How to check if a class implements a interface [duplicate]

Question

This question already has answers here:

Determine if a Class implements a interface in Java (5 answers)

Closed 8 years ago.

here we go,

I have an marker interface :

interface IPOJO {
}

And then i have a class implementing this interface :

class MePOJO implements IPOJO {
}

Now suppose i have a class object holding reference of class MePOJO :

Class<MePOJO> classObj = MePOJO.class;

So how can i determine if MePOJO class implements IPOJO just by using classObj ?

Answer 1

You should use Class#isAssignableFrom(Class<?> cls) method here:

Class<?> classObj = MePOJO.class;
System.out.println(IPOJO.class.isAssignableFrom(classObj));

Output:

true

Answer 2

There are several approaches for this.

Either by using the Class type of the object you've created and retrieve the list with all the interfaces that it implements.

MePOJO test = new MePOJO();
Class[] interfaces = test.getClass().getInterfaces();
for (Class c : interfaces) {
   if ("IPOJO".equals(c.getSimpleName()) {
      System.out.println("test implements IPOJO");
   }
}

Or using the Class#isAssignableFrom(Class clazz) method:

Class<?> clazz = MePOJO.class;
System.out.println(IPOJO.class.isAssignableFrom(clazz));

Answer 3

The correct way to do this is not with marker interfaces at all but with annotations:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public static @interface Pojo {
}

@Pojo
public static class MePojo {
}

public static void main(String[] args) throws Exception {
    System.out.println(MePojo.class.isAnnotationPresent(Pojo.class));
}

Output:

true

This way you don't pollute the namespace with marker interfaces.

Answer 4

Check with the keyword instanceOf

if(classObj  instanceOf IPOJO ){

}

The instanceof operator evaluates to true if and only if the runtime type of the object is assignment compatible with the class or interface.

Answer 5

Use method isInstance from Class :

if (classObj.isInstance(myObj)){
     ...
}

http://docs.oracle.com/javase/6/docs/api/java/lang/Class.html

Answer 6

There are also class members that allow you to get the names of the super class of the object. http://docs.oracle.com/javase/7/docs/api/java/lang/Class.html