• You can only access the methods defined on the interface through a variable declared as having that interface, even if the concrete class has additional methods. That's why you can call sysout, but not displayName. link
• It can be use where two subclasses have different type of action  on a single method of superclass.
• A reference variable can be declared as a class type or an interface type.If the variable is declared as an interface type,
• A reference variable can be declared as a class type or an interface typw. If the variable is declared as an  interface type than you can access only the method defined in interface from which it is declared.
• One interface extends other interface.Because one interface contain stubs can extends in only and we could not define its implementation
• Why an interface can not implement another interface?link

Interface for the feeding method: feeding
public interface Feeding {


 String feeds();
}

class isHarbivor implements Feeding{
 public String feeds()
 {
    return "Eat plants";
 }
}

class isCarnivor implements Feeding{
 public String feeds()
 {
    return "Eat both plants and meat";
 }
}

pet:
public class pets {


 public Feeding feedType;
 private String name;
 private int number;
 
 
 public void setName( String name){
  this.name=name;
 }
 public String getName(){
  return name;
 }
 
 public void setNumber( int number){
  this.number=number;
 }
 public int getNumber(){
  return number;
 }
 
 public String whatToFeed()
 {
  return feedType.feeds();
 }
 public void changeFeed(Feeding feedType)
 {
  this.feedType=feedType;
 }
 
}

More : Link

realted questions
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Usage in android:

Java property: Interface reference variable can refer to a subclass object

so here in the above design pattern I can use the reference to call method inside subclass to communicate with other fragment.