Java - How to call method class with interface without know class name

Question

I'm new in java, I want to call method class from implemented Class with interface without know class name "ClassA", which only know Object c and I have 2 file.

File (1) CobaInterface.java

package cobainterface;

public class CobaInterface {

    public static void main(String[] args) {

        ImplementedClass implementedClass = new ImplementedClass();

        ClassA clsA = new ClassA();
        implementedClass.myMethodFromClassA(clsA);
    }

}

class ClassA{
    public Integer getTwo(){
        return 2;
    }
}

interface MyInterface {
    public void myMethod();

    //here interface
    public void myMethodFromClassA(Object c);
}

File (2) : ImpementedClass.java

package cobainterface;

public class ImplementedClass extends CobaInterface {
    public void myMethodFromClassA(Object c) {
        //System.out.println(c.getTwo()); <- wrong when call method c.getTwo()
    }
}

How about if I want to call method getTwo() from ClassA without know Class Name, which only know Object c from file (2) as describe in code above. Thanks for advance.

Darren Forsythe · Accepted Answer · 2018-09-28 00:46:59Z

2

You should use generic types so the implementation knows what the object will be,

interface MyInterface<T> {
    public void myMethod();

    //here interface
    public void myMethodFromClassA(T c);
}

The impl becomes,

package cobainterface;

public class ImplementedClass Implements MyInterface<ClassA> {
    public void myMethodFromClassA(ClassA c) {
        //System.out.println(c.getTwo()); <- wrong when call method c.getTwo()
    }
}

All together,

class Scratch {
    public static void main(String[] args) {

        ImplementedClass implementedClass = new ImplementedClass();

        ClassA clsA = new ClassA();
        implementedClass.myMethodFromClassA(clsA);
    }

}

class ImplementedClass implements MyInterface<ClassA> {
    @Override
    public void myMethod() {

    }

    @Override
    public void myMethodFromClassA(ClassA c) {
        System.out.println(c.getTwo());
    }
}

class ClassA {
    public Integer getTwo() {
        return 2;
    }
}

interface MyInterface<T> {
    void myMethod();

    void myMethodFromClassA(T c);
}

You could also do a cast

System.out.println((MyClass)c.getTwo());

but you will lose all benefit of type saftey.

answered Sep 28, 2018 at 0:46

Darren Forsythe

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3 Comments

Scary Wombat Over a year ago

You could also do a cast - maybe should check with instanceof

Darren Forsythe Over a year ago

completely slipped my mind, there's a few ways to be safer with casting. @ScaryWombat. mentions.

Anz Over a year ago

@DarrenForsythe Thanks for help,, that's opened my mind about java,,

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