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Static method in a generic class?
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I'm new to generics, and am struggling with applying to my situation:
I currently have several SyncData Classes, one for each object type that I wish to sync. I'm looking at changing this to a single generic class SyncData. Within this class some parts of the code will use objects of type T, but I'd also like to reference static Methods of type T. This would enable me for example to specify a notification action (specific to the data type) to be called on completion of the sync.
I'm getting myself caught in a loop: to access the methods I need to define an interface that all the types will implement, in the generic T extends interface, and then I can reference the relevant methods of type T within my generic SyncData Class. However I can't specify a static method in an interface, and because I can't in the interface, I cant access it in my generic SyncData Class.
I've looked at various tutorials, questions here etc, maybe I'm missing the obvious, but I can't see the way forward - everything that comes up seams to refer to different issues with generics and static nethods. Any advice would be very much appreciated.
Edit:
As requested some sample code added to illustrate what I tried to do (I'm actually looking to use static methods at various other points within the code, but this illustrates the point).
Is it possible to access the static method of class T from within the SyncAction class (without making T extend DataType1 by which point there is no point using generics as T cannot be DataType2)?
public interface Syncable {
//following needed or I can't access onSyncComplete in SyncAction
public void onSyncComplete();
}
public class DataType1 implements Syncable {
//constructor etc removed as irrelevant to question
//Eclipse warns 'this static method cannot hide instance method from Syncable'
public static void onSyncComplete(){
//do stiff here
}
}
public class SyncAction<T extends Syncable>{
//do all the syncing etc using multiple DataType1 objects
T.onSyncComplete();
}
Even though the onSyncComplete() method could be static, you must make them instance (ie non-static) methods for this to work.
The reason is that static methods (and fields) are not "inherited".
If you really want to have a static method, you could have your onSyncComplete() instance method call another (perhaps private) static method in its implementation.
Few things:
Eclipse is correct in stating that your static version of onSyncComplete() is incorrect. From JLS seciton 8.4.8.2:
It is a compile-time error if a static method hides an instance method.
In this case, your static method is hiding the instance method defined in your interface, so you get an error.
This is because if such a thing were allowed there would be some ambiguity as to whether the static method or instance method were to be called, as you can invoke static methods on an instance (although that isn't really a good idea for readability, and is considered a mistake by some).
The reason you cannot call static methods on a generic type is that static methods are bound at compile time. Because generic types are erased during compilation to their bounding type (Syncable in your case), the compiler will not be able to figure out which static method to bind, and so emits an error.
I'm learning Java and one of the things that suprised me is you don't create an object from a class. For example:
class helloworld{
public static void main(String[] args){
System.out.println("Hello world!");
}
}
What I don't understand is that, I have always thought of a class as a blueprint for objects, whereas here you don't create an object and the program simply runs from the class. Is the creation of an object to run themain method from implicit? I haven't exactly phrased this very well and assume that I am missing some piece of understanding - could someone explain?
"A class as a blueprint for objects" is a Java 101 way of describing the reality. It helps to teach you what classes and objects are, but it is not the whole truth.
A class is also a holder of static code and data, which exist on their own, independent of any class instances. You can view it as a kind of namespaced global data.
In your example, the main method itself is such a static method, which can be executed with no existing objects, and System.out refers to an object which exists on its own, attached to a static variable in the System class.
If you are using only static methods and variables in java then all the object of that class share the same variables and methods, you dont have to create an object and call.
Also the main method is the entry point for the code to run
You do create objects in java. The static keyword shows that the variable, method or nested class belongs to the class/superclass, and not to an object. The main method is declared static so that it can be accessed at runtime; it is not part of an object, it is part of the class. The JVM looks for the main method at runtime.
public mean it can be access from any class static
static mean you don't have to create object or in other word no need of
the new and that what you are asking about
mean it will not return a value
Can we override the method as mentioned below:-
"public static void main"
No. main is a static method, and is thus not polymorphic. You may hide it be defining another static main method in a subclass, though.
No and you are losing the focus here. The main method has a single purpose and is declared logically for that sole purpose:
The main method in Java belongs to a class but not to an Object. Objects are created at The main() in Java is the start point in your application, there is no way to start your application from an instance specific method. That is why the static keyword make perfect sense with the main method. In fact all the parts of the main method declaration make perfect sense when you think like the 'jvm' and picture what the main method does (starts the application):
public, because this method must be accessible by the jvm (not written by you).
static, implying this method can be accessed without having an object (because it's representation never changes), but here the logic is easy understood if you think like the jvm again; "I don't have any objects to create (instantiate) objects, so I need a static method to start the application as there simple isn't any logical way to get an instance specific method up yet as I don't have anything up yet to create objects".
void This method can't logically return anything because there is nothing up yet to return anything to. It is the start point of the application.
main I am the main method as without me you won't have an application.
String[] args Send me data you may feel useful for my startup.
We can not override the static method because static metod is a class method and the scope of this method within the same class itself. So if you want to override forcefully then you have to define it outside of that class scope which does not make sense.
No. You can't override a static method.
It wouldn't really make sense to anyway. Since you don't need an instance of the class, you don't need polymorphic behavior. You would just change the all from SomeParent.main() to SomeChild.main()
MAIN is a class method (since its static by definition). Hence, it does not makes sense to "override" it (or for that matter any static method).
The concept of "overriding" is only for instance methods.
This is a good read with regards to the same.
No, it is not possible to override static methods, since they are not instance level methods, but class level methods.
I tried this, don't know if its correct way to override method, but I am able to override main method like this,
class MainOverride {
public static void main(String[] args) {
MainOverridden mo = new MainOverridden();
String [] s = {"a","b"};
mo.main(s);
}
}
class MainOverridden extends MainOverride {
public static void main(String[] args) {
System.out.println("Main overridden");
}
}
I do not understand why the main method has to be static. I understand static variables but static methods are difficult for me to grasp. Do static method exists so that one can create two methods with the same name in two different classes that won't clash with each other?
Also, I don't understand why I can't create a static constructor.
Could anyone help explain this concept?
Java has [static constructors] static initialization blocks which can be viewed as a "static constructor":
class Foo {
static String Bar;
static {
// "static constructor"
Bar = "Hello world!";
}
}
In any case, the only method in the main class which must be static is the main method. This is because it is invoked without first creating an instance of the "main class". A common technique, and the one I prefer, is to quickly get out of static context:
class Main {
int argCount;
// constructor
public Main (String[] args) {
// and back to boring ol' non-static Java
argCount = args.length;
}
void runIt () {
System.out.println("arg count: " + argCount);
}
// must be static -- no Main instance created yet
public static void main (String[] args) {
Main me = new Main(args);
me.runIt();
}
}
Also, static has nothing to do with "name clashes". A static method (or variable) is simply a method (or variable) that is not associated with a specific instance of a type. I would recommend reading through the Classes and Objects Java Tutorial and the section Understanding Instance and Class Variables.
Happy coding.
I am sharing one of the reason "why not a java constructor be static".
Simply to say, "A java constructor is always non static" because,
The purpose of the constructor is only to initialize/construct the object, and to make inheritance possible. To do these we need to use the two useful java keywords (cum non-static variables) such as this and super.
We will use 'this' to initialize the object.
We/Java will use super(ofcourse super()) to invoke super class constructor so that super object(or Object class) created first then the child object(hence the inheritance)
If the constructor is static then we cant use that two keywords(non-static variables) inside the constructor(As we know non-static stuff cant be referenced from static context)
So java constructors should not static.
Static methods belong to a class, not an object. The main method must be static because it is called first, before any other code has executed to instantiate any objects. It provides an entry point to the program. Static methods are called from outside of the container of an object. The same is true of static class variables. Only one copy exists for the entire class, as opposed to a member variable, which is created once for each object created from a class. They are used to store data for the class, such as the number of object instances have been created and not destroyed. This data belongs with the class. A good example of a static method is in the singleton pattern, where the constructor is private and can only be accessed by a static member function. A function outside the class would be unable to replicate this functionality. This method acts on class data and objects, so logically belongs to the same class. This all boils down to encapsulation. A class is responsible only for itself and knows only itself.
On the other hand, object methods are meant to operate on the data associated with a single instance of a class, an object. Constructors are the code that is used to initialize an object and set it's data to an initial state. They are executed immediately (and automatically) after the memory has been allocated to store a new object. Even if you do not explicitly define a constructor, a kind of "default constructor" is executed in order to map the object's member variables and the object's method code to the new object.
Hope this helps.
Constructor is used to create Objects.
Static is generally which is same for all objects.
So, if we have had static constructors creation of one object would affect all the other existing objects.
Static methods only reference to static variables. Therefore all the initial parameters which you are giving to create an object would change for all objects. It is no point creating similar objects for no use.
Hope this helps.... :)
Constructor is the property of an object while static has nothing to do with object. That's why there is nothing like static constructor. But we have static block to do the similar task as constructor i.e. initialization of fields etc.
On page 272 of Thinking In Java, 4th Edition, by Bruce Eckel, it says:
²The constructor is also a static method even though the static keyword is not explicit. So to be precise, a class is first loaded when any of its static members is accessed.
A little bit more context.
... the compiled code for each class exists in its own separate file. That file isn't loaded until the code is needed. In general you can say "class code is loaded at the point of first use." This is usually when the first object of that class is constructed, but loading also occurs when a static field or static method is accessed.²
This makes a lot of sense, if you think about the rule that says that a static method can't use non-static methods of the same class. I had this doubt a couple weeks ago when I couldn't understand how, using the Singleton Pattern, you could access the constructor inside the static method that is used to create a new instance of that class. Today I was flipping through the book and I came across this explanation.
It also makes sense in a way that, if the constructor wasn't static, you'd first need an instance of that class to be able to access it, but I guess this could spark up the old discussion about the chicken or the egg.
Hope it helped!
Constructors are neither entirely static (class level) or entirely non-static (instance level).
Unlike instance methods, constructors are not inherited.
Unlike static methods, a constructor can refer to this.
So, why can't you declare a constructor static?
Well, my take is that a (redundant) static keyword would be confusing and would not serve any purpose. Therefore they decided not to allow it.
The explanation that static initialization blocks can be viewed as constructors is (IMO) conceptually wrong. (It is analogous to saying that an instance initialization block is a regular constructor. Which is equally wrong.)
The key distinctions between static initialization and construction1 are:
static initialization happens at an indeterminate time2; there is no equivalent to new for class initialization,
there is no straight-forward way to pass (constructor) parameters to the initialization code
there is no practical way to recover from errors occurring during static initialization.
1 - Hypothetically, if class initialization was explicit, then it would make sense to have static constructors. But the downsize would be that applications would need to explicitly "construct" all of the classes that they used ... which would be horrible.
2 - You have a degree of control if you load a class dynamically, but even then if the class has already been loaded and initialized in the current classloader, then attempting to control initialization will fail.
I do not understand why the main method has to be static.
It has to be if you want the main method to act as an entrypoint for your application.
The problem is that if main was an instance method, then there would need to be an instance of your entrypoint class to call the main method on. But how do you create it? Which constructor would you choose? What if there was no public constructor?
The bottom line is that this is the way that Java was designed ... back in the 1990's ... and so far they have not seen the need to change this.
a) static is belongs to class not object and constrictor is called during the object creation.
b) if we create a constructor as static then it can't be call by subclass as static is accessible only from class not by sub class. So during subclass object creation it can't be call the present class constructor.
c) static members are executed first in the program, so if we declare constructor as static then it will executed before object creation which is oppose the purpose of the constructor.
If we declare constructor as static then it will give compile time error.
If we want to initialize static member then need to use of static block.
I wrote a simple example as an answer to a related question yesterday which may help make things more understandable: what's the point of java constructor?
The point of Static methods is that they can be called without creating an instance of a class, while "normal" instance methods are related to an instance, and can not be called without one.
Since the Main method of the Main class is the entry point of the program, no instance can possibly have been created yet, so naturally, you can not access it via an instance. Therefore, it is Static, so it can be run as the start of the program.
Just take a look on this link, it will definately help you to understand:
Why can't make a constructor static?
AND
Constructor is called at Run-time when we create Objects.
Static is same for all objects but all objects have their own state and properties.
So, if we have had static constructors creation of one object would affect all the other existing objects.
Note: static is class level while constructors related to the objects.
e.g.
public class Foo
{
String name;
int id;
// define constructors
Foo (String name, int id)
{
this.name = name;
this.id = id;
}
p s v m(String[] arg)
{
Foo f1 = new Foo("Amit",001);
Foo f2 = new Foo("Rahul",002);
}
}
If we create static constructor then both objects(f1 also) will contain the last updated value regarding name and id as Rahul and 002.
A constructor cannot be static, because in an OO language, the process for creating an object is as follows:
allocate the object
call the constructor to initialise the newly-allocated object
Constructors are not used anywhere else (and a type-safe language should enforce this), so it follows that a constructor will always be called in a non-static context.
If a constructor were static, it would not receive a reference to the newly-allocated object, and thus would not be able to initialise it.
Thus, a constructor can always be non-static (as it is always called from a non-static context) and must always be non-static (otherwise it would be unable to perform its task).
The main(String[]) method has a specific prototype that is dictated by how the Java runtime environment works. When you invoke java MyApplication from the command line, the Java VM will look for a static main(String[]) method contained in that class in order to execute the application. If that method is not found, then the Java VM can't run the class as an application. That's just how the language is defined. It also means that the Java VM doesn't create an instance of your application class in order to run it.
Now, if you want your class to be usable either as a standalone application or as an instance that's created by something else, then you can have your class implement the Runnable interface, and also provide a main method that executes the run method on a new instance.
public class MyRunnableThing implements Runnable
{
// Define whatever variables your runnable thing needs here as
// private instance fields.
/** Fulfills requirements of Runnable interface. */
public void run()
{
System.out.println( "I'm running..." ) ;
}
/** Also makes the class runnable from the console. */
public static void main( String[] args )
{
MyRunnableThing runMeNow = new MyRunnableThing() ;
runMeNow.run() ;
}
}
Now any class could potentially create an instance of MyRunnableThing and use its run() method to produce the same behavior that would have been seen by executing java MyRunnablething.
See also: Working with Static Constructor in Java. Some highlights from that Q&A:
A constructor is used to create an instance of the class, so it's an instance method, not a static method.
You can create a static method that creates an instance of the class, using the constructor. This is how the trendy new "builder" classes work.
You can create a static method that returns a persistent, unique singleton instance.
If your class has static members, then you can create a static initializer to initialize the values of those members.
The purpose of Constructor is to Construct an Object i.e. to initialize class's instance variables either their default values or by their initialized values. non-static Instance variables can't be accessed by static methods . So constructor is not static.
The method declared as static requires no object creation .As we don't create object for the main method it is declared as static.
constructor is implicitly called to initialize an object, so there is no purpose in having a static constructor.
First, the key word static means that everything marked static must be the class-level thing and belongs to the class only.While constructors belong to object and they may usually be called when we use the new operator.So we now know that a constructor is not even a class property,how could we possibly mark it as static?
Second,static constructor violates the whole purpose of inheritance in java.Every time just before we create an subclass object ,JVM automatically calls the superclass constructor to make it ready for the subclass object to be created.But if we mark the constructor static,the subclass will not be able to access the constructor of its superclass because it's marked static thus belongs to class only.
Java does not permit to declare a constructor as static. Following are the reasons.
Static means for the same class. i.e, static methods cannot be inherited.
With static, "this" reference (keyword) cannot be used. "this" is always linked to an object. A constructor always belongs to some object.
If a constructor is static, an object of subclass cannot access. If static is allowed with constructor, it is accessible within the class but not by subclass.
Static Belongs to Class, Constructor to Object
We know that static methods, block or variables belong to the class. Whereas a Constructor belongs to the object and called when we use the new operator to create an instance. Since a constructor is not class property, it makes sense that it’s not allowed to be static.
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.