how to implement a interface method in Java

Question

When the below code is compiled the compiler shows an error:

InterfaceTest.java:19: error: cannot find symbol knightObj.dispBK();

public class InterfaceTest{
    public static interface Knight{
        public void embark();
    }
    public static class BraveKnight implements Knight{
        private int id;
        public BraveKnight(int id){
            this.id = id;
        }
        public void dispBK(){
            System.out.println("ID: "+id);
        }
        public void embark(){
            System.out.println("ID: "+id);
        }
    }
    public static void main(String[] args){
        Knight knightObj = new BraveKnight(101);
        knightObj.dispBK();
    }
}

What may be the possible cause?

Answer 1

Knight is your interface, it does not define a method called dispBK(). Your knightObj object is of type Knight, so you need to cast it to BraveKnight to be able to call the dispBK() method.

Alternatively you could add the dispBK() method into your interface.

Your third option is to initialize your object like this: BraveKnight knightObj = new BraveKnight(101);.

I warmly recommend using some IDE such as Eclipse, they will catch easy errors like this.

Answer 2

As stated in the documentation:

When you define a new interface, you are defining a new reference data type. You can use interface names anywhere you can use any other data type name. If you define a reference variable whose type is an interface, any object you assign to it must be an instance of a class that implements the interface.

dispBK() is method of class. & Knight is interface and method is not declared in interface so you can't.

add :

public void dispBK(); in your interface and then it will work.

An Interface reference can hold Object of IMPL if only that all methods declared in interface.

OR you ll have to access that method using Impl object.

Answer 3

Add public void dispBK() to interface and then you can implement it

Answer 4

Method from child is not visible to parent. Here knightObj is an instance of parent interface Knight and you are trying to invoke a method of its child which is not visible. You should either add that dispBK() method to Kinght interface or use casting like ((BraveKnight)knightObj).dispBK();

Answer 5

An interfaace defines the minimum set of methods that the concrete class has to implement. That way you know that if you have a range of Knight objects (such as BraveKnight, CowardlyKnight and GoodKnight) they must all define the embark() method.

If you define your KnightObj variable as the Knight interface, then the methods of Knight are the only ones that are directly accessible.

However, you have instantiated your KnightObj as a new BraveKinght(101), this means that you can still access the additional methods of BraveKnight, if you cast your variable using

(BraveKnight)KnightObj.dispBK();

More detailed EXAMPLE:

You use this when passing parameters to functions or inside arrays, collections, such as

ArrayList<Knight> allKnight = new ArrayList<Knight>();
allKnight.add(new BraveKnight(105));
allKnight.add(new CowardlyKnight(13));
allKnight.add(new GoodKnight(88));

To test which concrete class you actually have, you use instanceof to evaluate it, ie

for (Knight eachKnight : allKnight) {
    // No need to cast this, as all Knightly objects must implement it.
    eachKnight.embark();

    if (eachKnight instanceof BraveKnight) {
         (BraveKnight)eachKnight.dispBK();
    } else if (eachKnight instanceof CowardlyKnight) {
         (CowardlyKnight)eachKnight.runaway();
    } // no test for GoodKnight - he's always good
}