Can one class extend two classes

Question

My class should extend two classes at the same time   public class Preferences extends AbstractBillingActivity    public class Preferences extends PreferenceActivity     How to do so   Upd  Since this is not possible  how should I use that AbstractBillingActivity with Preferences then   Upd2  If I go with interfaces  should I create    BillingInterface  public interface BillingInterface extends PreferenceActivity  AbstractBillingActivity       PreferenceActivity  public interface PreferenceActivity       AbstractBillingActivity  public interface AbstractBillingActivity            void onCreate Bundle savedInstanceState         and then   public class Preferences implements BillingInterface

User · Answer

Java doesn t support multiple inheritance  You can implement multiple interfaces  but not extend multiple classes

User · Answer

If you are interested in using methods from multiple classes  the best way to approach to it is to use Composition instead of Inheritence

User · Answer

Java does not support multiple inheritance  However  your problem may be solved using interfaces   The easiest solution would be to create an interface for AbstractBillingActivity and PreferenceActivityand implement them both

User · Answer

Java does not support multiple inheritance   There are a few workarounds I can think of   The first is aggregation  make a class that takes those two activities as fields   The second is to use interfaces   The third is to rethink your design  does it make sense for a Preferences class to be both a PreferenceActivity and an AbstractBillingActivity

User · Answer

In Groovy  you can use trait instead of class  As they act similar to abstract classes  in the way that you can specify abstract methods  but you can still implement others   you can do something like   trait EmployeeTrait       int getId              return 1000   Default value           abstract String getName     Required    trait CustomerTrait       String getCompany             return  Internal     Default value           abstract String getAddress      class InternalCustomer implements EmployeeTrait  CustomerTrait       String getName               String getAddress              def internalCustomer   new InternalCustomer   println internalCustomer id    1000 println internalCustomer company   Internal    Just to point out  its not exactly the same as extending two classes  but in some cases  like the above example   it can solve the situation  I strongly suggest to analyze your design before jumping into using traits  usually they are not required and you won t be able to nicely implement inheritance  for example  you can t use protected methods in traits   Follow the accepted answer s recommendation if possible

User · Answer

Also  instead of inner classes  you can use your 2 or more classes as fields   For example   Class Man  private Phone ownPhone  private DeviceInfo info    sets  gets   Class Phone  private String phoneType  private Long phoneNumber    sets  gets    Class DeviceInfo   String phoneModel  String cellPhoneOs  String osVersion  String phoneRam    sets  gets      So  here you have a man who can have some Phone with its number and type  also you have DeviceInfo for that Phone   Also  it s possible is better to use DeviceInfo as a field into Phone class  like  class Phone   DeviceInfo info  String phoneNumber  Stryng phoneType    sets  gets

User · Answer

Another solution is to create a private inner class that extends the second class   e g a class that extends JMenuItem and AbstractAction    public class MyClass extends JMenuItem         private class MyAction extends AbstractAction              This class can access everything from its parent

User · Answer

I can think of a workaround that can help if the classes you want to extend include only methods   Write these classes as interfaces  In Java  you can implements any number of interfaces  and implement the methods as default methods in the interfaces   https   www geeksforgeeks org default-methods-java

User · Answer

Java does not support multiple inheritance  that s why you can t extend a class from two  different classes at the same time    Rather  use a single class to extend from  and use interfaces to include additional functionality

User · Answer

No you cannot make a class extend to two classes   A possible solution is to make it extend from another class  and make that class extend from another again

User · Answer

Familiar with multilevel hierarchy   You can use subclass as superclass to your another class   You can try this   public class PreferenceActivity extends AbstractBillingActivity      then  public class Preferences extends PreferenceActivity      In this case  Preferences class inherits both PreferencesActivity and AbstractBillingActivity as well

User · Answer

Java 1 8  as well as Groovy and Scala  has a thing called  Interface Defender Methods   which are interfaces with pre-defined default method bodies    By implementing multiple interfaces that use defender methods  you could effectively   in a way  extend the behavior of two interface objects   Also  in Groovy  using the  Delegate annotation  you can extend behavior of two or more classes  with caveats when those classes contain methods of the same name    This code proves it   class Photo       int width     int height       class Selection        Delegate Photo photo         String title     String caption       def photo   new Photo width  640  height  480  def selection   new Selection title   Groovy   caption   Groovy   photo  photo  assert selection title     Groovy  assert selection caption     Groovy      assert selection width    640 assert selection height    480

User · Answer

What you re asking about is multiple inheritance  and it s very problematic for a number of reasons   Multiple inheritance was specifically avoided in Java  the choice was made to support multiple interface implementation  instead  which is the appropriate workaround

[java] Can one class extend two classes?

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