Creating object with reference to Interface
Solution 1
But in my code is displaying displayName()method undefined.
Right, because displayName
is not defined in the Printable
interface. 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
.
The reason for this is more apparent if you consider an example like this:
class Bar {
public static void foo(Printable p) {
p.sysout();
p.displayName();
}
}
class Test {
public static final void main(String[] args) {
Bar.foo(new Parent());
}
}
The code in foo
must not rely on anything other than what is featured in the Printable
interface, as we have no idea at compile-time what the concrete class may be.
The point of interfaces is to define the characteristics that are available to the code using only an interface reference, without regard to the concrete class being used.
Solution 2
The displayName()
method is displayed as undefined because objParent
declared as type Printable
and the interface does not have such method. To be able to use method displayName()
, you can declare it in interface Printable
:
interface Printable {
void sysout();
void displayName();
}
Or cast objParent
to type Parent
first before calling method displayName()
:
Printable objParent = new Parent();
objParent = (Parent) objParent;
objParent.displayName();
Solution 3
You need to type cast it to get the access to the Parent
methods
((Parent)objParent).displayName();
Solution 4
Compiler doesn't care about run-time. as far as the compiler is concerned, it checks if the reference type has a method called display in your interface type.
methods declared in your sub-class or implementing class are not part of your super class/interface. thus you cannot invoke those methods which are declared in sub-class with super class/interface reference type.
Java Beginner
Updated on September 03, 2020Comments
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Java Beginner over 3 years
A reference variable can be declared as a class type or an interface type.If the variable is declared as an interface type, it can reference any object of any class that implements the interface.
Based on the above statement I have made a code on understanding. As said above declared as an interface type, it can reference any object of any class that implements the interface.
But in my code is displaying
displayName()
method undefined atobjParent.displayName()
:public class OverridenClass { public static void main(String[] args) { Printable objParent = new Parent(); objParent.sysout(); objParent.displayName(); } } interface Printable { void sysout(); } class Parent implements Printable { public void displayName() { System.out.println("This is Parent Name"); } public void sysout() { System.out.println("I am Printable Interfacein Parent Class"); } }
I am sure I have understood the wrong way. Can someone explain the same?
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Java Beginner about 11 yearsBut in kathy bates its like If the variable is declared as an interface type, it can reference any object of any class that implements the interface.What do they exactly mean by any object of any class that implements the interface
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Java Beginner about 11 yearsThanks for Reply Eclipse suggested the same
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asifsid88 about 11 yearsBy that they mean it can hold any object of which implements that interface, but to access that additional methods which is defined in the implementor you need to type-cast it. See my answer
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asifsid88 about 11 yearsWelcome :) Hope that helps
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T.J. Crowder about 11 years@JavaBeginner: It can reference any object implementing the interface, but it can only use features of that object that are defined by the interface. This is the whole point of having an interface reference: Avoiding coupling to any specific concrete class.
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T.J. Crowder about 11 years@JavaBeginner: There is no point to doing this. You may as well simply declare
objParent
as aParent
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Java Beginner about 11 years@T.J. Crowder - You mean like with out Casting
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T.J. Crowder about 11 years@JavaBeginner: Right, but if you have to resort to casting, the odds are your code needs to be refactored. The point of using an interface is to code to the interface and not rely on features of the concrete class. If you need to use the features of the concrete class, declare your variable using that class, not the interface.