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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.
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.
I have a class A. I define another class B within a method (even main) of the class A and class B can access all the variables within the scope of the method it is defined in. What is the terminology for such classes (as B)? Some people have been saying Nested classes or Inner classes but IIRC, those are the classes where they have another class as their data members (kinda like composition in C++).
The second part of my question is that some people have been saying that when you have a class defined within a method of another class, then the variables of the first class (A) that are accessed by the later class (B) need to be declared final. Is this to be followed strictly and why so?
According to the Java Language Specification these are "local classes":
A local class is a nested class (§8 (Classes)) that is not a member of
any class and that has a name (§6.2, §6.7).
or "anonymous [inner] classes", which are just the ones that don't have a name (e.g. Interface x = new Interface() { ...).
These are special cases of inner classes which is generally what I've heard people refer to them as.
As for your second question, "Any local variable, formal parameter, or exception parameter used but not declared in an inner class must either be declared final or be effectively final (§4.12.4), or a compile-time error occurs where the use is attempted." So it is a compile-time error if you try to access non-final local variables. Obviously this part must be followed strictly if you want your class to compile.
I have not personally heard the advice that you should not access non-final fields of the enclosing class, and I'm pretty sure it's allowed. While arguments could be made that fields should be final unless they can't be, I don't see any stylistic reason this should be more important in inner classes. There is a technical difference that accessing a field from an inner class may cause the compiler to create and call synthetic getters and setters, but this is generally a minor performance concern.
My question is about handling and life cycle of the anonymous object in JVM 1.8.
As far as I read, in JDK 1.8, the underlying mechanism of lambda expression isn't purely function based. I.e. it still create an anonymous object with the method we defined in the code and call that method upon the anonymous object. Also, because the lambda expression doesn't introduce any new variable scope, calling "this" in the lambda expression would refer to the original object instead of such anonymous method.
Naturally the question follows: how does JVM handle the life cycle of such anonymous object? Define the object method containing such lambda expression as "outer object method", At least I have the following questions.
If the outer object method is a normal method, does this anonymous object belong to instance level or class level? What if the outer method is static?
If the outer object method is called multiple times, would this anonymous object be re-used or re-created?
Is such object subject to JVM GC? If yes is the GC rule remaining the same as the other objects?
Is there any tool or API to track the life cycle of such anonymous object, given it cannot be referred in the code directly?
Any help or comment or documentation is appreciated.
I don't know what you mean by "belong". An object doesn't "belong" to any level.
If the lambda is a closure, i.e. if it captures one or more local variables from the surrounding scope (including this (which is kinda like an implicit final local variable), OuterClass.this (which is implicitly accessed through a hidden field of this), or unqualified instance variables (which are implicitly accessed through this or OuterClass.this)), then in different times the function containing the lambda expression is evaluated, it will likely have to create different objects, because the values of the captured variables are stored as part of the lambda object, and since in different runs of the function (or even different times within one run of the function), the captured variables can have different values, different lambda objects must be created so that each lambda remembers its separate set of captured values.
However, if the lambda is not a closure, then any two lambda objects created from that lambda expression are semantically undistinguishable. So then one object can be re-used for all evaluations of that lambda expression. I believe in this case the virtual machine will statically allocate one object for that lambda which lives for the duration of the program.
Yes. If an object is created when the lambda expression is run, then it is dynamically allocated like other objects in Java, and it is subject to GC. However, if one object is created for the whole life of the program (see discussion for (2) above), then it would not be memory-managed, similar to string literals.