public class Foo {
    private String name;
    public Foo(@NonNull String name){...};
    ...
}

Here @NonNull is annotation which is processed compile time by the android studio to warn you that the particular function needs non null parameter.

There are three types of annotations.

@interface CustomAnnotation {}

@interface CustomAnnotation {  
    int value();  
}

@interface CustomAnnotation{  
    int value1();  
    String value2();  
    String value3();  
}

For creating custom annotations we need to decide

For this, we have built in custom annotations. Check out these mostly used ones:

@Target

(opens new window)

@Retention

(opens new window)

Creating Custom Annotation

@Retention(RetentionPolicy.SOURCE) // will not be available in compiled class   
@Target(ElementType.METHOD) // can be applied to methods only
@interface CustomAnnotation{  
      int value();    
}

Using Custom Annotation

class Foo{  
  @CustomAnnotation(value = 1)  // will be used by an annotation processor
  public void foo(){..}  
}

the value provided inside @CustomAnnotation will be consumed by an Annotationprocessor may be to generate code at compile time etc.