Interface

[John Paterson]

Hi folks,

I was writing out some code to test my understanding of interfaces when I ran into some trouble. Following are the the 3 files:

package interfacetest;

public interface Computer {

    void multiply();
    
    void divide();

    void subtract();

    void add();
    
}

package interfacetest;

public class Calculator implements Computer{
    
    public void multiply()
    {
       
    }
    
    public void divide();

    public void subtract();

    public void add();    
    
}

package interfacetest;

public class SecondCalculator extends Calculator {
    
     public void divide()
    {
       
    }
     
      public void subtract()
    {
        
    }
      
     public void add()
     {
         
     }
    
}

I am getting three error messages in the 'Calculator' file for the 3 methods divide, subtract and add. The error message is

missing method body, or declare abstract public void 'methodName'();

I don't understand why I am getting this. My underdstaning is that I can choose not to implement the methods in the first concrete implementing class as long as it's subclass implements them. Hope someone can help me here. Thanks.

regards
John

[Matthew Brown]

John Paterson wrote:My underdstaning is that I can choose not to implement the methods in the first concrete implementing class as long as it's subclass implements them.

No, if it's a concrete class then it can be instantiated. And that means somebody could create one, and then call the method. So the method body has to exist.

You can choose to not implement all the methods and force a subclass to do that. But in this case you have to declare the class as abstract so that it can't be instantiated.

[E Armitage]

John Paterson wrote:... My underdstaning is that I can choose not to implement the methods in the first concrete implementing class as long as it's subclass implements them..

That's the incorrect part. The first concrete class must implement all of then unless if it's (abstract) super class has implemented them.

[Helen Ma]

John Paterson wrote:Hi folks,

I was writing out some code to test my understanding of interfaces when I ran into some trouble. Following are the the 3 files:

package interfacetest;

public interface Computer {

    void multiply();
    
    void divide();

    void subtract();

    void add();
    
}

package interfacetest;

public class Calculator implements Computer{
    
    public void multiply()
    {
       
    }
    
    public void divide();

    public void subtract();

    public void add();    
    
}

package interfacetest;

public class SecondCalculator extends Calculator {
    
     public void divide()
    {
       
    }
     
      public void subtract()
    {
        
    }
      
     public void add()
     {
         
     }
    
}

I am getting three error messages in the 'Calculator' file for the 3 methods divide, subtract and add. The error message is

missing method body, or declare abstract public void 'methodName'();

I don't understand why I am getting this. My underdstaning is that I can choose not to implement the methods in the first concrete implementing class as long as it's subclass implements them. Hope someone can help me here. Thanks.

regards
John

In the Calculator class, provide some implementations for the divide, subtract and add methods.

That reminds me of a rule : if you declare one abstract method in a class, the class should be declare abstract. So, if you have an abstract method in Calculator, declare this class as abstract.

[John Paterson]

Thanks guys