Related
I'd like to know:
Why can't static methods be overridden in Java?
Can static methods be overloaded in Java?
Static methods can not be overridden in the exact sense of the word, but they can hide parent static methods
In practice it means that the compiler will decide which method to execute at the compile time, and not at the runtime, as it does with overridden instance methods.
For a neat example have a look here.
And this is java documentation explaining the difference between overriding instance methods and hiding class (static) methods.
Overriding: Overriding in Java simply means that the particular method would be called based on the run time type of the object and
not on the compile time type of it (which is the case with overriden
static methods)
Hiding: Parent class methods that are static are not part of a child class (although they are accessible), so there is no question of
overriding it. Even if you add another static method in a subclass,
identical to the one in its parent class, this subclass static method
is unique and distinct from the static method in its parent class.
Static methods can not be overridden because there is nothing to override, as they would be two different methods. For example
static class Class1 {
public static int Method1(){
return 0;
}
}
static class Class2 extends Class1 {
public static int Method1(){
return 1;
}
}
public static class Main {
public static void main(String[] args){
//Must explicitly chose Method1 from Class1 or Class2
Class1.Method1();
Class2.Method1();
}
}
And yes static methods can be overloaded just like any other method.
Static methods cannot be overridden because they are not dispatched on the object instance at runtime. The compiler decides which method gets called.
This is why you get a compiler warning when you write
MyClass myObject = new MyClass();
myObject.myStaticMethod();
// should be written as
MyClass.myStaticMethod()
// because it is not dispatched on myObject
myObject = new MySubClass();
myObject.myStaticMethod();
// still calls the static method in MyClass, NOT in MySubClass
Static methods can be overloaded (meaning that you can have the same method name for several methods as long as they have different parameter types).
Integer.parseInt("10");
Integer.parseInt("AA", 16);
Parent class methods that are static are not part of a child class (although they are accessible), so there is no question of overriding it. Even if you add another static method in a subclass, identical to the one in its parent class, this subclass static method is unique and distinct from the static method in its parent class.
Static methods can not be overridden because they are not part of the object's state. Rather, they belongs to the class (i.e they are class methods). It is ok to overload static (and final) methods.
Overloading is also called static binding, so as soon as the word static is used it means a static method cannot show run-time polymorphism.
We cannot override a static method but presence of different implementations of the same static method in a super class and its sub class is valid. Its just that the derived class will hide the implementations of the base class.
For static methods, the method call depends on the type of reference and not which object is being referred, i.e. Static method belongs only to a class and not its instances , so the method call is decided at the compile time itself.
Whereas in case of method overloading static methods can be overloaded iff they have diff number or types of parameters. If two methods have the same name and the same parameter list then they cannot be defined different only by using the 'static' keyword.
If I m calling the method by using SubClass name MysubClass then subclass method display what it means static method can be overridden or not
class MyClass {
static void myStaticMethod() {
System.out.println("Im in sta1");
}
}
class MySubClass extends MyClass {
static void myStaticMethod() {
System.out.println("Im in sta123");
}
}
public class My {
public static void main(String arg[]) {
MyClass myObject = new MyClass();
myObject.myStaticMethod();
// should be written as
MyClass.myStaticMethod();
// calling from subclass name
MySubClass.myStaticMethod();
myObject = new MySubClass();
myObject.myStaticMethod();
// still calls the static method in MyClass, NOT in MySubClass
}
}
No,Static methods can't be overriden as it is part of a class rather than an object.
But one can overload static method.
Static methods are a method whose single copy is shared by all the objects of the class. A static method belongs to the class rather than objects. since static methods are not dependent on the objects, Java Compiler need not wait till the creation of the objects so to call a static method we use syntax like ClassName.method() ;
In the case of method overloading, methods should be in the same class to overload.even if they are declared as static it is possible to overload them as,
Class Sample
{
static int calculate(int a,int b,int c)
{
int res = a+b+c;
return res;
}
static int calculate(int a,int b)
{
int res = a*b;
return res;
}
}
class Test
{
public static void main(String []args)
{
int res = Sample.calculate(10,20,30);
}
}
But in the case of method overriding, the method in the super class and the method in the sub class act as a different method. the super class will have its own copy and the sub class will have its own copy so it does not come under method overriding.
class SuperType {
public static void classMethod(){
System.out.println("Super type class method");
}
public void instancemethod(){
System.out.println("Super Type instance method");
}
}
public class SubType extends SuperType{
public static void classMethod(){
System.out.println("Sub type class method");
}
public void instancemethod(){
System.out.println("Sub Type instance method");
}
public static void main(String args[]){
SubType s=new SubType();
SuperType su=s;
SuperType.classMethod();// Prints.....Super type class method
su.classMethod(); //Prints.....Super type class method
SubType.classMethod(); //Prints.....Sub type class method
}
}
This example for static method overriding
Note: if we call a static method with object reference, then reference type(class) static method will be called, not object class static method.
Static method belongs to class only.
static methods are class level methods.
Hiding concept is used for static methods.
See : http://www.coderanch.com/how-to/java/OverridingVsHiding
The very purpose of using the static method is to access the method of a class without creating an instance for it.It will make no sense if we override that method since they will be accessed by classname.method()
No, you cannot override a static method. The static resolves against the class, not the instance.
public class Parent {
public static String getCName() {
return "I am the parent";
}
}
public class Child extends Parent {
public static String getCName() {
return "I am the child";
}
}
Each class has a static method getCName(). When you call on the Class name it behaves as you would expect and each returns the expected value.
#Test
public void testGetCNameOnClass() {
assertThat(Parent.getCName(), is("I am the parent"));
assertThat(Child.getCName(), is("I am the child"));
}
No surprises in this unit test. But this is not overriding.This declaring something that has a name collision.
If we try to reach the static from an instance of the class (not a good practice), then it really shows:
private Parent cp = new Child();
`enter code here`
assertThat(cp.getCName(), is("I am the parent"));
Even though cp is a Child, the static is resolved through the declared type, Parent, instead of the actual type of the object. For non-statics, this is resolved correctly because a non-static method can override a method of its parent.
You can overload a static method but you can't override a static method. Actually you can rewrite a static method in subclasses but this is not called a override because override should be related to polymorphism and dynamic binding. The static method belongs to the class so has nothing to do with those concepts. The rewrite of static method is more like a shadowing.
I design a code of static method overriding.I think It is override easily.Please clear me how its unable to override static members.Here is my code-
class Class1 {
public static int Method1(){
System.out.println("true");
return 0;
}
}
class Class2 extends Class1 {
public static int Method1(){
System.out.println("false");
return 1;
}
}
public class Mai {
public static void main(String[] args){
Class2 c=new Class2();
//Must explicitly chose Method1 from Class1 or Class2
//Class1.Method1();
c.Method1();
}
}
It’s actually pretty simple to understand – Everything that is marked static belongs to the class only, for example static method cannot be inherited in the sub class because they belong to the class in which they have been declared. Refer static keyword.
The best answer i found of this question is:
http://www.geeksforgeeks.org/can-we-overload-or-override-static-methods-in-java/
As any static method is part of class not instance so it is not possible to override static method
From Why doesn't Java allow overriding of static methods?
Overriding depends on having an instance of a class. The point of polymorphism is that you can subclass a class and the objects implementing those subclasses will have different behaviors for the same methods defined in the superclass (and overridden in the subclasses). A static method is not associated with any instance of a class so the concept is not applicable.
There were two considerations driving Java's design that impacted this. One was a concern with performance: there had been a lot of criticism of Smalltalk about it being too slow (garbage collection and polymorphic calls being part of that) and Java's creators were determined to avoid that. Another was the decision that the target audience for Java was C++ developers. Making static methods work the way they do have the benefit of familiarity for C++ programmers and were also very fast because there's no need to wait until runtime to figure out which method to call.
Definitely, we cannot override static methods in Java.
Because JVM resolves correct overridden method based upon the object at run-time by using dynamic binding in Java.
However, the static method in Java is associated with Class rather than the object and resolved and bonded during compile time.
I understand the "Cannot make a static reference to the non-static method" error, but I came across with this:
public class MyClass {
public static void main(String[] args) {
MyClass myClassInstance = new MyClass();
while (true) {
myClassInstance.myMethod();
myMethod();//Cannot make a static reference to the non-static method myMethod()
}
}// END main
void myMethod() {
try {
//Stuff
}
} catch (Exception e) {
myMethod();
}
}// END myMethod
}// END MyCLass
I can't just call myMethod() from main but I can do it from inside the method itself (in this case I want to call myMethod() again if an exception happens).
How does this work? is myClassInstance still somehow there because at that point I'm still inside myMethod()?
Would it be better to have static MyClass myClassInstance = new MyClass() at class level and then call myClassInstance.myMethod() every time?
First of all you should know more about static and non-static methods:
A static method belongs to the class
and a non-static method belongs to an
object of a class. That is, a
non-static method can only be called
on an object of a class that it
belongs to. A static method can
however be called both on the class as
well as an object of the class. A
static method can access only static
members. A non-static method can
access both static and non-static
members because at the time when the
static method is called, the class
might not be instantiated (if it is
called on the class itself). In the
other case, a non-static method can
only be called when the class has
already been instantiated. A static
method is shared by all instances of
the class. These are some of the basic
differences. I would also like to
point out an often ignored difference
in this context. Whenever a method is
called in C++/Java/C#, an implicit
argument (the 'this' reference) is
passed along with/without the other
parameters. In case of a static method
call, the 'this' reference is not
passed as static methods belong to a
class and hence do not have the 'this'
reference.
Reference:Static Vs Non-Static methods
How does this work? is myClassInstance still somehow there because at
that point I'm still inside myMethod()?
myMethod is an instance method that belongs to myClassInstance. so when you call myMethod() inside myMethod() it's equivalent to this.myMethod()
Would it be better to have static MyClass myClassInstance = new MyClass() at class level and then call myClassInstance.myMethod() every time?
When we declare a member of a class as static it means no matter how many objects of the class are created, there is only one copy of the static member. A static member is shared by all objects of the class. All static data is initialized to zero when the first object is created, if no other initialization is present. For more information visit Static Variables : Good or Bad?.
I'm beginner in Java and I have a basic question about main class and main method.I try to create method such as addition under the main method . throw the error like "non-static method". what is reason ? thanks...
I guess you using a code like this.
public class TestClass {
public static void main(String[] args) {
doSth();
}
public void doSth() {
}
You cannot call a non-static method from the main class.
If you want to call a non-static method from your main class, instance your class like this:
TestClass test = new TestClass();
test.doSth();
and call the method.
A static method means that you don't need to invoke the method on an instance(Object). A non-static (instance) method requires that you invoke it on an instance. So think about it: if I have a method changeThisItemToTheColorBlue() and I try to run it from the main method, what instance would it change? It doesn't know. You can run an instance method on an instance, like someItem.changeThisItemToTheColorBlue().
More information at http://en.wikipedia.org/wiki/Method_(computer_programming)#Static_methods
The only way to call a non-static method from a static method is to have an instance of the class containing the non-static method. By definition, a non-static method is one that is called ON an instance of some class, whereas a static method belongs to the class itself.
Is like when you try to invoke the non-static method startsWith of class String without an instance:
String.startsWith("Hello");
What you need is to have an instance and then invoke the non-static method:
String greeting = new String("Hello World");
greeting.startsWith("Hello"); // returns true
So you need to create and instance to invoke it.
And to be more clear about Static methods you can refer
https://softwareengineering.stackexchange.com/questions/211137/why-can-static-methods-only-use-static-data
You defined your method without the keyword 'static' I think.
You cannot call a non-static method in a static context such as the main method.
See Java Object Oriented Programming
The main method is a static method, so it does not exist inside an object.
To call non-static methods (methods without the "static" keyword in front of their definitions), you need to create an object of the class, using new.
You can just make the other method static, that will fix the immediate problem. But it may or may not be good Object Oriented design to do this. It would depend on what you were trying to do.
You can't call a non-static method from a static method without instantiation of the class. If you'd like to call another method without creating a new instance (a new object) of the main class you have to use the static keyword for the another method also.
package maintestjava;
public class Test {
// static main method - this is the entry point
public static void main(String[] args)
{
System.out.println(Test.addition(10, 10));
}
// static method - can be called without instantiation
public static int addition(int i, int j)
{
return i + j;
}
}
If you would like to call non static methods you have to instatiate the class, this way creating a new instance, an object of the class:
package maintestjava;
public class Test {
// static main method - this is the entry point
public static void main(String[] args)
{
Test instance = new Test();
System.out.println(instance.addition(10, 10));
}
// public method - can be called with instantiation on a created object
public int addition(int i, int j)
{
return i + j;
}
}
See more:
Static keyword on wikipedia
Static on about.com
I'm still learning about methods in Java and was wondering how exactly you might use an instance method. I was thinking about something like this:
public void example(String random) {
}
However, I'm not sure if this is actually an instance method or some other type of method. Could someone help me out?
If it's not a static method then it's an instance method. It's either one or the other. So yes, your method,
public void example(String random) {
// this doesn't appear to do anything
}
is an example of an instance method.
Regarding
and was wondering how exactly you might use an instance method
You would create an instance of the class, an object, and then call the instance method on the instance. i.e.,
public class Foo {
public void bar() {
System.out.println("I'm an instance method");
}
}
which could be used like:
Foo foo = new Foo(); // create an instance
foo.bar(); // call method on it
class InstanceMethod
{
public static void main(String [] args){
InstanceMethod obj = new InstanceMethod();// because that method we wrote is instance we will write an object to call it
System.out.println(obj.sum(3,2));
}
int f;
public double sum(int x,int y){// this method is instance method because we dont write static
f = x+y;
return f;
}
}
*An instance method * is a method is associated with objects, each instance method is called with a hidden argument that refers to the current object.
for example on an instance method :
public void myMethod {
// to do when call code
}
Instance method means the object of your class must be created to access the method. On the other hand, for static methods, as its a property of Class and not that of its object/instance, it is accessed without creating any instance of the class. But remember static methods can only access static variables, where as instance method can access the instance variables of your class.
Static methods and static variables are useful for memory management as it does not require to declare objects which would otherwise occupy memory.
Example of instance method and variable :
public class Example {
int a = 10; // instance variable
private static int b = 10; // static variable (belongs to the class)
public void instanceMethod(){
a =a + 10;
}
public static void staticMethod(){
b = b + 10;
}
}
void main(){
Example exmp = new Example();
exmp.instanceMethod(); // right
exmp.staticMethod(); // wrong..error..
// from here static variable and method cant be accessed.
}
Instance methods are the methods that require an object to access them where as static methods do not. The method that you mentioned is an instance method since it does not contain static keyword.
Example of instance method:
class main
{
public void instanceMethod()//instance method
{
System.out.println("Hello world");
}
}
The above method can be accessed with an object:
main obj=new main();//instance of class "main"
obj.instanceMethod();//accessing instance method using object
Hope this might help you.
Instance block with Cases { Static , constructor , Local method )
OutPut will be :
This question already has answers here:
Cannot make a static reference to the non-static method
(8 answers)
Closed 5 years ago.
I'm getting an error when I try to call a non-static method in a static class.
Cannot make a static reference to the non-static method methodName() from the type playback
I can't make the method static as this gives me an error too.
This static method cannot hide the instance method from xInterface
Is there any way to get round calling an non-static method in another static method? (The two methods are in seperate packages and seperate classes).
The only way to call a non-static method from a static method is to have an instance of the class containing the non-static method. By definition, a non-static method is one that is called ON an instance of some class, whereas a static method belongs to the class itself.
You could create an instance of the class you want to call the method on, e.g.
new Foo().nonStaticMethod();
Firstly create a class Instance and call the non-static method using that instance.
e.g,
class demo {
public static void main(String args[]) {
demo d = new demo();
d.add(10,20); // to call the non-static method
}
public void add(int x ,int y) {
int a = x;
int b = y;
int c = a + b;
System.out.println("addition" + c);
}
}
public class StaticMethod{
public static void main(String []args)throws Exception{
methodOne();
}
public int methodOne(){
System.out.println("we are in first methodOne");
return 1;
}
}
the above code not executed because static method must have that class reference.
public class StaticMethod{
public static void main(String []args)throws Exception{
StaticMethod sm=new StaticMethod();
sm.methodOne();
}
public int methodOne(){
System.out.println("we are in first methodOne");
return 1;
}
}
This will be definitely get executed. Because here we are creating reference which nothing but "sm" by using that reference of that class which is nothing
but (StaticMethod=new Static method()) we are calling method one (sm.methodOne()).
I hope this will be helpful.
You need an instance of the class containing the non static method.
Is like when you try to invoke the non-static method startsWith of class String without an instance:
String.startsWith("Hello");
What you need is to have an instance and then invoke the non-static method:
String greeting = new String("Hello World");
greeting.startsWith("Hello"); // returns true
So you need to create and instance to invoke it.
It sounds like the method really should be static (i.e. it doesn't access any data members and it doesn't need an instance to be invoked on). Since you used the term "static class", I understand that the whole class is probably dedicated to utility-like methods that could be static.
However, Java doesn't allow the implementation of an interface-defined method to be static. So when you (naturally) try to make the method static, you get the "cannot-hide-the-instance-method" error. (The Java Language Specification mentions this in section 9.4: "Note that a method declared in an interface must not be declared static, or a compile-time error occurs, because static methods cannot be abstract.")
So as long as the method is present in xInterface, and your class implements xInterface, you won't be able to make the method static.
If you can't change the interface (or don't want to), there are several things you can do:
Make the class a singleton: make the constructor private, and have a static data member in the class to hold the only existing instance. This way you'll be invoking the method on an instance, but at least you won't be creating new instances each time you need to call the method.
Implement 2 methods in your class: an instance method (as defined in xInterface), and a static method. The instance method will consist of a single line that delegates to the static method.
The only way to call a non-static method from a static method is to have an instance of the class containing the non-static method.
class A
{
void method()
{
}
}
class Demo
{
static void method2()
{
A a=new A();
a.method();
}
/*
void method3()
{
A a=new A();
a.method();
}
*/
public static void main(String args[])
{
A a=new A();
/*an instance of the class is created to access non-static method from a static method */
a.method();
method2();
/*method3();it will show error non-static method can not be accessed from a static method*/
}
}
There are two ways:
Call the non-static method from an instance within the static method. See fabien's answer for an oneliner sample... although I would strongly recommend against it. With his example he creates an instance of the class and only uses it for one method, only to have it dispose of it later. I don't recommend it because it treats an instance like a static function.
Change the static method to a non-static.
You can't get around this restriction directly, no. But there may be some reasonable things you can do in your particular case.
For example, you could just "new up" an instance of your class in the static method, then call the non-static method.
But you might get even better suggestions if you post your class(es) -- or a slimmed-down version of them.
The easiest way to use a non-static method/field within a a static method or vice versa is...
(To work this there must be at least one instance of this class)
This type of situation is very common in android app development eg:- An Activity has at-least one instance.
public class ParentClass{
private static ParentClass mParentInstance = null;
ParentClass(){
mParentInstance = ParentClass.this;
}
void instanceMethod1(){
}
static void staticMethod1(){
mParentInstance.instanceMethod1();
}
public static class InnerClass{
void innerClassMethod1(){
mParentInstance.staticMethod1();
mParentInstance.instanceMethod1();
}
}
}
Note:- This cannot be used as a builder method like this one.....
String.valueOf(100);
I use an interface and create an anonymous instance of it like so:
AppEntryPoint.java
public interface AppEntryPoint
{
public void entryMethod();
}
Main.java
public class Main
{
public static AppEntryPoint entryPoint;
public static void main(String[] args)
{
entryPoint = new AppEntryPoint()
{
//You now have an environment to run your app from
#Override
public void entryMethod()
{
//Do something...
System.out.println("Hello World!");
}
}
entryPoint.entryMethod();
}
public static AppEntryPoint getApplicationEntryPoint()
{
return entryPoint;
}
}
Not as elegant as creating an instance of that class and calling its own method, but accomplishes the same thing, essentially. Just another way to do it.
It is not possible to call non-static method within static method. The logic behind it is we do not create an object to instantiate static method, but we must create an object to instantiate non-static method. So non-static method will not get object for its instantiation inside static method, thus making it incapable for being instantiated.
Constructor is a special method which in theory is the "only" non-static method called by any static method. else its not allowed.
You can call a non static method within a static one using:
Classname.class.method()