CODEWITHSUNDEEP
javareflection
user489041
user489041

Reputation: 28294

Determine if a Class implements a interface in Java

I have a Class object. I want to determine if the type that the Class object represents implements a specific interface. I was wondering how this could be achieved?

I have the following code. Basically what it does is gets an array of all the classes in a specified package. I then want to go through the array and add the Class objects that implement an interface to my map. Problem is the isInstance() takes an object as a parameter. I can't instantiate an interface. So I am kind of at a loss with this. Any ideas?

Class[] classes = ClassUtils.getClasses(handlersPackage);
for(Class clazz : classes)
{
    if(clazz.isInstance(/*Some object*/)) //Need something in this if statement
    {
        retVal.put(clazz.getSimpleName(), clazz);
    }
}

Upvotes: 113

Views: 86844

Answers (5)

luiscla27
luiscla27

Reputation: 6409

A contribution for all the other answers, when possible do not use the most upvoted answer of method isAssignableFrom, even the "not great" answer of using clazz.getInterfaces() has better performance than isAssignableFrom.

A common mistake for developers when looking for an answer to the OP question, is to prefer isAssignableFrom when an instance is available, wrongly doing this:

if (IMyInterface.isAssignableFrom(myObject.getClass())) {
    ...

When possible, use IMyInterface.class.isInstance or instanceof as both of those have way better performance. Of course, as the OP stated; they have the drawback that you must have an instance and not just the class.

if (IMyInterface.class.isInstance(myObject)) {
    ...
if (myObject instanceof IMyInterface) { // +0.2% slower than `isInstance` (*see benchmark)
    ...

An even faster, but ugly solution would be to store an static Set with all the "valid" classes instead of checking them, this ugly solution is only preferred when you need to test classes a lot, as its performance outperforms all the other approaches for direct class check.

public static final Set<Class<?>> UGLY_SET = Stream.of(MyClass1.class, MyClass2.class, MyClass3.class).collect(Collectors.toCollection(HashSet::new));
if (UGLY_SET.contains(MyClass)) {
    ...
(*) JMH Benchmark for +0.2%

Please visit this answer from users @JBE, @Yura and @aleksandr-dubinsky, credits for them. Also, there's plenty of detail in that answer for the benchmark results to not be valid, so please take a look into it.

Upvotes: 0

Flavio
Flavio

Reputation: 11977

You should use isAssignableFrom:

if (YourInterface.class.isAssignableFrom(clazz)) {
    ...
}

Upvotes: 261

Roddy of the Frozen Peas
Roddy of the Frozen Peas

Reputation: 15184

You can use class.getInterfaces() and then check to see if the interface class is in there.

Class someInterface; // the interface you want to check for 
Class x; // 
Class[] interfaces = x.getInterfaces();

for (Class i : interfaces) {
    if (i.toString().equals(someInterface.toString()) {
        // if this is true, the class implements the interface you're looking for
    }
}

Upvotes: 11

Manu Navarro
Manu Navarro

Reputation: 740

You can also set the instance adding ".class"

Class[] classes = ClassUtils.getClasses(handlersPackage);
for(Class clazz : classes)
{
    if(Interface.class.isAssignableFrom(clazz))
    {
        retVal.put(clazz.getSimpleName(), clazz);
    }
}

Upvotes: 1

Ankur
Ankur

Reputation: 12774

you can use the below function to get all the implemented interfaces

Class[] intfs = clazz.getInterfaces();

Upvotes: 19

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