About Java method & a parameter which implements multiple interfaces

Question

I'm writing a Java application, and have a question. How can I implement a function with one parameter which should require an argument which implements multiple interfaces? For example:

interface Father
{
}

interface Teacher
{
}

public void foo(FatherAndTeacher person) // How do I do this?
{
    // Do something
}

I know I can use two parameters, such as:

public void foo(Father person1, Teacher person2)
{
    // Do something
}

but I think maybe having a single parameter which implements both interfaces would be better.

I think you'll need to show a good code example. I don't find this very clear as-is. — Carcigenicate
– Carcigenicate, Commented May 8, 2017 at 15:40
Yes,it is possible to implement two interfaces in Java. But still it is not clear what you asking for, could you be a little bit more specific? May be by example? — Vasily Vlasov
– Vasily Vlasov, Commented May 8, 2017 at 15:40
I've rewritten your question to hopefully make it clearer. If I've misinterpreted your question, feel free to edit your question again :) — Michael
– Michael, Commented May 8, 2017 at 15:43
Thanks very much, that's right and great! Sorry about my English. — WaterMoon
– WaterMoon, Commented May 9, 2017 at 4:38

Thomas · Accepted Answer · 2017-05-08 15:52:09Z

11

Two enforce a parameter to have 2 interfaces you have 2 basic options:

Create a common interface that extends both and use that as your parameter type:
```
interface FatherAndTeacher extends Father, Teacher { 
}
```
The problem with that is that objects that don't implement that interface don't match.
Use generics where you can require matching objects to implement both interfaces:
```
public  <T extends Father & Teacher> void foo(T person) {
  // Do something
}
```
Note that this only works with interfaces, i.e. you can't do T extends Number & String (2 classes). It works with one object boundary though, in which case the class must be first: T extends ConcretePerson & Father is ok but T extends Father & ConcretePerson is not (where ConcretePerson is a class).

edited May 8, 2017 at 15:52

answered May 8, 2017 at 15:49

Thomas

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4 Comments

Thomas Over a year ago

@Michael you're right, he didn't add it. I adapted the answer accordingly.

WaterMoon Over a year ago

Thank you very very much! : ) ^v^, I get the success, and thank you very very much : ) ^v^.

Thomas Over a year ago

@WaterMoon you're welcome. Note that you should accept one of the working answers then. :)

WaterMoon Over a year ago

Sorry, first use, don't know about this.

Michael · Accepted Answer · 2017-05-08 15:48:46Z

2

You can use a composite interface by extending both of your Teacher and Father interfaces.

Here's an example:

interface Teacher
{
    void teach();
}

interface Father
{
    void makeBadJoke();
}

// ----- Composite interface! Doesn't add any methods.
interface TeacherAndFather extends Teacher, Father
{
    /*This can be empty*/
}

class Bob implements TeacherAndFather
{
    public void teach() { System.out.println("1 + 1 = 2"); }
    public void makeBadJoke() { System.out.println("Knock knock..."); } 
}

class Main
{
    private static void foo(TeacherAndFather foo)
    {
        foo.teach();
        foo.makeBadJoke();
    }

    public static void main (String... args)
    {
        foo(new Bob());
    }
}

answered May 8, 2017 at 15:48

Michael

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2 Comments

WaterMoon Over a year ago

That's a great way, thank you very much.I thought this way and sometimes I want create less interface.Thanks. : )

Michael Over a year ago

@WaterMoon You're welcome. Remember to click the check mark next to whichever answer solved your problem.

treesong · Accepted Answer · 2017-05-08 15:44:45Z

0

just use Object as your function parameter type:

void doSomething(Object param);

or using generic method:

<T> void doSomething(T param);

or let the two interface both inherit from one same interface.

public interface Person {}
public interface Teacher extends Person {}
public interface Father extends Person {}
public class Test {
    public void foo(Person p) {
        //
    }
}

answered May 8, 2017 at 15:44

treesong

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3 Comments

Michael Over a year ago

Except in that case you can pass things to foo that are neither a Teacher or a Father.

treesong Over a year ago

yes, the foo function itself should check where parameter is instance of Teathcer or Father

Michael Over a year ago

Nice to be able to enforce it at compile-time though :)

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