Extending from two classes

Question

How can I do this   public class Main extends ListActivity   ControlMenu    Also  I would like to know that is this approach is okay that I have made the menus in class which is ControlMenu and I am extending in rest of the activities

User · Answer

you can t do multiple inheritance in java  consider using interfaces   interface I1    interface I2    class C implements I1  I2      or inner classes   class Outer       class Inner1 extends Class1        class Inner2 extends Class2

User · Answer

Yea  as everyone else wrote  you cannot do multiple inheritance in Java  If you have two classes from which you d like to use code  you d typically just subclass one  say class A   For class B  you abstract the important methods of it to an interface BInterface  ugly name  but you get the idea   then say Main extends A implements BInterface  Inside  you can instantiate an object of class B and implement all methods of BInterface by calling the corresponding functions of B   This changes the  is-a  relationship to a  has-a  relationship as your Main now is an A  but has a B  Depending on your use case  you might even make that change explicit by removing the BInterface from your A class and instead provide a method to access your B object directly

User · Answer

Why Not Use an Inner Class  Nesting    class A extends B       private class C extends D             Classes A   B   C   D accessible here

User · Answer

Yes  slandau is right  Java does not allow extending from several classes   What you want is probably public class Main extends ListActivity implements ControlMenu  I am guessing you are trying to make a list   Hope that helps

User · Answer

Java does not support multiple inheritance  but you can try to implement two or more interface

User · Answer

Extending from multiple classes is not allowed in java   to prevent Deadly Diamond of death

User · Answer

The creators of java decided that the problems of multiple inheritance outweigh the benefits  so they did not include multiple inheritance  You can read about one of the largest issues of multiple inheritance  the double diamond problem  here    The two most similar concepts are interface implementation and including objects of other classes as members of the current class  Using default methods in interfaces is almost exactly the same as multiple inheritance  however it is considered bad practice to use an interface with only default methods

User · Answer

Make an interface  Java doesn t have multiple inheritance    http   csis pace edu  bergin patterns multipleinheritance html

User · Answer

You can only Extend a single class  And implement Interfaces from many sources    Extending multiple classes is not available  The only solution I can think of is not inheriting either class but instead having an internal variable of each class and doing more of a proxy by redirecting the requests to your object to the object that you want them to go to    public class CustomActivity extends Activity         private AnotherClass mClass        protected void onCreate Bundle savedInstanceState             super onCreate savedInstanceState            mClass   new AnotherClass this                  Implement each method you want to use       public String getInfoFromOtherClass                  return mClass getInfoFromOtherClass                this is the best solution I have come up with  You can get the functionality from both classes and Still only actually be of one class type   The drawback is that you cannot fit into the Mold of the Internal class using a cast

User · Answer

As everyone else has said  No  you can t  However even though people have said many times over the years that you should use multiple interfaces they haven t really gone into how  Hopefully this will help   Say you have class Foo and class Bar that you both want to try extending into a class FooBar  Of course  as you said  you can t do   public class FooBar extends Foo  Bar   People have gone into the reasons for this to some extent already  Instead  write interfaces for both Foo and Bar covering all of their public methods  E g   public interface FooInterface        public void methodA         public int methodB                   public interface BarInterface        public int methodC int i                  And now make Foo and Bar implement the relative interfaces   public class Foo implements FooInterface              public class Bar implements BarInterface               Now  with class FooBar  you can implement both FooInterface and BarInterface while keeping a Foo and Bar object and just passing the methods straight through   public class FooBar implements FooInterface  BarInterface        Foo myFoo      Bar myBar          You can have the FooBar constructor require the arguments for both         the Foo and the Bar constructors     public FooBar int x  int y  int z           myFoo   new Foo x           myBar   new Bar y  z                 Or have the Foo and Bar objects passed right in     public FooBar Foo newFoo  Bar newBar           myFoo   newFoo          myBar   newBar             public void methodA            myFoo methodA               public int methodB            return myFoo methodB               public int methodC int i           return myBar methodC i                         The bonus for this method  is that the FooBar object fits the moulds of both FooInterface and BarInterface  That means this is perfectly fine   FooInterface testFoo  testFoo   new FooBar a  b  c   testFoo   new Foo a    BarInterface testBar  testBar   new FooBar a  b  c   testBar   new Bar b  c     Hope this clarifies how to use interfaces instead of multiple extensions  Even if I am a few years late

User · Answer

it is possible  public class ParallaxViewController lt T extends View  amp  Parallaxor gt  extends ParallaxController lt T gt  implements AbsListView OnScrollListener      blah

User · Answer

Like another alternative  maybe you can use an interface with a default implementation of a method  That depends of course of what you want to do   For example  you can create an abstract class and an interface   public abstract class FatherClass        abstract void methodInherit                   do something          public interface InterfaceWithDefaultsMethods       default void anotherMethod                   do something               maybe a method with a callable for call another function            So  after that  you can extend and implements both classes and use both methods   public class extends FatherClass implements InterfaceWithDefaultsMethods        void methode             methodInherit            anotherMethod              Hope this helps you

User · Answer

You will want to use interfaces  Generally  multiple inheritance is bad because of the Diamond Problem   abstract class A    abstract string foo       class B extends A    string foo      return  bar        class C extends A     string foo    return  baz        class D extends B  C    string foo     return super foo        What do I do  Which method should I call      C   and others have a couple ways to solve this  eg  string foo     return B  foo        but Java only uses interfaces    The Java Trails have a great introduction on interfaces  http   download oracle com javase tutorial java concepts interface html You ll probably want to follow that before diving into the nuances in the Android API

[java] Extending from two classes

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