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 ?

You could do cl obj = quest(); which will allow you to use both. — StackFlowed
– StackFlowed, Commented Sep 17, 2015 at 20:12
You should follow the naming conventions to make this a bit more readable. — Bubletan
– Bubletan, Commented Sep 17, 2015 at 20:14
obj.method2(); //exception - it's not an exception: that's something that happens at runtime. This would be a compiler error. — Andy Turner
– Andy Turner, Commented Sep 17, 2015 at 20:30

Paul Boddington · Accepted Answer · 2015-09-17 20:16:09Z

3

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.

answered Sep 17, 2015 at 20:16

Paul Boddington

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3 Comments

Jazzwave06 Over a year ago

This is a bad practice. You should investigate on dependency injection, or inversion of control. Inject your Test class with 2 dependencies: an Interface1 and an Interface2. Your DI container would resolve both to the same object.

Paul Boddington Over a year ago

@sturcotte06 I agree. I was just answering the question directly, but you're right it's generally not a very good idea to use instanceof to check if an instance of one interface is also another.

red_asparagus Over a year ago

@sturcotte06 how i can do it with DI framework, for example Spring. can you show short example?

Wojciech Wirzbicki · Accepted Answer · 2018-03-09 11:52:55Z

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

Jazzwave06 · Accepted Answer · 2015-09-18 13:29:50Z

If you want to do this in spring, the general idea would be:

// Test.java
public class Test {
    private final Interface1 i1;
    private final Interface2 i2;
    public Test(Interface1 i1, Interface2 i2) {
        this.i1 = i1;
        this.i2 = i2;
    }
}

<!-- application-context.xml -->
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
    <bean id="implementer" class="org.mypackage.Implementer" />
    <bean id="test" class="org.mypackage.Test">
        <constructor-arg ref="implementer"/>
        <constructor-arg ref="implementer"/>
    </bean>
</beans>

// Main.java
public static void main(String[] args) {
    ApplicationContext context = new ClassPathXmlApplicationContext("application-context.xml");
    Test t = (Test) context.getBean("test");
}

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