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Java : If A extends B and B extends Object, is that multiple inheritance
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Closed 3 years ago.
All classes in Java are inherited from Object class by default.
Then how the class inherited can inherit other classes?
Java do not support Multiple inheritance, right?
Classes inherit from at least and at most one class,
Either from Object implicitly (without writing extends), or from other class explicitly (as extends YourParentClass)
Excepting Object, which has no superclass, every class has one and only one direct superclass (single inheritance). In the absence of any other explicit superclass, every class is implicitly a subclass of Object
Because each class inherit from Object or from class inherit from Object, each class still inherit from Object, for example toString() method
If your custom class inherit from different class, still at the end of the hierarchy, the parent class will be Object, notice that classes hierarchy isn't multiple inheritance:
The Object class, defined in the java.lang package, defines and implements behavior common to all classes—including the ones that you write. In the Java platform, many classes derive directly from Object, other classes derive from some of those classes, and so on, forming a hierarchy of classes
At the top of the hierarchy, Object is the most general of all classes. Classes near the bottom of the hierarchy provide more specialized behavior.
As you already correctly stated, all classes in Java are inherited from Object class by default. That means that any class that does NOT have an extends clause defined, implicitly has written extends Object. Example
class Animal // implicitly written extends Object
{}
On the other hand, if a class has an extends clause, then it does not directly inherit from Object. But its parent class inherits from object. As inheritance is transitive, therefore the class also inherits the functions of Object. Example
class Dog extends Animal {}
class Animal // implicitly written extends Object
{}
This means Dog is an Animal and Animal is an Object. Therefore Dog is an Object. You could write Object o = (Object) new Dog(); and that would totally be fine.
This can go arbitrarily deep. For example you could now have a class Poodle which inherits Dog which inherits Animal which inherits Object. Therefore a Poodle is an Object.
By using inheritance , a child class will inherit object class through parent class.
What you are talking about is known as - Multilevel inheritance.
Multilevel inheritance refers to a mechanism in OO technology where one can inherit from a derived class, thereby making this derived class the base class for the new class.
i.e. - A is Object class, B is the base class that you want to inherit and C is your child class.
btw Java 8 has introduced default method to handle basic multiple inheritance problem ( though I don't agree that it solves the actual problem) - check default methods in Java 8
Related
I was reading an article on Inheritance . Few facts were listed on Inheritance but one point i couldn't understand what is Default superclass and its explanation. What is Default superclass?
Default Superclass : Except Object class, which has no superClass, every class has one and only one direct superclass(single inheritance). In the absence of any other explicit superclass, every class is implicitly a subclass of the Object class.
The default superclass is Object (more precisely java.lang.Object). If a class does not define a direct superclass explicitly (via extends), then Object is implicitly a superclass of that class.
Have a look at the following example which graphically shows this:
public class A {}
public class B extends A {}
public class C {}
Note that his rule does not apply to Object itself since this would produce a cyclic inheritance. In other words, java.lang.Object is the root of the class hierarchy.
Except Object class, which has no Super Class, every class has one and only one direct superclass(single inheritance). In the absence of any other explicit superclass, every class is implicitly a subclass of the Object class
Points to understand first:
There is a class Object in java which is already available with the JRE libraries.
When you define a class without an extends keyword, your class will by default extend the Object class in Java.
Even if you extend another class in your new class, the parent or its parent transitively inherits the Object class.
Simple way to understand - after you define a class with / without a parent class, create an object for it. If you are using an IDE, you can see that there are some method suggestions which is not implemented in your class (or parent ). Methods like clone() equals() hashCode() wait() etc. Where did these methods/ behaviors come from to your object ? - Yes it came from the ultimate parent Object
The default inheritance is implicit and handled by the Java itself. Hope this makes your understanding better.
object class is base class for every class you create.
when you create an object of your class then constructor of object class is called.
The Object class in Java is the default superclass. Object class is inherited into a newly created class by default if it does not explicitly inherited from any other class. So every class that you create in Java programming is inherently a child class of the Object.
All classes in Java extend Object class by default, and they are subclass of object class, the only exception of this convention is the Object class itself, object class does not extend any class by default, this is the whole idea.
This question already has answers here:
Java Multiple Inheritance
(17 answers)
Closed 7 years ago.
For eg:
Case 1: Does every Java class we create will extend Object class and if so then how come we an able to extend one more class to our class may be silly question just got it in mind i am posting this question
Pretty simple:
The hierarchy of java-classes is tree-like, with Object as root. If a class doesn't extend any other class it directly extends Object by default. So what about classes that extend some other class, like class A extends B. Now A extends B and Object. But this is not multiple inheritance. A extends B, thus A aswell extends any superclass of B, including Object. Just imagine the inheritance this way:
Object
/ | \
B ... SomeObject
|
A
Yes every class extends Object. However, Object is not necessarily the direct superclass. If a class Child extends Parent, then Child's direct superclass is Parent. In this case, the class hierarchy is:
Child < Parent < Object
So there is no multiple inheritance for Child. Object is inherited transitively by its direct superclass Parent.
No offense, but your english makes your question very difficult to read. I think I understood it though. Multiple inheritance is not the contrary of inheriting only from one class. You can inherit from several classes and still not have multiple inheritance.
For example, C inherit from A and B if A extends B, B extends C. However, the inheritance is a linear chain here, whereas multiple inheritance allows for trees, for example A extends B, A extends C. This is different, as you see. In java, you can inherit from as many classes as you like by transitivity but directly extend only one class.
Multiple inheritance is when you try to inherit more than a class at the same time,
I mean like
class Foo extends Faa, Fii {
}
that is not posible in Java but we can have this
class Faa{
}
class Fii extends Faa{
}
class Foo extends Fii{
}
that is not the same, in that case we have 2 parents (i mean Fii because we are extending from Fii and Faa because Fii extends from it)
All classes inherit from java.lang.Object, although extends Object is (generally) not written out anywhere. How is this possible?
if you don't explicitly write extends Object the compiler does it for you. So knowing that a class can only extend one super class, the compiler will look at the hierarchy and extend the highest super class to Object. So every class will directly or indirectly inherit the Object class.
The Object class however is a special case because it doesn't extend anything.
Lastly if you were to compile a simple class and decompile it, you will see the compiler inserts extends
java.lang.Object (or
the bytecode equivalent)
into the class
The Object is implicitly direct/indirect super class of all class.
From Oracle Java doc:
Definitions: A class that is derived from another class is called a
subclass (also a derived class, extended class, or child class). The
class from which the subclass is derived is called a superclass (also
a base class or a parent class).
Excepting Object, which has no superclass, every class has one and
only one direct superclass (single inheritance). In the absence of any
other explicit superclass, every class is implicitly a subclass of
Object.
I have confusion in java inheritance. As I studied Java does not support Multiple Inheritance. So that it does not have diamond problem of inheritance at all.
But, every class of Java inherits Object class by default, and if we extend a class then there are two inherited classes first is Object and second is our inherited class. As I know if any class inherits two or more classes, its known as Multiple Inheritance. It shows that java supports multiple inheritance because its inheriting two classes at same time.
Then why its said that java does not support multiple inheritance and is there any possibility to have diamond problem in java?
Since all classes extend Object, the base class you're extending is by definition already extending Object. Your class doesn't need to add Object itself since it already exists in the hierarchy.
That means, Object will still only exist once in your hierarchy as the very root, so no "diamond".
See this
OBJECT CLASS
|
|
CLASS A{} //Class A extends class Object
|
|
CLASS B{} //Class B extends class A
|
|
CLASS C{}//Class C extends class B
Since every Class is inherited from Object class it is not in the case When you inherit from other class.Thus the object class will become super or the root class for all other inherited classes.
So whats the problem?
But, every class of Java inherits Object class by default, and if we extend a class then there are two inherited classes first is Object and second is our inherited class
That's partially correct.
If you extend a class explicitely, then it won't extend Object class.
if you have a class
class A
{
}
then, compiler will change it into
class A extends Object
{
}
But, if you extends a class explicitely, as following,
class A extends AnotherClass
{
}
compiler won't add anything now, hence No multiple inheritance.
You have misunderstood the concept of Class extending Object in case of Inheritance.
Only the top level class in the inheritance hierarchy, extends from Object class, and rest of the class lower in the hierarchy, extend the immediate super class. Thus they have all the methods of Object class through this hierarchy of inheritance, and there is no multiple inheritance involved.
public class A { // Extends from `Object` class
}
class B extends A { // Extends from `A`
}
class C extends B { // Extends from `B`
}
So, there will be no diamond problem in the hierarchy.
Java supports only syntactical multiple inheritance. Java does not
supports implementation of multiple inheritance.
Some people says java supports multiple inheritance through interfaces but it's not correct here the explanation:
inheritance ::
Getting the properties from one class object to another class object:
Class A{}
Class B extends A {}
Here an object of class B getting the properties(methods/functions/ & data members/class variables)
of class A.
Why java does not supports multiple inheritance using classes:
Class A{}
Class B{}
Class C extends A,B{}
X--this statement causes error because Class A getting object of object class from both sides A and B...
Every java class by default extending Object of object class and Object of object class is the root object means super class for all classes.
But here Class C having two super classes of object so giving error means java does not support multiple inheritance by using classes.
Is java supports multiple inheritance using Interfaces:
Because of this interface concept only few of us saying that java supports multiple inheritance but it is wrong.
Here the explanation::
interface A{}
interface B{}
interface C implements A , B{}
(or)
interface A{}
interface B{}
Class C implements A , B{}
Here its look like multiple inheritance but inheritance means getting the properties from one class object to another class object.
Here in interface concept we are not at all getting any properties rather we are implementing the unimplemented methods of interface in class.
So inheritance and intefaces are quite opposite.......
So finally java supports only syntax of multiple inheritance does not supports implementation of multiple inheritance.
Inheritance is like debit and interface is like credit but interface has its own importance in other concepts like server side programming.
Not exactly in Java you can only inherit from 1 distinct class at a time. That distinct class might inherit from another class however you don't have a single class inheriting from multiple distinct classes...
You can have many ancestors but only one parent. Here ancestors mean Object class where as parent mean the class which you are extending.
By specifying a class to inherit you override the default extends Object. So only when you don't specify a class to inherit does a class extend Object.
The class you extend from already extends from Object. When you extend from the new class, you in effect stop extending directly from Object, and instead inherit it via the new direct ancestor.
So it's not that you're extending from two different classes, it's that you're extending from a class that itself already extends from another class, namely the Object class.
Visually, multiple inheritance would be:
A Object
| |
C
But the situation you describe is actually:
Object
|
A
|
C
While i was reading java book, i came across "Every class extends class Object"...but if want a class B to extend class A.....but Class B will be now having multiple inheritance,one from Object class and one from Class A.How the conflict is resolved.
Can anyone explain?
Its multi-level inheritance, not multiple:
class A extends Object
class B extends A
First of all, Object class is the super/base/parent class of every class including user-defined classes.
So even if we don't mention it explicitly, the user-defined classes extends Object class by default.
Morevoer, Object class implements a set of methods and variables which are common to all the objects being created in the application. This is the main reason why we have Object class as a base class for all the other classes.
For eg:
hashCode() - this method creates a unique identity for each of the object being created in JVM.
ClassB extends from ClassA which also extends from Object. Therefore ClassB extends Object indirectly, through classA
"Every class extends class Object" just means if you don't specify the parent class, it takes on Object as the parent class
The book was trying to explain that every class is either a direct or indirect subclass of Object. Among other things, this means that every class inherits the public methods of Object: toString(), hashcode(), wait(), etc. It also means that whatever class variable a happens to be, you can always assign a to a variable of class Object.
There is no such thing as multiple inheritance in Java. The closest Java comes to that is interfaces, which is a whole subject in itself.
there is no conflict.. take a look at this structure
animal
bird
sparrow
parrot
dog
poodle
cat
the parrot class gets all attributes/methods of its super-class bird and from its super-class animal. this is called multiple inheritance.
You get traits from your parents right? You also get traits from their parents also.