[[validity]]
= Validity

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== Properties

Property edits can also be validated:

[source,java]
----
import lombok.Getter;
import lombok.Setter;

@Property(editing = Editing.ENABLED)            // <.>
@Getter @Setter
private String status;

public String validateStatus(String status) {   // <.>
    // ...
}
----
<.> if required
<.> "validate" prefix, suffix matches up with the property name, takes and returns a string.
A non-null value is the reason why the

== Action parameters

Action arguments can be validated either singly or as a set.
For example:

[source,java]
----
public Customer placeOrder(Product p, int quantity) {
    // ...
}
public String validate0PlaceOrder(Product p) {                  // <.>
    // ...
}
public String validate1PlaceOrder(int quantity)  {              // <.>
    // ...
}
public String validatePlaceOrder(Product p, int quantity)  {    // <.>
    // ...
}
----
<.> "validate" prefix, N-th param; suffix matches action, param of correct type. +
Validates the 0th argument of the action, ie `Product`. +
A non-`null` return string is taken as the _reason_ why the action argument is invalid.
<.> similarly, N-th param. +
Validates the 1st argument of the action, ie `int quantity`
<.> validates all the arguments of the action together. +
A non-`null` return string is taken as the _reason_ why the action arguments taken together are invalid.

The framework validates each argument separately; only if all are valid does it check all the arguments together.




== For more information

For more information, see the xref:refguide:applib-methods:prefixes.adoc#validate[validate...()] section in the appropriate reference guide.
The reference guide also explains how to define validation declaratively, using the xref:refguide:applib:index/annotation/Parameter.adoc#mustSatisfy[@Parameter#mustSatisfy()] or xref:refguide:applib:index/annotation/Property.adoc#mustSatisfy[@Property#mustSatisfy()] attributes.

It's also possible to return xref:userguide:btb:i18n.adoc[locale-specific] strings, by returning xref:refguide:applib:index/services/i18n/TranslatableString.adoc[TranslatableString] instead of `java.lang.String`.


