I know that we cannot override static methods in Java, but can someone explain the following code?
class A {
public static void a() {
System.out.println("A.a()");
}
}
class B extends A {
public static void a() {
System.out.println("B.a()");
}
}
How was I able to override method a() in class B?
You didn't override anything here. To see for yourself, Try putting @Override annotation before public static void a() in class B and Java will throw an error.
You just defined a function in class B called a(), which is distinct (no relation whatsoever) from the function a() in class A.
But Because B.a() has the same name as a function in the parent class, it hides A.a() [As pointed by Eng. Fouad]. At runtime, the compiler uses the actual class of the declared reference to determine which method to run. For example,
B b = new B();
b.a() //prints B.a()
A a = (A)b;
a.a() //print A.a(). Uses the declared reference's class to find the method.
You cannot override static methods in Java. Remember static methods and fields are associated with the class, not with the objects. (Although, in some languages like Smalltalk, this is possible).
I found some good answers here: Why doesn't Java allow overriding of static methods?
That's called hiding a method, as stated in the Java tutorial Overriding and Hiding Methods:
If a subclass defines a class method with the same signature as a class method in the superclass, the method in the subclass hides the one in the superclass.
static methods are not inherited so its B's separate copy of method
static are related to class not the state of Object
You didn't override the method a(), because static methods are not inherited. If you had put @Override, you would have seen an error.
A.java:10: error: method does not override or implement a method from a supertype
@Override
^
1 error
But that doesn't stop you from defining static methods with the same signature in both classes.
Also, the choice of method to call depends on the declared type of the variable.
B b = null;
b.a(); // (1) prints B.a()
A a = new B();
a.a(); // (2) prints a.a()
At (1), if the system cared about the identity of b, it would throw a NPE. and at (2), the value of a is ignored. Since a is declared as an A, A.a() is called.
Your method is not overridden method. you just try to put @Override annotation before your method in derived class. it will give you a compile time error. so java will not allow you to override static method.
While goblinjuice answer was accepted, I thought the example code could improved:
public class StaticTest {
public static void main(String[] args) {
A.print();
B.print();
System.out.println("-");
A a = new A();
B b = new B();
a.print();
b.print();
System.out.println("-");
A c = b;
c.print();
}
}
class A {
public static void print() {
System.out.println("A");
}
}
class B extends A {
public static void print() {
System.out.println("B");
}
}
Produces:
A
B
-
A
B
-
A
If B had overridden print() it would have write B on the final line.
Static methods will called by its Class name so we don't need to create class object we just cal it with class name so we can't override static
for example
class AClass{
public static void test(){
}
}
class BClass extends AClass{
public static void test(){}
}
class CClass extends BClass{
public static void main(String args[]){
AClass aclass=new AClass();
aclass.test(); // its wrong because static method is called
// by its class name it can't accept object
}
}
we just call it
AClass.test();
means static class can't be overridden if it's overridden then how to cal it .
Static members belong to class not to any objects. Therefore static methods cannot be overriden. Also overiding happens at run time therefore compiler will not complain.
Howeve, you can add @Override annotation to method. This will flag compiler error.
