Related
I am wondering when to use static methods? Say if I have a class with a few getters and setters, a method or two, and I want those methods only to be invokable on an instance object of the class. Does this mean I should use a static method?
Example:
Obj x = new Obj();
x.someMethod();
...or:
Obj.someMethod(); // Is this the static way?
I'm rather confused!
One rule-of-thumb: ask yourself "Does it make sense to call this method, even if no object has been constructed yet?" If so, it should definitely be static.
So in a class Car you might have a method:
double convertMpgToKpl(double mpg)
...which would be static, because one might want to know what 35mpg converts to, even if nobody has ever built a Car. But this method (which sets the efficiency of one particular Car):
void setMileage(double mpg)
...can't be static since it's inconceivable to call the method before any Car has been constructed.
(By the way, the converse isn't always true: you might sometimes have a method which involves two Car objects, and still want it to be static. E.g.:
Car theMoreEfficientOf(Car c1, Car c2)
Although this could be converted to a non-static version, some would argue that since there isn't a "privileged" choice of which Car is more important, you shouldn't force a caller to choose one Car as the object you'll invoke the method on. This situation accounts for a fairly small fraction of all static methods, though.
Define static methods in the following scenarios only:
If you are writing utility classes and they are not supposed to be changed.
If the method is not using any instance variable.
If any operation is not dependent on instance creation.
If there is some code that can easily be shared by all the instance methods, extract that code into a static method.
If you are sure that the definition of the method will never be changed or overridden. As static methods can not be overridden.
There are some valid reasons to use static methods:
Performance: if you want some code to be run, and don't want to instantiate an extra object to do so, shove it into a static method. The JVM also can optimize static methods a lot (I think I've once read James Gosling declaring that you don't need custom instructions in the JVM, since static methods will be just as fast, but couldn't find the source - thus it could be completely false). Yes, it is micro-optimization, and probably unneeded. And we programmers never do unneeded things just because they are cool, right?
Practicality: instead of calling new Util().method(arg), call Util.method(arg), or method(arg) with static imports. Easier, shorter.
Adding methods: you really wanted the class String to have a removeSpecialChars() instance method, but it's not there (and it shouldn't, since your project's special characters may be different from the other project's), and you can't add it (since Java is somewhat sane), so you create an utility class, and call removeSpecialChars(s) instead of s.removeSpecialChars(). Sweet.
Purity: taking some precautions, your static method will be a pure function, that is, the only thing it depends on is its parameters. Data in, data out. This is easier to read and debug, since you don't have inheritance quirks to worry about. You can do it with instance methods too, but the compiler will help you a little more with static methods (by not allowing references to instance attributes, overriding methods, etc.).
You'll also have to create a static method if you want to make a singleton, but... don't. I mean, think twice.
Now, more importantly, why you wouldn't want to create a static method? Basically, polymorphism goes out of the window. You'll not be able to override the method, nor declare it in an interface (pre-Java 8). It takes a lot of flexibility out from your design. Also, if you need state, you'll end up with lots of concurrency bugs and/or bottlenecks if you are not careful.
After reading Misko's articles I believe that static methods are bad from a testing point of view. You should have factories instead(maybe using a dependency injection tool like Guice).
how do I ensure that I only have one of something
only have one of something
The problem of “how do I ensure that I
only have one of something” is nicely
sidestepped. You instantiate only a
single ApplicationFactory in your
main, and as a result, you only
instantiate a single instance of all
of your singletons.
The basic issue with static methods is they are procedural code
The basic issue with static methods is
they are procedural code. I have no
idea how to unit-test procedural code.
Unit-testing assumes that I can
instantiate a piece of my application
in isolation. During the instantiation
I wire the dependencies with
mocks/friendlies which replace the
real dependencies. With procedural
programing there is nothing to "wire"
since there are no objects, the code
and data are separate.
A static method is one type of method which doesn't need any object to be initialized for it to be called. Have you noticed static is used in the main function in Java? Program execution begins from there without an object being created.
Consider the following example:
class Languages
{
public static void main(String[] args)
{
display();
}
static void display()
{
System.out.println("Java is my favorite programming language.");
}
}
Static methods in java belong to the class (not an instance of it). They use no instance variables and will usually take input from the parameters, perform actions on it, then return some result. Instances methods are associated with objects and, as the name implies, can use instance variables.
No, static methods aren't associated with an instance; they belong to the class. Static methods are your second example; instance methods are the first.
If you apply static keyword with any method, it is known as static method.
A static method belongs to the class rather than object of a class.
A static method invoked without the need for creating an instance of a class.
static method can access static data member and can change the value of it.
A static method can be accessed just using the name of a class dot static name . . . example : Student9.change();
If you want to use non-static fields of a class, you must use a non-static method.
//Program of changing the common property of all objects(static field).
class Student9{
int rollno;
String name;
static String college = "ITS";
static void change(){
college = "BBDIT";
}
Student9(int r, String n){
rollno = r;
name = n;
}
void display (){System.out.println(rollno+" "+name+" "+college);}
public static void main(String args[]){
Student9.change();
Student9 s1 = new Student9 (111,"Indian");
Student9 s2 = new Student9 (222,"American");
Student9 s3 = new Student9 (333,"China");
s1.display();
s2.display();
s3.display();
} }
O/P: 111 Indian BBDIT
222 American BBDIT
333 China BBDIT
Static methods are not associated with an instance, so they can not access any non-static fields in the class.
You would use a static method if the method does not use any fields (or only static fields) of a class.
If any non-static fields of a class are used you must use a non-static method.
Static methods should be called on the Class, Instance methods should be called on the Instances of the Class. But what does that mean in reality? Here is a useful example:
A car class might have an instance method called Accelerate(). You can only Accelerate a car, if the car actually exists (has been constructed) and therefore this would be an instance method.
A car class might also have a count method called GetCarCount(). This would return the total number of cars created (or constructed). If no cars have been constructed, this method would return 0, but it should still be able to be called, and therefore it would have to be a static method.
Use a static method when you want to be able to access the method without an instance of the class.
Actually, we use static properties and methods in a class, when we want to use some part of our program should exists there until our program is running. And we know that, to manipulate static properties, we need static methods as they are not a part of instance variable. And without static methods, to manipulate static properties is time consuming.
Static:
Obj.someMethod
Use static when you want to provide class level access to a method, i.e. where the method should be callable without an instance of the class.
Static methods don't need to be invoked on the object and that is when you use it. Example: your Main() is a static and you don't create an object to call it.
Static methods and variables are controlled version of 'Global' functions and variables in Java. In which methods can be accessed as classname.methodName() or classInstanceName.methodName(), i.e. static methods and variables can be accessed using class name as well as instances of the class.
Class can't be declared as static(because it makes no sense. if a class is declared public, it can be accessed from anywhere), inner classes can be declared static.
Static methods can be used if
One does not want to perform an action on an instance (utility methods)
As mentioned in few of above answers in this post, converting miles to kilometers, or calculating temperature from Fahrenheit to Celsius and vice-versa. With these examples using static method, it does not need to instantiate whole new object in heap memory. Consider below
1. new ABCClass(double farenheit).convertFarenheitToCelcium()
2. ABCClass.convertFarenheitToCelcium(double farenheit)
the former creates a new class footprint for every method invoke, Performance, Practical. Examples are Math and Apache-Commons library StringUtils class below:
Math.random()
Math.sqrt(double)
Math.min(int, int)
StringUtils.isEmpty(String)
StringUtils.isBlank(String)
One wants to use as a simple function. Inputs are explictly passed, and getting the result data as return value. Inheritence, object instanciation does not come into picture. Concise, Readable.
NOTE:
Few folks argue against testability of static methods, but static methods can be tested too! With jMockit, one can mock static methods. Testability. Example below:
new MockUp<ClassName>() {
#Mock
public int doSomething(Input input1, Input input2){
return returnValue;
}
};
I found a nice description, when to use static methods:
There is no hard and fast, well written rules, to decide when to make a method static or not, But there are few observations based upon experience, which not only help to make a method static but also teaches when to use static method in Java. You should consider making a method static in Java :
If a method doesn't modify state of object, or not using any instance variables.
You want to call method without creating instance of that class.
A method is good candidate of being static, if it only work on arguments provided to it e.g. public int factorial(int number){}, this method only operate on number provided as argument.
Utility methods are also good candidate of being static e.g. StringUtils.isEmpty(String text), this a utility method to check if a String is empty or not.
If function of method will remain static across class hierarchy e.g. equals() method is not a good candidate of making static because every Class can redefine equality.
Source is here
Static methods are the methods in Java that can be called without creating an object of class. It is belong to the class.
We use static method when we no need to be invoked method using instance.
A static method has two main purposes:
For utility or helper methods that don't require any object state.
Since there is no need to access instance variables, having static
methods eliminates the need for the caller to instantiate the object
just to call the method.
For the state that is shared by all
instances of the class, like a counter. All instance must share the
same state. Methods that merely use that state should be static as
well.
You should use static methods whenever,
The code in the method is not dependent on instance creation and is
not using any instance variable.
A particular piece of code is to be shared by all the instance methods.
The definition of the method should not be changed or overridden.
you are writing utility classes that should not be changed.
https://www.tutorialspoint.com/When-to-use-static-methods-in-Java
In eclipse you can enable a warning which helps you detect potential static methods. (Above the highlighted line is another one I forgot to highlight)
I am wondering when to use static methods?
A common use for static methods is to access static fields.
But you can have static methods, without referencing static variables. Helper methods without referring static variable can be found in some java classes like java.lang.Math
public static int min(int a, int b) {
return (a <= b) ? a : b;
}
The other use case, I can think of these methods combined with synchronized method is implementation of class level locking in multi threaded environment.
Say if I have a class with a few getters and setters, a method or two, and I want those methods only to be invokable on an instance object of the class. Does this mean I should use a static method?
If you need to access method on an instance object of the class, your method should should be non static.
Oracle documentation page provides more details.
Not all combinations of instance and class variables and methods are allowed:
Instance methods can access instance variables and instance methods directly.
Instance methods can access class variables and class methods directly.
Class methods can access class variables and class methods directly.
Class methods cannot access instance variables or instance methods directly—they must use an object reference. Also, class methods cannot use the this keyword as there is no instance for this to refer to.
Whenever you do not want to create an object to call a method in your code just declare that method as static. Since the static method does not need an instance to be called with but the catch here is not all static methods are called by JVM automatically. This privilege is enjoyed only by the main() "public static void main[String... args]" method in java because at Runtime this is the method Signature public "static" void main[] sought by JVM as an entry point to start execution of the code.
Example:
public class Demo
{
public static void main(String... args)
{
Demo d = new Demo();
System.out.println("This static method is executed by JVM");
//Now to call the static method Displ() you can use the below methods:
Displ(); //By method name itself
Demo.Displ(); //By using class name//Recommended
d.Displ(); //By using instance //Not recommended
}
public static void Displ()
{
System.out.println("This static method needs to be called explicitly");
}
}
Output:-
This static method is executed by JVM
This static method needs to be called explicitly
This static method needs to be called explicitly
This static method needs to be called explicitly
The only reasonable place to use static methods are probably Math functions, and of course main() must be static, and maybe small factory-methods. But logic should not be kept in static methods.
I am wondering when to use static methods? Say if I have a class with a few getters and setters, a method or two, and I want those methods only to be invokable on an instance object of the class. Does this mean I should use a static method?
Example:
Obj x = new Obj();
x.someMethod();
...or:
Obj.someMethod(); // Is this the static way?
I'm rather confused!
One rule-of-thumb: ask yourself "Does it make sense to call this method, even if no object has been constructed yet?" If so, it should definitely be static.
So in a class Car you might have a method:
double convertMpgToKpl(double mpg)
...which would be static, because one might want to know what 35mpg converts to, even if nobody has ever built a Car. But this method (which sets the efficiency of one particular Car):
void setMileage(double mpg)
...can't be static since it's inconceivable to call the method before any Car has been constructed.
(By the way, the converse isn't always true: you might sometimes have a method which involves two Car objects, and still want it to be static. E.g.:
Car theMoreEfficientOf(Car c1, Car c2)
Although this could be converted to a non-static version, some would argue that since there isn't a "privileged" choice of which Car is more important, you shouldn't force a caller to choose one Car as the object you'll invoke the method on. This situation accounts for a fairly small fraction of all static methods, though.
Define static methods in the following scenarios only:
If you are writing utility classes and they are not supposed to be changed.
If the method is not using any instance variable.
If any operation is not dependent on instance creation.
If there is some code that can easily be shared by all the instance methods, extract that code into a static method.
If you are sure that the definition of the method will never be changed or overridden. As static methods can not be overridden.
There are some valid reasons to use static methods:
Performance: if you want some code to be run, and don't want to instantiate an extra object to do so, shove it into a static method. The JVM also can optimize static methods a lot (I think I've once read James Gosling declaring that you don't need custom instructions in the JVM, since static methods will be just as fast, but couldn't find the source - thus it could be completely false). Yes, it is micro-optimization, and probably unneeded. And we programmers never do unneeded things just because they are cool, right?
Practicality: instead of calling new Util().method(arg), call Util.method(arg), or method(arg) with static imports. Easier, shorter.
Adding methods: you really wanted the class String to have a removeSpecialChars() instance method, but it's not there (and it shouldn't, since your project's special characters may be different from the other project's), and you can't add it (since Java is somewhat sane), so you create an utility class, and call removeSpecialChars(s) instead of s.removeSpecialChars(). Sweet.
Purity: taking some precautions, your static method will be a pure function, that is, the only thing it depends on is its parameters. Data in, data out. This is easier to read and debug, since you don't have inheritance quirks to worry about. You can do it with instance methods too, but the compiler will help you a little more with static methods (by not allowing references to instance attributes, overriding methods, etc.).
You'll also have to create a static method if you want to make a singleton, but... don't. I mean, think twice.
Now, more importantly, why you wouldn't want to create a static method? Basically, polymorphism goes out of the window. You'll not be able to override the method, nor declare it in an interface (pre-Java 8). It takes a lot of flexibility out from your design. Also, if you need state, you'll end up with lots of concurrency bugs and/or bottlenecks if you are not careful.
After reading Misko's articles I believe that static methods are bad from a testing point of view. You should have factories instead(maybe using a dependency injection tool like Guice).
how do I ensure that I only have one of something
only have one of something
The problem of “how do I ensure that I
only have one of something” is nicely
sidestepped. You instantiate only a
single ApplicationFactory in your
main, and as a result, you only
instantiate a single instance of all
of your singletons.
The basic issue with static methods is they are procedural code
The basic issue with static methods is
they are procedural code. I have no
idea how to unit-test procedural code.
Unit-testing assumes that I can
instantiate a piece of my application
in isolation. During the instantiation
I wire the dependencies with
mocks/friendlies which replace the
real dependencies. With procedural
programing there is nothing to "wire"
since there are no objects, the code
and data are separate.
A static method is one type of method which doesn't need any object to be initialized for it to be called. Have you noticed static is used in the main function in Java? Program execution begins from there without an object being created.
Consider the following example:
class Languages
{
public static void main(String[] args)
{
display();
}
static void display()
{
System.out.println("Java is my favorite programming language.");
}
}
Static methods in java belong to the class (not an instance of it). They use no instance variables and will usually take input from the parameters, perform actions on it, then return some result. Instances methods are associated with objects and, as the name implies, can use instance variables.
No, static methods aren't associated with an instance; they belong to the class. Static methods are your second example; instance methods are the first.
If you apply static keyword with any method, it is known as static method.
A static method belongs to the class rather than object of a class.
A static method invoked without the need for creating an instance of a class.
static method can access static data member and can change the value of it.
A static method can be accessed just using the name of a class dot static name . . . example : Student9.change();
If you want to use non-static fields of a class, you must use a non-static method.
//Program of changing the common property of all objects(static field).
class Student9{
int rollno;
String name;
static String college = "ITS";
static void change(){
college = "BBDIT";
}
Student9(int r, String n){
rollno = r;
name = n;
}
void display (){System.out.println(rollno+" "+name+" "+college);}
public static void main(String args[]){
Student9.change();
Student9 s1 = new Student9 (111,"Indian");
Student9 s2 = new Student9 (222,"American");
Student9 s3 = new Student9 (333,"China");
s1.display();
s2.display();
s3.display();
} }
O/P: 111 Indian BBDIT
222 American BBDIT
333 China BBDIT
Static methods are not associated with an instance, so they can not access any non-static fields in the class.
You would use a static method if the method does not use any fields (or only static fields) of a class.
If any non-static fields of a class are used you must use a non-static method.
Static methods should be called on the Class, Instance methods should be called on the Instances of the Class. But what does that mean in reality? Here is a useful example:
A car class might have an instance method called Accelerate(). You can only Accelerate a car, if the car actually exists (has been constructed) and therefore this would be an instance method.
A car class might also have a count method called GetCarCount(). This would return the total number of cars created (or constructed). If no cars have been constructed, this method would return 0, but it should still be able to be called, and therefore it would have to be a static method.
Use a static method when you want to be able to access the method without an instance of the class.
Actually, we use static properties and methods in a class, when we want to use some part of our program should exists there until our program is running. And we know that, to manipulate static properties, we need static methods as they are not a part of instance variable. And without static methods, to manipulate static properties is time consuming.
Static:
Obj.someMethod
Use static when you want to provide class level access to a method, i.e. where the method should be callable without an instance of the class.
Static methods don't need to be invoked on the object and that is when you use it. Example: your Main() is a static and you don't create an object to call it.
Static methods and variables are controlled version of 'Global' functions and variables in Java. In which methods can be accessed as classname.methodName() or classInstanceName.methodName(), i.e. static methods and variables can be accessed using class name as well as instances of the class.
Class can't be declared as static(because it makes no sense. if a class is declared public, it can be accessed from anywhere), inner classes can be declared static.
Static methods can be used if
One does not want to perform an action on an instance (utility methods)
As mentioned in few of above answers in this post, converting miles to kilometers, or calculating temperature from Fahrenheit to Celsius and vice-versa. With these examples using static method, it does not need to instantiate whole new object in heap memory. Consider below
1. new ABCClass(double farenheit).convertFarenheitToCelcium()
2. ABCClass.convertFarenheitToCelcium(double farenheit)
the former creates a new class footprint for every method invoke, Performance, Practical. Examples are Math and Apache-Commons library StringUtils class below:
Math.random()
Math.sqrt(double)
Math.min(int, int)
StringUtils.isEmpty(String)
StringUtils.isBlank(String)
One wants to use as a simple function. Inputs are explictly passed, and getting the result data as return value. Inheritence, object instanciation does not come into picture. Concise, Readable.
NOTE:
Few folks argue against testability of static methods, but static methods can be tested too! With jMockit, one can mock static methods. Testability. Example below:
new MockUp<ClassName>() {
#Mock
public int doSomething(Input input1, Input input2){
return returnValue;
}
};
I found a nice description, when to use static methods:
There is no hard and fast, well written rules, to decide when to make a method static or not, But there are few observations based upon experience, which not only help to make a method static but also teaches when to use static method in Java. You should consider making a method static in Java :
If a method doesn't modify state of object, or not using any instance variables.
You want to call method without creating instance of that class.
A method is good candidate of being static, if it only work on arguments provided to it e.g. public int factorial(int number){}, this method only operate on number provided as argument.
Utility methods are also good candidate of being static e.g. StringUtils.isEmpty(String text), this a utility method to check if a String is empty or not.
If function of method will remain static across class hierarchy e.g. equals() method is not a good candidate of making static because every Class can redefine equality.
Source is here
Static methods are the methods in Java that can be called without creating an object of class. It is belong to the class.
We use static method when we no need to be invoked method using instance.
A static method has two main purposes:
For utility or helper methods that don't require any object state.
Since there is no need to access instance variables, having static
methods eliminates the need for the caller to instantiate the object
just to call the method.
For the state that is shared by all
instances of the class, like a counter. All instance must share the
same state. Methods that merely use that state should be static as
well.
You should use static methods whenever,
The code in the method is not dependent on instance creation and is
not using any instance variable.
A particular piece of code is to be shared by all the instance methods.
The definition of the method should not be changed or overridden.
you are writing utility classes that should not be changed.
https://www.tutorialspoint.com/When-to-use-static-methods-in-Java
In eclipse you can enable a warning which helps you detect potential static methods. (Above the highlighted line is another one I forgot to highlight)
I am wondering when to use static methods?
A common use for static methods is to access static fields.
But you can have static methods, without referencing static variables. Helper methods without referring static variable can be found in some java classes like java.lang.Math
public static int min(int a, int b) {
return (a <= b) ? a : b;
}
The other use case, I can think of these methods combined with synchronized method is implementation of class level locking in multi threaded environment.
Say if I have a class with a few getters and setters, a method or two, and I want those methods only to be invokable on an instance object of the class. Does this mean I should use a static method?
If you need to access method on an instance object of the class, your method should should be non static.
Oracle documentation page provides more details.
Not all combinations of instance and class variables and methods are allowed:
Instance methods can access instance variables and instance methods directly.
Instance methods can access class variables and class methods directly.
Class methods can access class variables and class methods directly.
Class methods cannot access instance variables or instance methods directly—they must use an object reference. Also, class methods cannot use the this keyword as there is no instance for this to refer to.
Whenever you do not want to create an object to call a method in your code just declare that method as static. Since the static method does not need an instance to be called with but the catch here is not all static methods are called by JVM automatically. This privilege is enjoyed only by the main() "public static void main[String... args]" method in java because at Runtime this is the method Signature public "static" void main[] sought by JVM as an entry point to start execution of the code.
Example:
public class Demo
{
public static void main(String... args)
{
Demo d = new Demo();
System.out.println("This static method is executed by JVM");
//Now to call the static method Displ() you can use the below methods:
Displ(); //By method name itself
Demo.Displ(); //By using class name//Recommended
d.Displ(); //By using instance //Not recommended
}
public static void Displ()
{
System.out.println("This static method needs to be called explicitly");
}
}
Output:-
This static method is executed by JVM
This static method needs to be called explicitly
This static method needs to be called explicitly
This static method needs to be called explicitly
The only reasonable place to use static methods are probably Math functions, and of course main() must be static, and maybe small factory-methods. But logic should not be kept in static methods.
I need to modify an argument to a function in Java. How do I add the argument to the callers using automation?
For example, In class A a method hello(int a ) exists. Now I am adding String to the same function argument so it becomes hello(String b , int a). If class B calls this method,how do I update the calling line in class B.
A a = new A();
int a; String b;
a.hello(a) % change to a.hello(b,a); //Assuming b is accessible.%
Eclipse provides this functionality via refactoring. Do they expose this API to external world or any other API is there?
You can right-click onto the method, select Refactor and select Change Method Signature. This opens a dialog that allows you to add new parameters, change parameter order, etc. When adding a new parameter you can provide a "Default Value" that will be used as argument in all classes that are currently calling the changed method.
Only classes with source that are currently in open Projects of the current Workspace are changed. The "outside world" will not get any notification of that. If you want to keep things runnable you should follow the advice to create a second method with the additional parameter and keep the old one (calling the new one with fitting default value). This can also automatically be done by mentioned refactor-dialog by checking the checkbox "keep original method as delegate to changed method".
Instead of trying to find all calling instances of a method, I would overload the method to include a way to handle passing in a string.
Class A(int a){}
Class A(String a){}
when you call these methods with the appropriate argument, you call that specific method. If you already have method calls using String a as an argument, a)you should get compiler errors, but more importantly b)this would fix all of them.
You can overload the method to add more parameters. All you have to do is define another method with the same name that takes different arguments.
public class A{
public void hello(int a){
//do things
}
public void hello(String b, int a){
//do things
}
}
The JVM will determine which method to use based on the given arguments during run time. If this isn't the functionality you need just give me a heads up.
Let's look at the method signature of main() in C, C++ and Java without paying much attention to it.
C and C++ take the syntax of the main() function something like the one shown below.
int main(void)
{
//Elegant code goes here.
return(0);
}
or it can simply be declared void as shown below.
void main(void)
{
//Elegant code goes here.
}
The main() function in C and C++ can optionally takes two command-line arguments, if needed.
The signature of the main() method in Java however, something like this.
public static void main(String []args)
{
//Elegant Java code goes here.
}
Java requires the main() method to be static simply because it was clearly mentioned that it is the first method to be invoked when no objects were created hence, it must be static. The same thing is also applicable to C and C++. There also the main() function is the first function to be invoked still they don't require the main() function to be static. Why? Why did the Java designers take somewhat different view to implement the main() method in Java?
C has no objects and all methods are static; in C++ one has to declare methods as virtual for them to be not static.
Because main is member method in case of Java.
Also See
why-is-the-java-main-method-static
In java, entire program is encapsulated within a single class. and that class name is usually the name of the program.
Static member function of the class has the ability to be called without using any of the object and only one copy of the static member function is shared by all the object.
Since main function is single for all the object of your java program and we are not using any object for calling the main function. For this reason main is declared static,
Because main is run without the object being instantiated. Therefore is static method.
If main() is not declared as static, one would have to instantiate the class first to be able to call the method. But to be able to instantiate, one would need to have a running program. This is a catch 22, so the only way to launch the method is having it static.
I have a class that will be subclassed. All subclasses must contain a static method with the same signature, but differnt for each one.
I would like to have an abstract instance method in the superclass that subclasses will override, but it seems mot possible in Java, I wonder why.
A silly example:
Image{
abstract String getExtension();...
RGBImage extends Image{
static String getExtension(){return "RGB"};..
PNGImage extends Image{
static String getExtension(){return "PNG"};...
Have a look at this explanation. You could use the Builder pattern for your purpose.
As others have said, Java does not support overriding or abstract for static methods. However, I don't really understand what you would achieve with an "abstract" static method anyway.
The normal use of an abstract method is to force every subclass to implement a method with the same signature. But why would it even it matter for a static (hence non-polymorphic) method? If you forget to provide the method for one of the subclasses, EITHER it doesn't matter because you don't call it, OR it does matter but you'll get a compilation error at the point you try to call the missing method.
The only use-case I can think of where it might matter is if you are calling the method reflectively in a pseudo-polymorphic way. But if you are doing that you'd be better off using real polymorphism and instance methods.
I don't do much Java, but (using C++ logic) you could make the base class implementation a dummy implementation that cannot be executed.
static void DoStuff(arg_type arg)
{
std::cerr << "Method DoStuff() must be overriden\n";
::abort();
}
That isn't as good as an abstract method, but it will achieve some of the same results.
Unfortunately, this is not possible in Java. You have to make them instance methods to be able to define as being abstract.
It's not possible to override static methods as they pertain to a class, not an instance.
Furthermore, an instance method cannot be made static in the sub-classes as you suggest in your example.
Can you provide more information as to why all the subclasses need to have the same static method?
You want to override instance method with static method ? That makes no sense. Static methods are not polymorphic. How would you like to call it? It is impossible in Java to enforce class to have a static method with given signature.
as others have said, static is not polymorphic... it means it is the same for the class, not specific to an instance. i am not sure what you would accomplish by using a static method... but you could have have your extended method implementation return a static value:
public String getExtension() {
return STATIC_CONSTANT;
}
I guess it depends on what you are trying to accomplish with the static method. From your post: "Let me say, everything works OK implemnting a instance getExtension() method in each subclass. But the point is that the method is in fact static in the subclass, has no dependence on the instance. I know I can't do that, I only saying it seems no so strange idea to me."
The problem is you are asking for polymorphic behavior (a change in the behavior of a method for each subclass) from something that defines behavior for the class, not the instance. By definition, static cannot be polymorphic. I hope this helps you with your understanding here of why this is not possible.
The reason I want to have the abstract method declared in the superclass is because the class has to do some extra work with the result of the abstract method.
Lets put it simple even the example has no sense:
Image{
abstract String getExtension();
int process(){
return getExtension().length
}
The algorithm in process() is common for all subclasses XXXImage of Image, so its right place is the superclass.
Let me say, everything works OK implemnting a instance getExtension() method in each subclass. But the point is that the method is in fact static in the subclass, has no dependence on the instance.
I know I can't do that, I only saying it seems no so strange idea to me.
Yes thaks, this is what I'm doing, retun a static constat. In may case it returns the class object of a inner enum that lists the name of sections of a given file format: the file format of the subclass.
But you put it very clear in your example:
public String getExtension() {
return STATIC_CONSTANT;
}
I was expecting to be able to put "static" before String...