There are many different types of interfaces that allow us to modify TestNG behavior. Listed next are these interfaces:

• ITestListener: A listener used for listening on running tests.
• ISuiteListener: A listener used specifically for test suites.
• IReporter: This is an interface to be implemented if a report has to be generated.
• IMethodInterceptor: This is used to change the collection of methods that TestNG is going to execute.
• IInvokedMethodListener: This gets into action before and after a test method is invoked by TestNG.

• IHookable: If this interface is implemented by a class, then the run() method will be invoked instead of the @test method. The test method will be performed when the callBack() method of the IHookCallBack parameter is invoked.
• IAnnotationTransformer: The transform method will be invoked by TestNG to give you a chance to modify a TestNG annotation read from your test classes. You can change the values by calling any of the setter methods on the ITest interface.
• IAnnotationTransformer2: If any other test annotation needs to be modified apart from the @Test annotation, one can use this interface.

We will mainly be looking at the iTestListener and iSuiteListener interface implementations.

There are two ways in which TestNG listeners can be used:

• By extending the TestListenerAdapter class
• By implementing the corresponding interface, iTestListener

Let's understand both methods.