Say I have an abstract class called "Subsystem.java" and I have a class that extends it called "Drive.java". In Subsystem.java, I have a method called initialize with no arguments. How do I Override that method in Drive.java and add arguments? In other words, I want to be able to override a method with no arguments into one that has arguments. Is this even possible? Thanks.
No, that is not an override, but an overload. You still couldn't create an instance from Drive, since it would have an unimplemented abstract method.
Overrides have the exact same signatures.
Overloads have the same name, but different parameter types.
You can not override as like you describe.
When a method in a subclass has the same name, same parameters or signature and same return type(or sub-type) as a method in its super-class, then the method in the subclass is said to override the method in the super-class.
you can refer below link for more information about overriding a method in Java.
Method overriding in Java
or you can also refer Oracle Java documentation
Oracle Java document
Related
I want to find the contents of the java.net.ServerSocket accept() method (because I am trying to override it so it returns a different type). Is this possible in Eclipse, or for that matter, in Java?
Thanks.
No, it isn't possible in Java. Because, you can not modify the return type of a function you override. The Java Tutorials Overriding and Hiding Methods says (in part)
An instance method in a subclass with the same signature (name, plus the number and the type of its parameters) and return type as an instance method in the superclass overrides the superclass's method.
I recommend you use the #Override annotation to protect yourself from acidentally overloading a method you intend to override.
let's say we have this method
public class Animal {
public void eat() { }
}
and on another class that extends Animal has this method
public String eat(){}
is this considered as method overriding? because I've heard that you can use different return types in method overriding providing they have the same method arguments
This is neither overloading , nor overriding ! It is compilation error .
In Java it is possible to define two or more methods within the same class that share the same name, as long as their parameter declarations are different. When this is the case, the methods are said to be overloaded, and the process is referred to as method overloading.
Following are rules of method overriding in java which must be followed while overriding any method. private, static and final method can not be overridden.
1)Method signature must be same including return type, number of method parameters, type of parameters and order of parameters.
2)Overriding method can not throw higher Exception than original or overridden method. This rule only applies to checked Exception in Java, overridden method is free to throw any unchecked Exception.
3)Overriding method can not reduce accessibility of overridden method , means if original or overridden method is public than overriding method can not make it protected.
You can only change the return type to a subclass of the original return type.
All answers are correct from a technical point of view because Void is a class in Java that no other class can extend and when methods are overridden the return-type has to be a sub-type of the original type (parentReturnType.isAsignableFrom(overriddenReturnType)).
But from a practical point of view the question is still valid and it would have made sense to make an excuse for the special case void. The only value a variable of the type Void can hold is null. Further no variable can have the primitive type void. Finally you can not compile this code:
Void result = object.notify();
So in the end it would have made sense to specify Java differently when 1.5 was introduced and allow overriding void return types with any other type.
Therefore the only reasonable answer is IMHO it is not possible because Java was designed such that this is not allowed.
This not correct Code, as two methods with same name should return same value, only arguments can be differents
Overriding is having same method with same arguments in the subclass
Overloading having same method name with different argumens
The return type of a method is not part of that method's signature.
When you call eat() on the subclass, java looks at the signature you provided (eat with no parameters) and starts looking for a method with this signature, starting in the class of the instance type of the object you call it on (and searches through its ancestors if it fails to find it).
So, methods called on a subclass will always run the overridden methods (they're found first).
A critical 'point' of OO (specifically polymorphism) is that a subclass can be passed in where the superclass was expected.
If a method wants to call the superclass' eat() method and get a void, but you passed a subclass into that method of which eat() returned a String; you can see that method will call the subclass' eat and get an unexpected value.
You can see why this could be a problem. :I
So, overriding methods must return the same type or a subclass of the type returned by the parent.
(Similar principles apply as to why you can return a subclass; I'll let you think about it c: )
It's neither overriding nor overloading, it creates compilation error.
Two methods with same name, different param numbers or different type of params known as overload method, return type may be same or different. It may be in same class or parent, child classes.
Two methods having same param numbers, same types, same return types but one is in parent class and other is in child class are known as overrided methods.
What is the difference between abstract methodand method overriding in java? Because same result can be found by using method overriding. so what is the necessity of abstract method.
An abstract method forces a programmer who inherits the method to define an implementation for it, i.e. the class says "I will need an implementation of this function, which the concrete implementation of this class must provide".
This is in contrast to overriden methods, which allow (rather than require) the inheriting class to change the implementation of the method for objects of that class.
In particular, overridable methods (or "virtual" methods) have a basic implementation (which can itself be empty) that the overriding method can call.
The abstract method needs to be implemented in order for the subclass to be utilized. You must also implement non-abstract methods, for instance, in the case of interface method signatures.
I think it is correct to say that you implement abstract methods in the same way that you implement method stubs of an interface. Overriding is not the same as implementing because you can override methods which are not stubs. Subclasses often override methods where the superclass is neither an interface nor an abstract class. So the term overriding has a broader meaning than implementation.
You may find this tutorial helpful as it has quite a clear definition.
http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html
Hope that helps!
The two have different usages which can't be compared.
An Abstract Class is created only to be sub classed by some other class. The implementing class necessarily has to implement all the abstract methods declared inside it. However Method Overriding is the ability of a subclass to override a method from an inherited superclass whose behavior is "close enough" and then to modify behavior as needed. An Abstract Method is one that is declared without an implementation and needs to be necessarily implemented in the subclass.
When the parent class have an add method with 2 parameters, If we add new add method with 3 parameters in child class, shall we call this as over loading?
Thanks in advance.
Yes, since the method with two parameters is inherited by the subclass, the method with three parameters is said to be an overloading method.
class A
add(param1, param2)
class B
add(param1, param2) <-- inherited
add(param1, param2, param3) <-- overloading the above method
A quote from the official trail on Overriding and Hiding Methods:
Note: In a subclass, you can overload the methods inherited from the superclass. Such overloaded methods neither hide nor override the superclass methods—they are new methods, unique to the subclass.
(As you probably already figured out, the method with three classes is not an overriding method.)
Yes, this is overloading. It would be overloading even if the method were in the same class as the method with two parameters.
Note that when there are different numbers of parameters (and no varargs parameters) overloading is reasonably simple. It gets a lot more complicated when you have methods with the same number of parameters - at that point the compiler has to choose the "best" method out of applicable candidate methods.
Also note that overloading is determined at compile time whereas which override is executed is determined at execution time based on the actual type of the object the method is called on.
Overloading is when methods have the SAME NAME but DIFFERENT SIGNATURE.
Overriding - when methods have IDENTICAL NAMES and IDENTICAL SIGNATURE.
Yes definitely a overloading and a nice feature of inheritance.
If the method name already exists but the parameters are different, then yes, this is overloading.
yep... it is overloading method even if the method is located in the same class.
ps: i assume the new method with three parameters has identical name and return type
This is a very naiveish question, but here goes:
An overriden method from a base class will mean that calls to the sub class will call the derived, overriden method, correct?
Thus, if there is no override annotation, the method in the base class will be called. So the override method will serve purely to document the intent - call one version of a method over the other.
Is this the case?
This leads me to the following question:
What is the difference between an abstract class which 5-6 classes may derive from but the methods inherited in the derived classes are not overriden, and one class (Static or not being irrelevant), used by those 5-6 classes?
The #Override annotation is intended ONLY to catch errors at compilation time. It does not affect override behavior at runtime. The idea is that you give the compiler the chance to inform you that your method name or signature is wrong.
The override annotation is simply a marker to show the the method overrides a superclass method.
It being there has no effect at runtime. See here:
http://download.oracle.com/javase/1.5.0/docs/api/java/lang/Override.html
Using abstract is an architectural design to ensure that users either must implement an abstract method (declared but not implemented in the base class) or that they all have access to the same base methods and data (this is the same as the latter example you gave).
Thus, if there is no override
annotation, the method in the base
class will be called.
No. The overriding method in the derived class will always be called.
So the override
method will serve purely to document
the intent - call one version of a
method over the other.
Is this the case?
No. As per the other answers, it tells the compiler to insist that there was something to override.