What is the method signature means? String value() default "";

Question

I came across this method signature in Spring's Component interface.

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Indexed
public @interface Component 
{
   String value() default "";
}

What does the method signature String value() default ""; mean? How and When should we define such syntax for our coding purposes?

Answer 1

This is no method signature. it means that you can pass a String as parameter to the Component annotation, like this:

@Component(value = "value")

If you don't specify a value your self, the default value "" will be used.

If it had been like this:

String value(); // without the default

value would have been a mandatory parameter. Trying to use Component like this:

@Component()

would lead to an Exception, since you didn't provide a value.

EDIT: when to use.

If you don't know much about this syntax, or annotations in general, you shouldn't use them. About everything that can be done using annotations, especially custom made ones, can also be done without annotations.

Let's say you want to create an annotation to validate the value of a field. I'll be using the example of Belgian postal codes. They all are 4 digits, and are between 1000 and 9999.

@Target( {ElementType.FIELD})
@Retention( RetentionPolicy.RUNTIME)
@Constraint( validatedBy = ValidatePostalCodeImpl.class)
public @interface ValidatePostalCode{
  String message() default "You have entered an invalid postal code";
  Class<?>[] groups() default {}; // needed for the validation
  Class<? extends Payload>[] payload() default{}; // needed for the validation

  int maxValue() default 9999; // so, by default, this will validate based
  int minValue() default 1000; // on these values, but you will be able to 
  // override these
}

/* Validation implementation */

public class ValidatePostalCodeImpl implements ConstraintValidator<ValidatePostalCode, Integer> {

    int upperValue;
    int lowerValue;

    @Override
    public void initialize(ValidatePostalCode validate) {
        this.upperValue = validate.maxValue(); // here you call them as if they indeed were regular methods
        this.lowerValue = validate.minValue();
    }

    @Override
    public boolean isValid(Integer integer, ConstraintValidatorContext context) {
        return integer >= lowerValue && integer <= upperValue;
    }

}

/* Usage */

@Entity
@Table(name = "addresses")
public class Addresses {

  // home address -> In Belgium, so has to be between the default values:
  @ValidatePostalCode
  Integer belgianPostalCode;

  // vacation home in another country, let's say the PC's there are between
  // 12000 and 50000
  @ValidatePostalCode(minValue = 12000, maxValue = 50000)
  Integer foreignPostalCode;

}

Sure, this is a very limited example, but it should get you an idea.