CODEWITHSUNDEEP
javaruntimecompile-timetypechecking
user1154664
user1154664

Reputation: 1392

Check in compile time if someClass.class is derived from a anotherClass.class?

this seems to be a newbie question, In Java, how can I check at compile time if someClass.class is derived from a anotherClass.class or at run-time if at compile time is not possible?

Upvotes: 1

Views: 220

Answers (3)

Adrian Shum
Adrian Shum

Reputation: 40036

Checking in runtime is straightforward as other people has already suggested. However, it is possible to do in compile time with a little trick.

You can make use of generics to have a method like this:

public static <A, B extends A> void isExtends(Class<A> clazzA, Class<B> clazzB) {
    // empty
}

If you want to make sure someClass.class is derived from a anotherClass.class, simply put in your code:

isExtends(AnotherClass.class, SomeClass.class);

Compiler will help you to verify if SomeClass extends from AnotherClass.

(But honestly, I wonder if this is actually something useful in real life :P )

Upvotes: 0

user207421
user207421

Reputation: 310884

You can check at runtime with Class.isAssignableFrom() if you only have classes, or the instanceof operator if you have instances.

As for compile time, I don't understand the question. And why wouldn't the person writing the code know before compile time?

Upvotes: 0

Bohemian
Bohemian

Reputation: 424993

In code:

anotherClass.class.isAssignableFrom( someClass.class );

If you want to also check that the other class is a real class (and not an interface) add in a call to isInterface():

!anotherClass.class.isInterface() && anotherClass.class.isAssignableFrom( someClass.class );

Example:

System.out.println( Number.class.isAssignableFrom( Integer.class )); // true

Note that instanceof won't work for you because you don't have instances, only classes.

Excerpt from Javadoc for Class.isAssignableFrom:

Determines if the class or interface represented by this Class object is either the same as, or is a superclass or superinterface of, the class or interface represented by the specified Class parameter. It returns true if so; otherwise it returns false. If this Class object represents a primitive type, this method returns true if the specified Class parameter is exactly this Class object; otherwise it returns false.

Upvotes: 3

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