Question

what is annotation class. what is the use of it in java/android.

In iphone Annotation is used to drop a pin on the map..

java has java.lang.Annotation package… what is the use of it? can i have a examples, tutorials,sample codes, etc?

Answer 1

The two concepts are unrelated:

• In the iPhone SDK, an annotation is a sort of note that you can put on a map.
• In Java (and the Android SDK), annotations are a feature of the Java language used to add specific kinds of metadata to classes.

In the Java world, annotations are compile-time tags which are attached to a piece of code. They add metadata to a snippet of code which tools or third parties (or even your own code) can then use later. But they usually don’t affect the way that code runs; that is, if you deleted the annotation, that snippet of code typically shouldn’t behave any differently.

A simple example is marking a piece of code with a copyright notice. Here’s an annotation that we might use for this:

public @interface Copyright {
    String value();
}

And here’s how we’d attach that metadata to code:

@Copyright(value = "2010, United States National Security Administration")
public class QuantumCryptographyDecoder { ... }

If there’s only one annotation property named value you can omit the value = specifier as a shortcut:

@Copyright("2010, United States National Security Administration")
public class QuantumCryptographyDecoder { ... }

Otherwise you have to specify it:

@Created(year = 2010)
public class ShinyNewClass { ... }

If we removed the @Copyright annotation, the QuantumCryptographyDecoder would still work the same as it did before. There just wouldn’t be any Copyright metadata attached to it. But a third-party tool which did some kind of validation (for instance, requiring that all classes whose names included “Quantum” have a @Copyright notice attached) could alert you to the missing @Copyright or take some other useful action.