Implement interface by inherited Class in Java

Question

I have interface Car:

public interface Car {
    void run();
}

Also i have a base class that implements this interface:

public class Base implements Car {
    @Override
    public void run() {
        //some implementation here
    }
}

Also i have one more class that must implement Car interface So why i can not do something like that?

public class Der extends Base implements Car  {
   //no implementation of interface here because Base class already implements it
}

Why my base class can not implement interface? What is internal reason of this?

It's implements, not implement. And extends must be before implements. So: public class Der extends Base implements Car — MrBackend
– MrBackend, Commented Jun 12, 2014 at 6:57
extends must come before implements, for example, public class Der extends Base implements Car {, but has already been pointed out, that would the same as using public class Der extends Base { — MadProgrammer
– MadProgrammer, Commented Jun 12, 2014 at 6:58

geoand · Accepted Answer · 2014-06-12 07:04:16Z

4

When expending from Base, your Der will of course have the run method because it implements the Car. So

public class Der extends Base {

}

is sufficient

If you need to override run, you can easily do that. The code would look like

public class Der extends Base {

   @Override
   public void run() {
      //do whatever here
   }
}

If you need to use Der wherever Car will be used, you can of course do that.

Finally if you need Car to also implement some other interface the syntax would be

public class Der extends Base implements SomeInterface{

}

edited Jun 12, 2014 at 7:04

answered Jun 12, 2014 at 6:55

geoand

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Comments

rachana · Accepted Answer · 2014-06-12 06:59:27Z

1

If your Base class has implemented Car Interface and this base class if extended by Der Class then there is no need to again implement Car interface in Der class.

like if

Class Base implements Car{

}

and then

Class Der extends Base{

}

Then all the methods in the interface implicitly access in your Der class.

answered Jun 12, 2014 at 6:59

rachana

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Comments

TrungTran05T3 · Accepted Answer · 2014-06-13 11:02:43Z

1

public class Der implements Car extends Base {
   //no implementation of interface here because Base class already implements it
}

you must use extends keyword before implements keywork

right :

public class Der extends Base implements Car {
       //no implementation of interface here because Base class already implements it
    }

answered Jun 13, 2014 at 11:02

TrungTran05T3

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Comments

Darshan Mehta · Accepted Answer · 2014-06-12 06:55:33Z

0

Base class (super class of Der) already implements Car interface, hence, there is no need to implement it explicitly.

answered Jun 12, 2014 at 6:55

Darshan Mehta

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Comments

benzonico · Accepted Answer · 2014-06-12 06:57:24Z

0

When extending Base your Der class will have access to the run method and will actually be able to be considered as of type Car

ie :

Car a = new Der();

Will be totally valid.

answered Jun 12, 2014 at 6:57

benzonico

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Comments

Sanjay Rabari · Accepted Answer · 2014-06-12 06:58:40Z

0

That is why there is no multiple inheritance in java.

see if you provide implementation for run method then what about overriden method of Base Class

so for redundancy its not allowed.

answered Jun 12, 2014 at 6:58

Sanjay Rabari

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Comments

Ranjith · Accepted Answer · 2014-06-12 11:57:06Z

0

Implements keyword should come after Extends keyword in your example, like below:

    public class Der extends Base implements Car  {
          //no implementation of interface here because
          // Base class already implements it
    }

No issues if you do the extends and implements in one class, even extending the class i.e. Base which had already implemented same interface i.e. Car.

answered Jun 12, 2014 at 11:57

Ranjith

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