This question already has answers here:
Multiple Inheritance and class Object
(5 answers)
Closed 9 years ago.
Well Java does not support multiple inheritance in java..
But wait in eclipse we can see any class extends OBJECT class by default, we can see all methods of Object class if we try to add unimplemented methods.
Now MY POINT IS i can make my class extend any class for example Thread.
So now my class extends Thread by user defined side and Object by default...
that means multiple class inheritance ?
Behavior similar to multiple inheritance can be seen with Java interfaces:
// implements BOTH Runnable AND ActionListener
public class MultipleInterfaces implements Runnable, ActionListener {
#Override public void run() {}
#Override public void actionPerformed(ActionEvent e) {}
}
Multiple inheritance would be like this:
// Not allowed, complete nuts
public class Amalgam extends ArrayList<Thread>, JPanel, Font {
public Amalgam() {
super(); // <- and what would this do?
}
}
A class can only have one superclass i.e. in java one class can only extend one class. If one is not specified, then it is implicitly extends to Object.
So suppose the class is MyClass and it extends MySuperClass. As MyClass extends MySuperClass so it will not extend directly Object. But MySuperClass class itself is not explicitly extending any class so it extends Object and in turn MyClass also extends Object in the hierarchy.
So it is not Multiple inheritance rather it is Multilevel inheritance. Hope it helps.
The behavior mentioned by you is multi-level inheritance which java supports by default
This is MultiLevel inheritance and not Multiple inheritance.
any class extends OBJECT class by default
That means Thead class extends Object too.
Related
I ran into a difficulty while doing an assignment for school. The problem is that the assignment requires us to create a class SummableSet that inherits from class IntSet, but I also need SummableSet to extend Applet in order to create an Applet that the Assignment also requires. I'm not sure how to go about doing this since I've done my research and it is not possible for a sub-class to inherit two classes. So how can I create an applet from these two classes if I can not extend Applet in my sub-class, SummableSet?
As you know, your class can only extend from one other class, and the solution is not to try to do the impossible. You'll need to create at least two classes here, one that extends Applet (why your teacher is having you use a dead technology like applets is beyond me), and the other new class that extends IntSet. Then the Applet extending class would use the other class in a "has-a" or "composition" relationship.
e.g..,
public class MyApplet extends Applet {
private SummableSet summableSet = new SummableSet();
#Override
public void init() {
// use summableSet here
}
}
public class SummableSet extends IntSet {
// ...... code for SummableSet here
}
This question already has answers here:
Final interface in Java?
(10 answers)
Closed 6 years ago.
As since in a class I can do:
public final class Foo{}
wich means no more classes can extends that Foo class... e.g. String class is final, so no custom class can extends the class String.
How can I prevent to do the same with an interface?
If I do
public interface ISome{
void fly();
}
I would like to allow that
class A implements ISome {}
but block that
public interface IHouse extends ISome{
void fly();
}
doing this
public final interface ISome{}
makes no sense... and will bring a compile error like:
Illegal modifier for the interface
You can't.
Supposedly the Java designers didn't think there would ever be an appropriate use case for this: if you don't want an interface to be extended then really you ought to declare those functions directly in a concrete class.
That said, you can achieve this in C++ as in this language an interface is more of a convention - consisting of only pure virtual functions, and you can enforce non-extensibility with techniques such as friendship.
This question already has answers here:
Java doesn't support multiple inheritance but implicitly every class in java extends Object and allows one more [duplicate]
(8 answers)
Closed 7 years ago.
In this tutorial (http://www.studytonight.com/java/object-and-classes) I read that a java class may optionally extend one parent class. By default, it will extend java.lang.Object.
Note: important statement that i was read that Java enums extend the java.lang.Enum class implicitly, so your enum types cannot extend another class.
according to note our normal java class should not extend other class like enum (enum types cannot extend another class).but we can able to inherit one class. is this multiple inheritance.?
in java class can derived by extends keyword. like this
class SomeClass
{ }
class MyClass extends SomeClass{}
How can all java classes by default extends java.lang.Object class without extends keyword in java?
When our class extends some base class, it becomes multiple inheritance. I searched in stackoverflow, but still I am not clear.
By default any class extends Object class. Doesn't it mean java supports multiple inheritance?
Can anybody clarify this with a simple example.
Every class, except for java.lang.Object, extends exactly one class.
If you write extends Something, then your class extends Something.
If you don't write extends Something, then your class extends java.lang.Object. (the same as if you wrote extends Object)
If you don't extend any class, you will still extend Object. If you explicitely extend some class, than you extend just this class, but the extended class will in other turn extend Object by default. This way Object is always in class hierarchy.
If every class in Java implicitly extends Object class and multiple inheritance is not possible in Java then how do we even extend any class?
If you extend a class A that class in turn extends Object, so B extends A implicitly also extends Object.
"Java doesn't have multiple inheritance" means that you can't have two different parents, not that your parent can't have a parent.
C++ is an example of a language that lets you do multiple inheritance: http://www.learncpp.com/cpp-tutorial/117-multiple-inheritance/
Multiple inheritance looks like this:
class Teacher: public Person, public Employee
which means 'Teacher extends Person and Employee, inheriting its fields and methods'.
Instead of multiple inheritance, you're expected to create and implement interfaces to represent all the behaviours (or contracts if you prefer) your object supports. Java uses this for interfaces like Closeable and Serializable.
Multiple Inheritance is the concept of a single class inheriting from two or more super classes. If a class inherits from a super class and if that super class inherits from another super class it does NOT qualify as Multiple Inheritance. It is still Single Inheritance.
once u create object of sub class the object hierachy would be create on order of
objectclaass–>superclass—>subclass;
It is true that every class in Java inherits from the Object class – either indirectly or directly.
so in this case the sub class indirectly inherits object class.
Every Class that dosn't extends any other class it extends Object class.
if you extends anther class example extends Vector class
look to the hierarchy of class Vector you will end with a simple class that dosn't extends any anther class which explicitly extends Object.
and Any class extends anther class it explicitly extends all class that the parent class extends.
There is no multiple inheritance in Java, but there is class hierarchy. Inheritance in Java is transitive: if class A extends Object and class B extends A, than by transitivity A extends Object.
I have an Android app with an Activity, a Thread and a class to hold data.
I have created an abstract class ActivityTemplate which extends the Activity class and implements some callbacks, e.g. onTouchListener. This also includes some abstract methods.
I have created an abstract class (ThreadTemplate) which extends the Thread class and includes some abstract methods.
I have created an abstract DataTemplate class which holds some data elements and some simple methods to manipulate it.
I can produce my app by deriving three classes, one from each of the 'Template's above, but I really want to be able to roll them all up into a single MyTemplate class from which I can derive my app from.
Sort of:
public class MyTemplate extends ActivityTemplate, ThreadTemplate, DataTemplate
then
public class MyApp extends MyTemplate
But the MyTemplate class will not compile as you can only extend one class.
Any thoughts, or am I asking the impossible?
Use object composition instead of inheritance:
class MyApp {
private ActivityTemplate activityTemplate;
private ThreadTemplate threadtemplate;
private DataTemplate dataTemplate;
}
When using inheritance always ask yourself the question whether your subclass has an "is-a" relationship with the parent. In this case, MyApp is not a template. It has-a template.