Why do we need function overriding?
Why use Method Overriding in java - Stack Overflow
method overriding in Java - Stack Overflow
Please explain how overloading and overriding methods works in Java
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I was asked to write a program on overriding , 3 classes with add methods where one added integers , other string and the other subtracted(name was still add).
If we are gonna write the function definition and another class then what is the purpose of overriding.
And is there any way to just call all the three add methods from the instance of one of the classes?
If you make a new method bark for your Dog class instead of overriding speak, then you also have to add code to detect that youre dealing with a dog, then cast it to a Dog type, then call bark. You get a program that has a bunch of very specific code that has to handle each concrete type individually, and the workflow has to be updated every time you add a new subclass. There is no abstraction or generality.
The idea behind polymorphism is that your program shouldn't have to worry about which objects are dogs and which are cats, it just handles animals, and lets the subclasses take care of the details for themselves. The program stays at a high level and tells the animals to speak, the specific animals' subclasses decide whether to bark or meow. So if you introduce a new animal subclass the code handling animals doesn't change.
You override a method when you have a class hierarchy, as you do, where there is a sensible default behaviour which many of your subclasses will want to use, but some subclasses need different behaviour and you want to call the method using a reference of the base class.
If I had code like:
Dog d = new Dog();
d.speak();
Then just implementing bark() would be fine, because d is always a Dog.
But when I have code like this:
Animal a;
if (...) {
a = new Dog();
} else {
a = new Cat();
}
a.speak();
Then we have to override a method. If the subclasses had a new method (like your bark()) which wasn't present on Animal we wouldn't be able to call it, because a is an Animal -- the actual object it refers to might be a Dog or a Cat.
To answer the question, which is specifically how overriding is implemented in the virtual machine, there's a write up available in Programming for the Java Virtual Machine (Google Books link).
The VM will look for an appropriate method definition in the referenced class, and then work its way up through the inheritance stack. Obviously at some stage various optimisations will apply.
See here for a description of the relevant bytecode instruction invokevirtual:
invokevirtual looks at the descriptor given in , and determines how many arguments the method takes (this may be zero). It pops these arguments off the operand stack. Next it pops objectref off the stack. objectref is a reference to the object whose method is being called. invokevirtual retrieves the Java class for objectref, and searches the list of methods defined by that class and then its superclasses, looking for a method called methodname, whose descriptor is descriptor.
As gustafc has highlighted below, various optimisations can apply, and no doubt the JIT will introduce further.
Method overriding in Java is a concept based on polymorphism OOPS concept which allows programmer to create two methods with same name and method signature on interface and its various implementation and actual method is called at runtime depending upon type of object at runtime. Method overriding allows you to write flexible and extensible code in Java because you can introduce new functionality with minimal code change.
There are few rules which needs to be followed while overriding any method in Java, failure to follow these rules result in compile time error in Java.
- First and most important rule regarding method overriding in Java is that you can only override method in sub class. You can not override method in same class.
- Second important rule of method overriding in Java that name and signature of method must be same in Super class and Sub class or in interface and its implementation.
- Third rule to override method in Java is that overriding method can not reduce accessibility of overridden method in Java. For example if overridden method is public than overriding method can not be protected, private or package-private; But opposite is true overriding method can increase accessibility of method in Java, i.e. if overridden method is protected than overriding method can be protected or public.
- Another worth noting rule of method overriding in Java is that overriding method can not throw checked Exception which is higher in hierarchy than overridden method. Which means if overridden method throws IOException than overriding method can not throw java.lang.Exception in its throws clause because java.lang.Exception comes higher than IOException in Exception hierarchy. This rule doesn't apply to RuntimeException in Java, which is not even need to be declared in throws clause in Java.
- You can not override private, static and final method in Java. private and static method are bonded during compile time using static binding in Java and doesn't resolve during runtime. overriding final method in Java is compile time error. Though private and static method can be hidden if you declare another method with same and signature in sub class.
- Overridden method is called using dynamic binding in Java at runtime based upon type of Object.
- If you are extending abstract class or implementing interface than you need to override all abstract method unless your class is not abstract. abstract method can only be used by using method overriding.
- Always use @Override annotation while overriding method in Java. Though this is not rule but its one of the best Java coding practice to follow. From Java 6 you can use @Override annotation on method inherited from interface as well.
I am trying to make sense of both of these and was wondering if someone had an interesting way of understanding it. It seems like a bizarre thing to have possible.