Class with multiple interfaces

Question

i have 2 interfaces inter1 and inter2 and class that implements both of them:

public interface Interface1 {
    method1();
}
public interface Interface2 {
    method2();
}
public class Implementer implements Interface1, Interface2 {
    method1() { 
         // something 
    }
    method2() { 
         // something 
    } 
}
public class Test { 
    public static void main(String[] args) {
        Interface1 obj = quest();
        obj.method1();
        if(obj instanceof Interface2) {
            obj.method2(); //exception
        }    
    }

    public static Interface1 quest() {
        return new cl();
    }
}

How to cast obj to Interface2 and call method2() or it is possible to call method2() without casting ?

Answer 1

If you write inter1 obj = ... then you will not be able to write obj.method2) unless you cast to inter2 or to a type that implements inter2.

For example

inter1 obj = quest();
if (obj instanceof class1)
    ((class1) obj).method2();

or

inter1 obj = quest();
if (obj instanceof inter2)
    ((inter2) obj).method2();

As an aside, when you write in Java you normally give classes and interfaces names that begin the a capital letter, otherwise you confuse people reading your code.

Answer 2

Using genecics it is possible to declare generic reference implementing more than one type. You can invoke method from each interface it implements without casting. Example below:

public class TestTwoTypes{

    public static void main(String[] args) {
        testTwoTypes();
    }

    static <T extends Type1 & Type2> void testTwoTypes(){
        T twoTypes = createTwoTypesImplementation();
        twoTypes.method1();
        twoTypes.method2();
    }

    static <T extends Type1 & Type2> T createTwoTypesImplementation(){
        return (T) new Type1AndType2Implementation();
    }
}

interface Type1{

    void method1();
}

interface Type2{
    void method2();
}

class Type1AndType2Implementation implements Type1, Type2{

    @Override
    public void method1() {
        System.out.println("method1");
    }

    @Override
    public void method2() {
        System.out.println("method2");
    }
}

The output is:

method1
method2