static method relates an action to a type of object, whereas the non static method relates an action to an instance of that type of object. Typically it is a method that does something with relation to the instance.
Ex:
class Car might have a wash method, which would indicate washing a particular car, whereas a static method would apply to the type car.
if a method is not static, that "tells" the compiler that the method requires access to instance-level data in the class, (like a non-static field). This data would not be available unless an instance of the class has been created. So the compiler throws an error if you try to call the method from a static method.. If in fact the method does NOT reference any non-static member of the class, make the method static.
In Resharper, for example, just creating a non-static method that does NOT reference any static member of the class generates a warning message "This method can be made static"
The compiler actually adds an argument to non-static methods. It adds a this pointer/reference. This is also the reason why a static method can not use this, because there is no object.
So you are asking for a very core reason?
Well, since you are developing in Java, the compiler generates an object code that the Java Virtual Machine can interpret. The JVM anyway is a binary program that run in machine language (probably the JVM’s version specific for your operating system and hardware was previously compiled by another programming language like C in order to get a machine code that can run in your processor). At the end, any code is translated to machine code. So, create an object (an instance of a class) is equivalent to reserve a memory space (memory registers that will be processor registers when the CPU scheduler of the operating system put your program at the top of the queue in order to execute it) to have a data storage place that can be able to read and write data. If you don’t have an instance of a class (which happens on a static context), then you don’t have that memory space to read or write the data. In fact, like other people had said, the data don’t exist (because from the begin you never had written neither had reserved the memory space to store it).
Sorry for my english! I'm latin!
The simple reason behind this is that Static data members of parent class
can be accessed (only if they are not overridden) but for instance(non-static)
data members or methods we need their reference and so they can only be
called through an object.
A non-static method is dependent on the object. It is recognized by the program once the object is created.
Static methods can be called even before the creation of an object. Static methods are great for doing comparisons or operations that aren't dependent on the actual objects you plan to work with.
Why is it that even if you have declared a private static final variable, a color - defaultC, let's say - then you still cannot use this.defaultC in the super constructor (i.e. you can only use super(defaultC) and not super(this.defaultC) even though it is equivalent to simply defaultC.
I was simply trying to extend a Tile class to a Wall class and the Wall class I stored all of the necessary private static final variables for all walls (such as their width and height, another int and the default color), and the Tile class already has some of the variables (but they are protected not private). I didn't want the warning Field hides another field to pop up (because there Tile.java and Wall.java have the same names for many variables), so I used this. for my private static final variables, and there were errors galore.
It's not particularly troubling (as I simply have a few warnings), but I was simply wondering why. I am guessing that the compiler simply doesn't like it because you cannot reference this before the supertype, but there are still exactly the same. Has such a feature not been added yet that overlooks such a thing or is there another reason I cannot see that you cannot use super(this.PRIVATE_STATIC_FINAL_VARIABLE);?
A static variable does not belong to any instance of an object. The this keyword should refer to an instance of an object and variables used within. While you can use the this keyword to access static variables, you should not.
Also, a variable marked private in a super class cannot be accessed within a child class.
To reference a variable declared in a super class from a child class using the this keyword, that variable must be declared protected or public.
You could try to tie the static final variable to the class you're in when passing it to super. I believe you can do this by doing something like super(SubClass.PRIVATE_STATIC_FINAL_VARIABLE);
Here we have two things to know,
what is the meaning of static keyword.
what is the meaning of this keyword
what is a constructor
in java, there are two types, class and instance of an class. We can use a class without creating an instance of it, or after creating a instance of the class itself.
object is an instance of a class.
The keyword static is used with the class. this is used with instances of class. And this keyword refers the instance of the class it is used within.
An instance of an class(object) is created after calling the class's constructor. We can use keyword this only after creating an instance of class (inside the object).
As the constructor method is called before creating the instance of the class, We can't use this keyword inside a constructor. That's why you got an error in super(this.defaultC) but not in super(defaultC)
And you can't use this keyword inside constructor for PRIVATE_STATIC_FINAL_VARIABLE as well.
But you are allowed to use PRIVATE_STATIC_FINAL_VARIABLE without using this
i.e. super(PRIVATE_STATIC_FINAL_VARIABLE)
This question already has answers here:
What is the reason behind "non-static method cannot be referenced from a static context"? [duplicate]
(13 answers)
Closed 8 years ago.
I have learned that fields are like global variables which can be accessed by the methods inside the same class. I have done it this way before and never had a problem. I have now a class where I have some fields but the methods cannot access them without having to make them static fields. I get the error "cannot make static reference to non-static..."
I thought static was to access fields on other classes without having to create an object reference to the class. The only difference I have with this code is that I have a single class and my main() method within this class. Does having main() inside this class make a difference?
A static member only exists once for the class itself as opposed to regular class members which are distinct per instance of your class.
Having a main() method does not impact the behavior of your static members, however static methods can only access static members while non-static methods can access both static and non-static class members.
You can't access non static instance inside static method. I think you are trying to access class variable inside main method directly, i.e.
class A
{
int x;
main() method
{
x;//Not accessible here,, create instance of class and access it.like
A a=new A();
a.x;
}
}
Static (methods,variables,classes, etc) belongs to Class not to the particular instance of the class. We define as static when the behaviour or state does not depend on any particular instance of the class. For Example "generate a Random Number " it does not depend on the instance, it always generate a number regardless of instance such behaviour can be defined as static.
Regarding the error, posting you code will helpful to give better solution.
Refer the below link to know more about Static and non-static
http://javarevisited.blogspot.in/2012/02/why-non-static-variable-cannot-be.html
This question already has answers here:
Non-static variable cannot be referenced from a static context
(15 answers)
Closed 8 years ago.
The community reviewed whether to reopen this question last year and left it closed:
Original close reason(s) were not resolved
The very common beginner mistake is when you try to use a class property "statically" without making an instance of that class. It leaves you with the mentioned error message:
You can either make the non static method static or make an instance of that class to use its properties.
What the reason behind this? Am not concern with the solution, rather the reason.
private java.util.List<String> someMethod(){
/* Some Code */
return someList;
}
public static void main(String[] strArgs){
// The following statement causes the error.
java.util.List<String> someList = someMethod();
}
You can't call something that doesn't exist. Since you haven't created an object, the non-static method doesn't exist yet. A static method (by definition) always exists.
The method you are trying to call is an instance-level method; you do not have an instance.
static methods belong to the class, non-static methods belong to instances of the class.
The essence of object oriented programming is encapsulating logic together with the data it operates on.
Instance methods are the logic, instance fields are the data. Together, they form an object.
public class Foo
{
private String foo;
public Foo(String foo){ this.foo = foo; }
public getFoo(){ return this.foo; }
public static void main(String[] args){
System.out.println( getFoo() );
}
}
What could possibly be the result of running the above program?
Without an object, there is no instance data, and while the instance methods exist as part of the class definition, they need an object instance to provide data for them.
In theory, an instance method that does not access any instance data could work in a static context, but then there isn't really any reason for it to be an instance method. It's a language design decision to allow it anyway rather than making up an extra rule to forbid it.
I just realized, I think people shouldn't be exposed to the concept of "static" very early.
Static methods should probably be the exception rather than the norm. Especially early on anyways if you want to learn OOP. (Why start with an exception to the rule?) That's very counter-pedagogical of Java, that the "first" thing you should learn is the public static void main thing. (Few real Java applications have their own main methods anyways.)
I think it is worth pointing out that by the rules of the Java language the Java compiler inserts the equivalent of "this." when it notices that you're accessing instance methods or instance fields without an explicit instance. Of course, the compiler knows that it can only do this from within an instance method, which has a "this" variable, as static methods don't.
Which means that when you're in an instance method the following are equivalent:
instanceMethod();
this.instanceMethod();
and these are also equivalent:
... = instanceField;
... = this.instanceField;
The compiler is effectively inserting the "this." when you don't supply a specific instance.
This (pun intended) bit of "magic help" by the compiler can confuse novices: it means that instance calls and static calls sometimes appear to have the same syntax while in reality are calls of different types and underlying mechanisms.
The instance method call is sometimes referred to as a method invocation or dispatch because of the behaviors of virtual methods supporting polymorphism; dispatching behavior happens regardless of whether you wrote an explicit object instance to use or the compiler inserted a "this.".
The static method call mechanism is simpler, like a function call in a non-OOP language.
Personally, I think the error message is misleading, it could read "non-static method cannot be referenced from a static context without specifying an explicit object instance".
What the compiler is complaining about is that it cannot simply insert the standard "this." as it does within instance methods, because this code is within a static method; however, maybe the author merely forgot to supply the instance of interest for this invocation — say, an instance possibly supplied to the static method as parameter, or created within this static method.
In short, you most certainly can call instance methods from within a static method, you just need to have and specify an explicit instance object for the invocation.
The answers so far describe why, but here is a something else you might want to consider:
You can can call a method from an instantiable class by appending a method call to its constructor,
Object instance = new Constuctor().methodCall();
or
primitive name = new Constuctor().methodCall();
This is useful it you only wish to use a method of an instantiable class once within a single scope. If you are calling multiple methods from an instantiable class within a single scope, definitely create a referable instance.
If we try to access an instance method from a static context , the compiler has no way to guess which instance method ( variable for which object ), you are referring to. Though, you can always access it using an object reference.
A static method relates an action to a type of object, whereas the non static method relates an action to an instance of that type of object. Typically it is a method that does something with relation to the instance.
Ex:
class Car might have a wash method, which would indicate washing a particular car, whereas a static method would apply to the type car.
if a method is not static, that "tells" the compiler that the method requires access to instance-level data in the class, (like a non-static field). This data would not be available unless an instance of the class has been created. So the compiler throws an error if you try to call the method from a static method.. If in fact the method does NOT reference any non-static member of the class, make the method static.
In Resharper, for example, just creating a non-static method that does NOT reference any static member of the class generates a warning message "This method can be made static"
The compiler actually adds an argument to non-static methods. It adds a this pointer/reference. This is also the reason why a static method can not use this, because there is no object.
So you are asking for a very core reason?
Well, since you are developing in Java, the compiler generates an object code that the Java Virtual Machine can interpret. The JVM anyway is a binary program that run in machine language (probably the JVM’s version specific for your operating system and hardware was previously compiled by another programming language like C in order to get a machine code that can run in your processor). At the end, any code is translated to machine code. So, create an object (an instance of a class) is equivalent to reserve a memory space (memory registers that will be processor registers when the CPU scheduler of the operating system put your program at the top of the queue in order to execute it) to have a data storage place that can be able to read and write data. If you don’t have an instance of a class (which happens on a static context), then you don’t have that memory space to read or write the data. In fact, like other people had said, the data don’t exist (because from the begin you never had written neither had reserved the memory space to store it).
Sorry for my english! I'm latin!
The simple reason behind this is that Static data members of parent class
can be accessed (only if they are not overridden) but for instance(non-static)
data members or methods we need their reference and so they can only be
called through an object.
A non-static method is dependent on the object. It is recognized by the program once the object is created.
Static methods can be called even before the creation of an object. Static methods are great for doing comparisons or operations that aren't dependent on the actual objects you plan to work with.