Java Annotations

Question

What is the purpose of annotations in Java? I have this fuzzy idea of them as somewhere in between a comment and actual code. Do they affect the program at run time?

Answer 1

Anders gives a good summary, and here's an example of a JUnit annotation

@Test(expected=IOException.class)
public void flatfileMissing() throws IOException {
    readFlatFile("testfiles"+separator+"flatfile_doesnotexist.dat");
}

Here the @Test annotation is telling JUnit that the flatfileMissing method is a test that should be executed and that the expected result is a thrown IOException. Thus, when you run your tests, this method will be called and the test will pass or fail based on whether an IOException is thrown.