In Java, you can extend from another interface using the extends keyword. But Java 8 has default methods. What happens if we extend another interface that has the same default methods as our interface?
It gets re-declared in the "child" interface.
Re-declare with different Implementation
Consider these codes. We have two (2) interfaces and one of them extends the other. The ParentInterface has a default method that returns the String"Parent interface". The other interface, ChildInterface, extends from the ParentInterface.
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packagecom.turreta.interface1;
publicclassDefaultMethodsDemo1{
publicstaticvoidmain(String[]args){
// Using anonymous class
ChildInterface ci=newChildInterface(){
};
System.out.println(ci.getId());
}
}
interfaceParentInterface{
defaultStringgetId(){
return"Parent interface";
}
}
interfaceChildInterfaceextendsParentInterface{
defaultStringgetId(){
return"Child interface";
}
}
Output
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1
Childinterface
Re-declare as an abstract method
If we do not wish to use any default implementation, we can re-declare the default method as an abstract method. It is now up to the users of the interface to provide their own implementations of the abstract method.
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publicclassDefaultMethodsDemo2{
publicstaticvoidmain(String[]args){
ChildInterface ci=newChildInterface(){
@Override
publicStringgetId(){
return"Programmer's own implementation";
}
};
System.out.println(ci.getId());
}
}
interfaceParentInterface{
defaultStringgetId(){
return"Parent interface";
}
}
interfaceChildInterfaceextendsParentInterface{
StringgetId();
}
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Output
1
Programmer'sown implementation
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