If I define a class which is full of static member function and variable(java), then something interesting may happen.
Can I use the class as a single instance of the class and have no need worrying about conveying the instance variable?
How will the construction function take effect if I don't instantiate an object? So I can't initialize the static member variable in construction function, right?
If I define a class which are full of static member function and variable(java), then something interesting may happen.
Not really, usually helper / utility classes don't have instance level fields.They only have static methods and static fields.
Can I use the class as a single instance of the class and have no need worrying about conveying the instance variable
No, marking all methods and fields as static won't make this a Singleton class.
How will the construction function take effect if I don't instantiate an object
The default constructor will be called when you do new MyClass().
So I can't initialize the static member variable in construction function, right?
You can. Unless it is marked as final. You might run into trouble if you have static blocks
It is working same as any other class with static members, no matter if all of them are static or only part of them. They are used as expected - bound to class, not to instance.
Also you can actually have "static class" with constructor, look for Singleton pattern (however be aware, that pattern can be misused very easily).
today i have stumbled on a code which i have seen in my project and was worried looking into it. I dont realize why they have made these as static methods as they change state of object within them.
below is the code
#Controller
CruiseController{
getCruiseSearchResults(){
//prepare cruise serach request, static method in CruiseHelper
CruiseSearchRequest cruiseReq = CruiseHelper.prepareRequest();
...futher impl
}
/** my helper class which has utlity methods */
CruiseHelper{
public static CruiseSearchRequest prepareRequest(){
CruiseSearchRequest cruiseRequest = new CruiseSearchRequest();
// all below methods are static
setCruiseTypeandDestination(cruiseRequest)
setStartAndEndDate(cruiseRequest)
setShipAndDeparturePort(cruiseRequest)
setDurationAndAccesiblity(cruiseRequest)
setPromoType(cruiseRequest)
setResultPreferences(cruiseRequest)
return cruiseSearchCriteriaDTO
}
static void setCruiseTypeandDestination(CruiseSearchRequest cruiseRequest){
/** changing the state of object in static method */
cruiseRequest.setCruiseType("ABC");
cruiseRequest.setCruiseType("Alaska");
}
//.... further static methods as above, all of them
//change the state of cruiseRequest
}
So i know that, above methods should not be static as they all have properties of each request. But the code is working and has not failed on any load test performed.
my important question is : "can this above code be considered ??" and "can this fail, if yes. then in what scenario ?"
Indeed, these methods change the state of an object, but it's an object that is given as a parameter to them, which is perfectly valid and makes sense.
static means that the method is bound to the object definition (the class) and not to any specific object instance. Thus static method cannot change the state of it's own object as it simply does not have an instance to work on (it does not have a this).
I suggest you read this about static and class variable : Understanding Class Members
Static methods are used to imply that that method doesn't need an instance of the class to be called. For example consider the String class. It can still change the state of any object.
replaceAll() is not a static method as it needs an instance to work on. where as valueOf() isn't as it doesn't need a String instance.
I suggest you revisit the basics of Java.
I dont realize why they have made these as static methods as they change state of object within them.
Because they're methods of a class called CruiseController. They are modifying instances of a CruiseSearchRequest, a different class. You could not do cruiseSearchRequest.setCruiseTypeandDestination();, because the method setCruiseTypeandDestination isn't on the CruiseSearchRequest class. So, instead, it receives the CruiseSearchRequest object as a parameter, and since it isn't tied to any instance of CruiseController, it is a static method of that class.
You could make the methods non-static if you moved them to the CruiseSearchRequest class. However, you don't need to. There is absolutely nothing technically wrong with modifying objects in a static method. It might or might not be a good design for your particular program, but it will not "fail".
Methods in Spring shouldn't be static not because they won't work but becouse it is a bad decision in terms of your application architecture. Static methods are wrong decision in terms of unit testing - it is harder to mock static methods than objects. Also I think intensive usage of static methods breaks the concept of dependency injection and the code becomes more tightly-coupled. Here is a nice article on this topic.
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Possible Duplicate:
Difference between static class and singleton pattern?
I was wondering,
Would a class such as Java's Math class, where all methods are static be considered a singleton? Or does a singleton have to have an instance, eg: Math.getInstance().abs(...) to qualify as a singleton?
Thanks
Having just static methods in a class does not qualify it being a Singleton, as you can still make as many instances of that class, if you have a public constructor in it.
For a class to qualify as Singleton, it should have private constructor, so that it can't be instantiated from outside the class, and have a static factory that returns the same instance everytime invoked.
If you really mean static class, then first of all, you can't have your top-level class as static. You can only have static nested class, in which case you don't need to create any instance of that class, but you can and you can create multiple instances and hence it as not Singleton.
Also, the class you mentioned - java.lang.Math, is not a static class. You should see the documentation of that.
Static classes in Java are just nested classes which aren't inner classes. (They're not like static classes in C#, for example.) They can still have instance methods, state etc - and there can be multiple instances.
java.lang.Math is not a static class.
And no, a class which never has an instance is not a singleton. The important difference is that a singleton can implement an interface (or even derive from an abstract class) whereas if you never create an instance of a class, any instance methods are pointless.
A class that is applied Singleton Pattern has one or none instance at any time on a JVM. That's why it's called single-ton. Having static or non-static members has no relationship with being singleton or non-singleton.
Static classes usually provide some helper methods. In fact i don't think its appropriate to compare Static classes with Singleton. Both are completely different.
We can create multiple instances of a static class but singleton guarantees(atleast in theory) only single instance.
It would have to have an instance with Singleton meaning to only be initialised, once.
Edit: Here's a link to the Wikipedia article
A singleton guarantees that there is only up to one instance of that class within the environment. It is different from a class with a bunch of static methods. A singleton carries state and can carry runtime values, while static methods of a class don't.
You can argue that you can have static values to do that, but the purpose of static variables is different (and sometimes abused like global variables). So a singleton operates in a different context than static methods.
A singleton also is usually associated with some resource (some connection to another server, access to some file, etc.) for which a static instance wouldn't be that ideal. Singletons also allow for lazy loading (if connecting or accessing a resource is expensive), while classes get loaded when the classloader encounters a reference to them.
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.