Java – Test if object implements interface

java

This has probably been asked before, but a quick search only brought up the same question asked for C#. See here.

What I basically want to do is to check wether a given object implements a given interface.

I kind of figured out a solution but this is just not comfortable enough to use it frequently in if or case statements and I was wondering wether Java does not have built-in solution.

public static Boolean implementsInterface(Object object, Class interf){
    for (Class c : object.getClass().getInterfaces()) {
        if (c.equals(interf)) {
            return true;
        }
    }
    return false;
}

EDIT: Ok, thanks for your answers. Especially to Damien Pollet and Noldorin, you made me rethink my design so I don't test for interfaces anymore.

Best Answer

The instanceof operator does the work in a NullPointerException safe way. For example:

 if ("" instanceof java.io.Serializable) {
     // it's true
 }

yields true. Since:

 if (null instanceof AnyType) {
     // never reached
 }

yields false, the instanceof operator is null safe (the code you posted isn't).

instanceof is the built-in, compile-time safe alternative to Class#isInstance(Object)

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Java – How to test a class that has private methods, fields or inner classes

Update:

Some 10 years later perhaps the best way to test a private method, or any inaccessible member, is via @Jailbreak from the Manifold framework.
@Jailbreak Foo foo = new Foo();
// Direct, *type-safe* access to *all* foo's members
foo.privateMethod(x, y, z);
foo.privateField = value;
This way your code remains type-safe and readable. No design compromises, no overexposing methods and fields for the sake of tests.

If you have somewhat of a legacy Java application, and you're not allowed to change the visibility of your methods, the best way to test private methods is to use reflection.

Internally we're using helpers to get/set private and private static variables as well as invoke private and private static methods. The following patterns will let you do pretty much anything related to the private methods and fields. Of course, you can't change private static final variables through reflection.

Method method = TargetClass.getDeclaredMethod(methodName, argClasses);
method.setAccessible(true);
return method.invoke(targetObject, argObjects);

And for fields:

Field field = TargetClass.getDeclaredField(fieldName);
field.setAccessible(true);
field.set(object, value);

Notes:
1. TargetClass.getDeclaredMethod(methodName, argClasses) lets you look into private methods. The same thing applies for getDeclaredField.
2. The setAccessible(true) is required to play around with privates.

Java – “implements Runnable” vs “extends Thread” in Java

Yes: implements Runnable is the preferred way to do it, IMO. You're not really specialising the thread's behaviour. You're just giving it something to run. That means composition is the philosophically "purer" way to go.

In practical terms, it means you can implement Runnable and extend from another class as well... and you can also implement Runnable via a lambda expression as of Java 8.

Best Answer

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Java – How to test a class that has private methods, fields or inner classes

Java – “implements Runnable” vs “extends Thread” in Java

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