is re-defining a non-static method in a subclass with the same everything but as static overriding or hiding it ?
http://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html says hiding. but when i declare the superclass method as final, i get an override error.
superclass declaration is
final static void display() { ... }
subclass:
void display() { ... }
gives override error.
Is re-defining a non-static method in a subclass with the same everything but as static overriding or hiding it?
It's neither, because doing so triggers a compilation error, rendering your program invalid.
class A {
void x();
}
class B extends A {
// ERROR!!!
static void x();
}
Hiding happens when both methods in the pair are static methods; overriding happens when both methods in the pair are instance methods. When one of the two is a static method and the other one is an instance method, Java considers it an error. It does not matter if the instance method is final or not; it also does not matter if the static method is in the base or in the derived class: Java calls it an error either way.
The compiler message that says "cannot override" is misleading, though: I think that "name collision" would have been a better name for such conditions, because "overriding" is reserved for situations with two instance methods.
The method you describe is an instance method, not a static method. You cannot hide instance methods, only static methods. An instance method declared final cannot be overridden in a subclass, and this is what you are trying to do.
final static void display() { ... }
The above method is having non-access modifier final, and a method which has been made final can't be overridden.
How can you override a method that is final.
The final methods can never be overrided in the subclass.
That is the compilation error..
You can't override a final method...
Ex:
class Super
{
final void display()
{
//do something
}
void show()
{
//Do Something
}
}
class Sub extends Super
{
//Not Valid hence Compile Error
void display()
{
//do something
}
//Valid
void show()
{
//Do Something
}
}
Related
Please have a look at following code.
class TestClass{
private void privMethod()
{
System.out.println("TestClass Method");
}
public static void main(String... args)
{
TestClass obj=new SubClass();
obj.privMethod();
}
}
class SubClass extends TestClass
{
private void privMethod()
{
System.out.println("SubClass Method");
}
}
when I compile this code its gets compiled fine.
when I run TestClass, the output is: TestClass method
The ref. variable obj is of TestClass type but it is referring to a Sub class' object so at run time it should bind with Sub's privMethod().
Can you explain this?
private methods are not overriden by methods in a child class. Only public, protected and package-private ones can be overriden. Therefore polymorphism behavior at runtime is not applied.
If you add the annotation #Override to the declared method in the subclass, you would get a compilation error:
#Override // error
private void privMethod()
{
System.out.println("SubClass Method");
}
private modifier—the field/method is accessible only within its own class.
For more on access specifier.
So although the SubClass extends TestClassbut privMethod() of TestClass is not visible to the SubClass at all. So when you are implementing the privMethod() method in SubClass, it is not overriding the method of TestClass.
To make sure that you are overriding the method of the superclass, you can use #Override Java annotation.If the method does not match a method in the superclass, the compiler will give you the below error
The method privMethod() of type SubClass must override or implement a supertype method
I am confused by the behaviour of the static method when it is overridden in the subclass.
Below is the code:
public class SuperClass {
public static void staticMethod() {
System.out.println("SuperClass: inside staticMethod");
}
}
public class SubClass extends SuperClass {
//overriding the static method
public static void staticMethod() {
System.out.println("SubClass: inside staticMethod");
}
}
public class CheckClass {
public static void main(String[] args) {
SuperClass superClassWithSuperCons = new SuperClass();
SuperClass superClassWithSubCons = new SubClass();
SubClass subClassWithSubCons = new SubClass();
superClassWithSuperCons.staticMethod();
superClassWithSubCons.staticMethod();
subClassWithSubCons.staticMethod();
}
}
Below is the output which we are getting :
1) SuperClass: inside staticMethod
2) SuperClass: inside staticMethod
3) SubClass: inside staticMethod
Why static method of superclass gets called here in the second case?
If method is not static, then according to polymorphism, method of the subclass is called when subclass object is passed on runtime.
static method resolution is always based on the Reference type.
The code
superClassWithSuperCons.staticMethod();
superClassWithSubCons.staticMethod();
subClassWithSubCons.staticMethod();
is converted to this after compilation
SuperClass.staticMethod();
SuperClass.staticMethod();
SubClass.staticMethod();
Accroding to this it is the call to SuperClass method not the subclass method.So you are getting the output of SuperClass method.
A method declared static cannot be overridden but can be re-declared. That's the answer.
A static method is not associated with any instance of a class so the concept is not applicable. Try the same with not static and you'll see the difference.
Your question is duplicated actually:
Why doesn't Java allow overriding of static methods?
Interesting question. I'm not familiar with the underlying mechanisms, but it seems that for static methods, the declared type (in your middle example, SuperClass), not the actual type SubClass is considered for resolving the method call. It actually makes sense because you're not looking at the actual instance of an object when calling a static function.
Static methods are redefined not overridden...
If you try to override the static method then it will be treated as a non overridden method of the sub class.
Static methods will be called based on the type of the reference not the object created.
For more information visit the page.
http://javaunturnedtopics.blogspot.in/2016/07/static-methods-are-redefined-not.html
So I know that in Java when you have a static method you are supposed to call it with the format ClassName.method() rather than use the same structure as you would for instance methods, namely:
ClassName myObject = new ClassName();
myObject.method();
However, if you were to do it this way it would still be valid code and would work. Let's say I decide to do this where the method in question is static, and have the following setup:
public SuperClass {
public static int foo(int x) {
return x;
}
}
public SubClass extends SuperClass {
public static int foo(int x) { // Overriding foo() in SuperClass
return x + 1;
}
}
public MyDriver {
public static void main(String[] args) {
SuperClass myObject = new SubClass(); // Upcasting.
System.out.println(myObject.foo(5)); // This should polymorphically print 6
}
}
What prints out on the screen, however, is 5 rather than 6. Why?
Static methods apply only to the class they are defined in, and they cannot be overridden.
When you call myObject.foo(5), you are calling SuperClass.foo(5) in reality, because you declared myObject as a SuperClass, regardless of whether you instantiated it as one.
The proper way to call a static method is to call it directly from the class it is declared in, so if you wanted to call SubClass.foo(), you must call it from an explicitly declared SubClass instance (meaning no upcasting), or you need to call SubClass.foo() like so.
The simple answer is that calling static methods from instances evaluates to calling those same methods from the declared type with no instance rather than the instance type.
I am not certain of this, but I would not be surprised if when the code is compiled into byte-code, that the instance static method call would actually be compiled into a direct call to the declared type.
Edit: An upvote brought my attention back to this and I cleaned up the explanation to make it more clear and fix some negligible grammatical mistakes on my part. I hope this helps future readers.
Using instances of a class to call that class's static methods is something you should avoid, since it can cause confusion. If you need to call any method polymorphically, make it an instance method. You cannot polymorphically call a static method. The reason the SuperClass invocation is called is because that is the apparent class of myObject at compile-time. This effect can also be seen in the following scenario:
public void doSomething(SuperClass param) {
System.out.println("SuperClass");
}
public void doSomething(SubClass param) {
System.out.println("SubClass");
}
public void test() {
SuperClass myObject = new SubClass();
doSomething(myObject);
}
If test() is called, SuperClass will be printed.
Static methods don't depends on the instance, they belong to the class and only to the class, in fact if you have a static method you'll always access to one unique instance always.
myObject.foo(5)
is only a shortcut, what you really are doing is
SuperClass.foo(5)
Static methods are treated as global by the JVM, they are not bound to an object instance at all. By the way, you can only overload static methods, but you can not override. So check for "Oracle Documentation for Overriding and Hiding Documents".
Defining a Method with the Same Signature as a Superclass's Method:
-----------------------------------------Superclass Instance MethodSuperclass Static Method
Subclass Instance Method Overrides Generates a compile-time error
Subclass Static Method Generates a compile-time error Hides
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Can we override static method in Java?
We cannot override the static methods of the base class.
Actually I tried something like this:
// Base class
public class StaticExampleImpl {
protected String name="overriding";
public static void display(){
System.out.println("static method display : base class");
}
}
Then the derived class is as follows:
//derived class
public class StaticDemo extends StaticExampleImpl {
// cannot override the static methods...
//#Override
public static void display(){
System.out.println("child!!! static method display");
}
public static void main(String[] args) {
StaticDemo d=new StaticDemo();
d.display(); // derived class display is called rather than Base class.
}
}
So, when I uncomment the #Override method, it gives error as "Static methods cannot be overriden". But with commenting it works fine. So, when we create the Objects and call the static methods with the instances, those work fine. so what is the difference??
because static methods are not get inherited.
When you uncomment #Override it means you are trying to override the
static method which is not possible thats why you are getting an
error.
But when you comment //#Override it means you are declaring a new
method in child class.
Static methods does not belong to an instance of a class, it belongs to the actual class.
When you call d.display();, you are really calling the static method of the StaticDemo d reference's static method.
if you did :
StaticExampleImpl d2 = new StaticDemo();d2.display(), you will find that it calls the base class's display.
However, don't do this. It leads to confusing code, and is a bad way to implement inheritance poorly.
Overriding depends the an instance of a class. polymorphismis that you can subclass a class and the objects implementing those subclasses will have different behaviors for those method defined in the superclass (and overridden in the subclasses) .static methods does not belong to an instance of a class so the concept is not applicable.
Static methods cannot be inherited. If you want to call the 'base' class static method, you have to explicitely call StaticExampleImpl.display().
Static methods are bound to class they can't be inherited thats why you can't have base class static method in derived class.
If you are trying to override a static method, there is probably something wrong with your design.
OOP and Polymorphism allows you to do the following:
public class MyClass1 {
public String toString() { return "MyClass1 Instance"; }
}
public class MyClass2 extends MyClass1 {
#Override
public String toString() { return "MyClass1 Instance"; }
}
public void printSomething(MyClass1 myclass1){
System.out.println(myclass1);
}
Inside printSomething, the toString method which is going to be called is the one on the runtime type of myClass1: when you pass inside printSomething an instance of MyClass2, its compile-type will be MyClass1 but its runtime type will be MyClass2
It is clear that to use polymorphism you need objects instances, where the actual runtime type could different from the compile type. Static methods however do not belong to any object instance, but to the class. Why don't you explain us what you are trying to achieve?
The following code:
StaticExampleImpl one = new StaticExampleImpl();
StaticDemo two = new StaticDemo();
StaticExampleImpl three = two;
one.display();
two.display();
three.display();
Will yield the following output:
static method display : base class
child!!! static method display
static method display : base class
As you can see, the method does not get inherited. This is why they are called 'static methods': they are called statically, not dynamically, as instance methods would be. The compile-time type of the class is what matters when calling static methods, not the runtime type.
This is also why you shouldn't call static methods through object instances. Always call them like this:
StaticExampleImpl.display();
StaticDemo.display();
This completely takes away the confusion that might (will) come up when people expect inheritance to work for these methods.
any static block in java, may be static variables, methods are loaded when the class is loaded. You probably know about class loader in java. So thing is static block (methods, variables or anything is static) is loaded once. So you can’t actually override any static block.
Commenting #Override means that you are writing another static method in sub class, but not just overriding base class method.
why should we widen the accessibility of overridden methods ? If the super class has a protected method and subclass has same method with public. Why should happen?
It's a different method! Subclasses don't inherit private methods! So you're not "overriding" at all. You are simply DEFINING a method with the same name as the private method in the superclass.
class A
{
private void myMethod() { }
}
class B extends A
{
public void myMethod() { } // a completely different method. Has nothing to do with the above method. It is not an override.
}
Because in an object hierarchy, JVM will always run the Overriden method. If your overriden method is not accessible, then it is useless.
public class A{
void A(){}
}
public class B extends A{
private void A(){} //this makes no sence and its impossible
PSV main(String ..){
A a = new B();
a.A(); //error as JVM cannot call overriden method which is private.
}
}
Methods declared as private or static can not be overridden!
Annotation #Override indicates that a method declaration is intended to override a method declaration in a superclass. If a method is annotated with this annotation type but does not override a superclass method, compilers are required to generate an error message.
Use it every time you override a method for two benefits. This way, if you make a common mistake of misspelling a method name or not correctly matching the parameters, you will be warned that you method does not actually override as you think it does. Secondly, it makes your code easier to understand because it is more obvious when methods are overwritten.
And in Java 1.6 you can use it to mark when a method implements an interface for the same benefits.