The difference between (a) import somePackage.someClass; and (b)someClass object = new someClass(); is that (a) will allow call the methods from the imported class without creating new instances of it, while (b) will create an object using the template class and therefore the methods for the class someClass will belong to the object object. So if I want to use a method someMethod() from someClass in (b) I'd call it through the object object. Is it how it works?
Yes you can use static methods from a class directly
Yes you can use methods from a class by creating an object
But more important thing than just the above options available is when to use which. First type of call is to class methods whereas the second class is to instance methods.
Instance Methods vs Class Methods: Each class represents a set of attributes and behaviour. Instance methods usually represent the behaviour. example if Person is a class and Robb is an object, then robb.weight can be attribute, robb.write() would be an instance method and Person.type() (ans: species) or Person.population (ans: total number of instances) can be class methods.
Also you represent instance methods in textual writing as ClassName#instanceMethod and ClassName.classMethods
No, you are wrong
Simplistically if the class that you want to use is not in the same package then you need to import it, or fully path the class e.g. java.util.ArrayList.
If the methods are not static, then you will need to create a new Instance of the class you want to use.
You can use methods from other class directly only if it is a static method. You will also have to add static in your import statement if you want to use the method name directly without prefixing it will the class name.
For non-static methods you have to create instance of the class and then call that method.
My friend is new to programming. He said he had a problem in his code and shared his screen on Skype. I saw he had instantiated his Main class to use all the functions, etc. It looked like this:
public static void main(String[] args) {
Main main = new Main();
main.Example();
}
I've also noticed this in a couple tutorials online. The thing is, since you shouldn't really have more than one main, why instantiate it?
You could argue that it's better that way.
You should probably not have a Main class to begin with but one with a proper object oriented purpose that encapsulates something.
The main method has to be unfortunately inside one of your classes.
But that doesn't mean that the poor class that has to house the main method isn't allowed to be instantiated.
In pure OOP there is no place for static at all. Not even utility classes: http://www.yegor256.com/2014/05/05/oop-alternative-to-utility-classes.html
This code initialize the Main class. Not public static void main() method !!!
It is a common thing in Java as per my knowledge as you can't always use static classes or methods.
you cannot access non-static variables or methods inside static methods (like main method). Hence the way to do is to create an instance of the class itself and use non-static variables or methods.
Refer this question
The difference between using static and non-static context can be found here
Assuming your asking why he would create a object of his main class the answer would be to access non-static methods from within static context. Without that main class object, he would only be able to access static methods and classes. I would recommend looking into Object Oriented Programming basics, which can be found here.
Main main = new Main(); //creates object of class Main
main.Example(); //calls method Example from object main
What your friend did was create a instance of the class and store it into memory. He is not instantiating the main method, but the class Main. This gives him the ability to alter information belonging to his main object of the class Main.
The purpose for doing this is to access non-static methods in the class Main.
The static methods can be easily accessed with the following format.
ClassName.methodname().
But instance method of the class can not be called without creating. If we have all the methods static, in that case there is no need of the the creation of the objects as we do it in utility classes.
I have a class called Base which has a method called execute(). There are about 100 classes which derive from this base class and provide their own implementation of execute(). Now, I have some common logic which I want to put in Base.SomeMethod(). This method needs to be called at the end of execute(). My question whether it is possible to call this without changing each and every derived class's execute() method?
public class Base {
public final void execute() {
doExecute();
someMethod();
}
protected abstract void doExecute();
public void someMethod() {
}
}
This solution prevents the super code smell.
Yes, but you have to change the callers then. Callers will have to call a doExecute() (find a better name for it though) method, which you define in your base class as final, and which calls execute(), then the common code.
Another option is aspect-oriented programming, but I wouldn't recommend it for this purpose, that is, to "hack" code.
The question is: why is changing the name of a method in a 100 or so classes such a problem? It's a click of the mouse with an IDE.
Not that I'm aware of. Next time you should consider that you might want to add some common action for all extended classes, and call for super.execute()!
Only by using something that instruments your code; this isn't possible with pure Java.
Let me state your problem as i understand : Animal class has Breath() method which has implementation and due to inheritance all the subclasses has this member and unless there is very different way of breathing nobody will override.
Now at the end of Breath method you want to call CloseEyes() method of animal class and may be that is true that some or all of the subclasses overrides CloseEyes() method.
So your problem : Everytime any animal breath you want to them to CloseEyes but from Animal class and not from the derived classes.
If there are already CloseEyes() methods in many derived classes then you are actually doing something wrong in calling base class's CloseEyes().
If you still want only base class's method to be called then why do you need same method name- you just say AnimalEyeClose() , make it private and have it in Animal class.
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 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.