Do all java classes extend object

Any way to extend two or more classes in java?

I know that Java does not allow us to extend more than one class. I know that interfaces also exist and I don’t want to use them this time. Is there some kind of trick or workaround to extend multiple classes in Java ? If yes, please include the sample code also.

Question edited — I also say «I know that interfaces also exist and I don’t want to use them this time.» Now what will the answer be ?

If you still want multiple Inheritance in spite of knowing about Interfaces perhaps you need to rethink your application design.

12 Answers 12

You can do it easily with interfaces and composition.

The question is why you would ask such a thing? Why do you not want to use interfaces «at this time»?

You have to know that Java only allows single inheritance of implementation. You’ve spent seven months at SO, so surely you must know how to use the search feature. The question has been asked here repeatedly.

Be more creative. There’s no reason that a JPanel has to be Observable. JPanel is for rendering. I agree that its model data might want to be Observable. By all means do it with composition.

How will i make a model given that — I want to make a reusable class called button holder which extends JPanel. This will hold a grid of buttons, each of which has a unique value/name. The button holder can be used by any device like elevator panel, calculator etc. These devices are panels which hold a text field and button holder. They will find out which button is pressed inside button holder and then put the name of button in text field.

Every button will have to have a Listener to respond to events. Wouldn’t the model be a good candidate to be that Listener, responding to every button’s press event? Now you also have text boxes? Aren’t those part of the model? It’s possible to do what you want without multiple inheritance, but I’m not going to design and implement it for you in a comment.

haha — dont expect you to do the work for me. Just needed something to get me started. I was confused.

No, unlike C++ you cannot extend multiple classes but you can implement multiple interfaces.

Question edited — I also say «I know that interfaces also exist and I don’t want to use them this time.» Now what will the answer be ?

It simply isn’t possible, its omission in Java was a conscious language design choice that they made for fear of causing «confusion».

go for interfaces .No multiple inheritance in Java.

Multiple inheritance can cause the diamond problem.

JAVA omits many rarely used, poorly understood, confusing features of C++ that in our experience bring more grief than benefit. This primarily consists of operator overloading (although it does have method overloading), multiple inheritance, and extensive automatic coercions _ Dr. James Gosling

Question edited — I also say «I know that interfaces also exist and I don’t want to use them this time.» Now what will the answer be ?

 class One < void oneMethod()< >> class Two extends One < void TwoMethod()< >> class Abc extends Two < @Override void oneMethod() < // TODO Auto-generated method stub super.oneMethod(); >@Override void TwoMethod() < // TODO Auto-generated method stub super.TwoMethod(); >> 

Multiple inheritance in java is usually done by implementing multiple interfaces. You can not extend more than one class. A possible workaround may be to use some kind of object composition for instance aggregation.

Question edited — I also say «I know that interfaces also exist and I don’t want to use them this time.» Now what will the answer be ?

@davidblaine you can not extend multiple classes. You could use composition for instance — put a member of the type of the class you want to inherit

@davidblaine depends on what you are trying to do. Composition is a way to combine simple objects or data types into more complex ones so if it works for you you may consider it a workaround

Java not support this for classes.You can use multiple interfaces.You can find what the problem with multiple inheritance in java

You can use inner classes like

 public class class1 extends class2 < class Inner extends class3< >> 

Question edited — I also say «I know that interfaces also exist and I don’t want to use them this time.» Now what will the answer be ?

Multiple inheritance adds complexity with little benefits, that’s why it is not present in java Why is Multiple Inheritance not allowed in Java or C#?

One way I can think of is to write your program needing multiple inheritance in a language that supports it e.g.C++ and then make your Java code interact with the output from that program e.g using files or databases.

Answer is No. But you can do this if helps:

Use interfaces instead. The following is how that would look in code:

public Class ClassName implements Interface1, Interface2

To avoid diamond problem Java does not support Multiple Inheritance through classes but it supports using interfaces. So you may use Association Relationship. e.g.

Class A <> Class B <> Class C implements SomeInterface < A a; B b; // setter getter for a and b and other methods >

You can use a lookup of capabilities.

class Vehicle < T lookup(Class klazz) < return map.get(klazz); // typically >> interface Flyable < void fly(); >Vehicle x; Flyable f = x.lookup(Flyable.class); if (f != null)

This decouples classes. It requires runtime checking though.

It uses delegation: local members implementing the lookup’ed class/interface.

You can derive a class from Vehicle that can both fly (Flyable) and dive (Diving). The flying can be implemented by an instance of a shared class. You can even dynamically compose capabilities (add wheels to a boat).

That often is better than implementing several interfaces, where fly() would either be a code copy, or delegate to a Flyable field (with shared implementation class).

Especially with multiple interfaces/classes involved you otherwise risk code on base class objects (Vehicle here) with cases and unsafe casts, forgetting if (x instanceof Boat) .

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Java do all classes extend object java

Consider to method of object class where object references are checked whether they refer to same object or not. That super class is also a sub class of Object class.

Classes Extending Object [duplicate]

Class A // extends Object < >Class B extends A

Class B extends Class A directly and Class Object transitively

Many things in programming work in accordance to real life situations. Think about Animals.

• Animals. Animals can eat.
• Mammals are Animals. Mammals produce milk.
• Felidaes are Mammals. Felidaes are Carnivores.
• Lions are Felidaes. Lions has attributes of all the above.

Implementing the above with Java classes.

class Animal < //can eat >class Mammal extends Animal < //can eat //produce milk >class Felidae extends Mammal < //can eat //produce milk //carnivore >class Lion extends Felidae < //can eat //produce milk //carnivore //roars >class PussyCat extends Felidae < //can eat //produce milk //carnivore //meows >

Since mammals are animals, if lion is a mammal, that automatically categorizes lions as animals too, thus receiving every attribute of an animal.

Lions are subset of Animal , Mammal and Felidae , it posses all attributes of its super class . PussyCat shares the same «ancestors» (super class) with lions , thus it inherits the same attributes and behaviours, except that it meows instead of roars.

Does this mean that the current class doesn’t extend Object anymore or does it mean it extends two classes?

In Java everything extends from Object. Even if you don’t do it, it implicitly extends Object for you.

Hence you can safely assume every classes you creates are subclasses of Object and they all includes methods from Object such as toString() and clone() .

You can’t extend more than one class in Java, but it is still a subclass of Object .

Every classes is a Sub class to Object Class. if you are extending your own class with another class. That super class is also a sub class of Object class.

Class A // extends Object < >Class B extends A

here i am extending Class B with A . Even though Class B has all the features of Object class through Class A

Java — Every class extends Object?, class Object < /* stuff */ >class Foo /* implicit extends Object */ <> class FooBar extends Foo /* and therefore extends Object */ <> The rules are described in the Java Language Specification : The class Object is …

Since all classes extend Object, and Object is a class, how can Object extend Object? [duplicate]

java.lang.Object is special in this way. The Java language specification, section 8.1.4 states:

The extends clause must not appear in the definition of the class Object, or a compile-time error occurs, because it is the primordial class and has no direct superclass.

If you look at any implementation of the Java standard library, you’ll find that the source of java.lang.Object does in fact not have an extends clause (and because the Object class is primordial and has special treatment in the spec, there’s no extends Object implicitly present).

Additionally, you may observe that the value of Object.class.getSuperclass() is precisely the null reference.

Digging into native code, it appears that this rule is enforced here, after a few layers of delegation.

Extending Java class twice, you can not extend more than one class in Java. But you may implement more than one interfaces. However, if your parent is a child from …

How JVM will extend the Object class for a custom class in java?

Object class contains all the common method. So implicit exetending by default gives you those common method, which reduce the overhead.

Methods are — equals, wait, notify, notifyall, hashcode,getClass,clone etc

If you need custom logic, so you can override that method .

Note: All objects, including arrays, implement the methods of Object class.

The answer to this is related to the idea of inheritance in general — the idea of inheritance is that you define a common set of behaviors, that apply to all subclasses. Anything defined in the «Object» class is available to all classes you create.

It contains a few things that are applicable to every peace of information you use in your code:

• equals and hashCode methods to establish an equality theory within the given abstraction.
• toString to represent an object in human-readable (probably, only programmer-readable) format.
• getClass to provide reflection capabilities on the given abstraction; some methods to organize object-oriented runtime.

Java chose to make a single class be the ultimate parent class for everything so that there is an easy way to pass around any arbitrary object, without needing to know its type (i.e. you can use the declared type of Object to refer to every single item in the type system, even primitives using their wrapper classes). However, there are OOP languages such as C++ where there is no universal base class as in Java. Another benefit to having a universal base class is that logic dealing with the superclass does not have to be special cased for top-level classes (with the exception of the universal base class, Object, itself).

To provide this functinality by default, every class extends Object class.

• Consider to equals method of object class where object references are checked whether they refer to same object or not. If you don’t provide an implementation of equals() method, then the default one will be called for custom objects also.
• The default toString method provides the class name and hashcode for the object. You don’t need to write a separate one if you don’t want other string representation of the object.
• The methods like wait(), notify(), notifyAll() provide functionality for thread communication for objects. So your classes do not have to do it externally. They can just extends Object class functionality.

Javac — How to not extend the Object class in Java?, There is no ‘dual inheritance’. You may extend soms user-defined class, which in turn has to extend Object down the line since that’s the default. …

How to not extend the Object class in Java?

int is a primitive, hence we cannot say it extends anything.

When it comes to your question:

Is there any class that does not extend Object class?

You have answered it literally line above:

every class in Java extends the Object class

P.S.: We cannot say, that there is a dual inheritance, because a class extends Object and some other class, because that «other class» also extends Object . In fact, a class is a subtype of all its supertypes, for example:

HashMap m = new LinkedHashMap(); AbstractMap m = new LinkedHashMap(); Object m = new LinkedHashMap(); 

Type parameters omitted for brevity.

Edit: referencing the discussion about what is an object and what is not, JLS §4.3.1 gives the answer:

An object is a class instance or an array.

Can we get all the classes that extends a specific class, Java does not define «project», but some IDE’s do. If you are not using an IDE, you have to define what you mean by «project». Also, given class A, do you …

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