Adding @NotNull or Pattern constraints on List

Question

How can we ensure the individual strings inside a list are not null/blank or follow a specific pattern

@NotNull
List emailIds;

Answer 1

You can create a simple wrapper class for the e-mail String:

public class EmailAddress {

    @Pattern("\b[A-Z0-9._%+-]+@[A-Z0-9.-]+.[A-Z]{2,4}\b.")
    String email;

    //getters and setters
}

Then mark the field @Valid in your existing object:

@NotNull
@Valid
List<EmailAddress> emailIds;

The validator will then validate each object in the list.

Answer 2

In my opinion, use a wrapper class for the object, and have your own verification on the methods:

public class ListWrapper<E> {

    private List<E> list = new ArrayList<>();
    private Pattern check = /*pattern*/;

    public boolean add(E obj) {
        if (this.verify(obj)) {
            return list.add(obj);
        }
        return false;
    }

    //etc

    public boolean verify(E obj) {
        //check pattern and for null
    }

Alternatively, just use a custom object for the list

Answer 3

You don’t have to use any wrapper class just to validate a list of strings. Just use @EachPattern constraint from validator-collection:

@NotNull
@EachPattern(regexp="\b[A-Z0-9._%+-]+@[A-Z0-9.-]+.[A-Z]{2,4}\b.")
List<String> values;

And that’s all. Easy, right? See this SO answer for more information.

Answer 4

Bean validation 2.0 (Hibernate Validator 6.0.1 and above) supports validating container elements by annotating type arguments of parameterized types. Example:

List<@Positive Integer> positiveNumbers;

Or even (although a bit busy):

List<@NotNull @Pattern(regexp="\\b[A-Z0-9._%+-]+@[A-Z0-9.-]+.[A-Z]{2,4}\\b") String> emails;

References:

• http://beanvalidation.org/2.0/
• https://docs.jboss.org/hibernate/stable/validator/reference/en-US/html_single/#_with_code_list_code