Using NotNull Annotation in method argument

Question

@Nullable and @NotNull do nothing on their own. They are supposed to act as Documentation tools.

The @Nullable Annotation reminds you about the necessity to introduce an NPE check when:

1. Calling methods that can return null.
2. Dereferencing variables (fields, local variables, parameters) that can be null.

The @NotNull Annotation is, actually, an explicit contract declaring the following:

1. A method should not return null.
2. A variable (like fields, local variables, and parameters) cannot should not hold null value.

For example, instead of writing:

/**
 * @param aX should not be null
 */
public void setX(final Object aX ) {
    // some code
}

You can use:

public void setX(@NotNull final Object aX ) {
    // some code
}

Additionally, @NotNull is often checked by ConstraintValidators (eg. in spring and hibernate).

The @NotNull annotation doesn't do any validation on its own because the annotation definition does not provide any ConstraintValidator type reference.

For more info see:

1. Bean validation
2. NotNull.java
3. Constraint.java
4. ConstraintValidator.java

As mentioned above @NotNull does nothing on its own. A good way of using @NotNull would be using it with Objects.requireNonNull

public class Foo {
    private final Bar bar;

    public Foo(@NotNull Bar bar) {
        this.bar = Objects.requireNonNull(bar, "bar must not be null");
    }
}

To make @NonNull active you need Lombok:

https://projectlombok.org/features/NonNull

import lombok.NonNull;

Follow: Which @NotNull Java annotation should I use?

SO @NotNull just is a tag...If you want to validate it, then you must use something like hibernate validator jsr 303

ValidatorFactory validatorFactory = Validation.buildDefaultValidatorFactory();
Validator validator = validatorFactory.getValidator();
 Set<ConstraintViolation<List<Searching>> violations = validator.validate(searchingList);

If you are using Spring, you can force validation by annotating the class with @Validated:

import org.springframework.validation.annotation.Validated;

More info available here: Javax @NotNull annotation usage

You could also use @NonNull from projectlombok (lombok.NonNull)