---
title: Field Components
order: 4
layout: page
---

[[components.fields]]
= Field Components

((("[classname]#Field#", id="term.components.fields", range="startofrange")))

_Fields_ are components that have a value that the user can change through the
user interface. <<figure.components.fields>> illustrates the inheritance relationships
and the important interfaces and base classes.

[[figure.components.fields]]
.Field Components
image::img/field-class-hierarchy.png[width=100%, scaledwidth=100%]

Field components are built upon the framework defined in the [classname]#HasValue#
interface.
[classname]#AbstractField# is the base class for all field components,
except those components that allow the user to select a value.
(see <<components-selection.asciidoc#components.selection,"Selection Components">>).

In addition to the component features inherited from
[classname]#AbstractComponent#, it implements the features defined in the
[interfacename]#HasValue# and [classname]#Component.Focusable# interfaces.

[[figure.components.fields.hasvalue]]
.Field components having values
image::img/field-interface-v8-hi.png[width=60%, scaledwidth=100%]

The description of the [interfacename]#HasValue# interface and field components extending [classname]#AbstractField] is broken down in the following sections.

[[components.fields.field]]
== The [interfacename]#HasValue# Interface

The [interfacename]#HasValue# interface marks a component that has a user editable value.
The type parameter in the interface is the type of the value that the component is editing.

You can set the value with the [methodname]#setValue()# and read it with the
[methodname]#getValue()# method defined in the [classname]#HasValue# interface.

The [classname]#HasValue# interface defines a number of properties, which you can
access with the corresponding setters and getters.


[methodname]#readOnly#:: Set the component to be read-only, meaning that the value is not editable.

[methodname]#requiredIndicatorVisible#:: When enabled, a required indicator
(the asterisk * character) is displayed on the left, above, or right the field,
depending on the containing layout and whether the field has a caption.
When the component is used in a form (see <<../datamodel/datamodel-forms.asciidoc#datamodel.forms.validation,"Validation">>),
it can be set to be required, which will automatically show the required indicator,
and validate that the value is not empty. Without validation, the required indicator
is merely a visual guide.

[methodname]#emptyValue#:: The initial empty value of the component.

[methodname]#clear#:: Clears the value to the empty value.


[[components.fields.valuechanges]]
== Handling Value Changes

[interfacename]#HasValue# provides [methodname]#addValueChangeListener# method for listening to changes to the field value. This method returns a [classname]#Registration# object that can be used to later
remove the added listener if necessary.

[source, java]
----
TextField textField = new TextField();
Label echo = new Label();

textField.addValueChangeListener(event -> {
    String origin = event.isUserOriginated()
        ? "user"
        : "application";
    String message = origin
        + " entered the following: "
        + event.getValue();
    Notification.show(message);
});
----


[[components.fields.databinding]]
== Binding Fields to Data

Fields can be grouped into _forms_ and coupled with business data objects with
the [classname]#Binder# class. When a field is bound to a property using
[classname]#Binder#, it gets its default value from the property, and
is stored to the property either manually via the [methodname]#Binder.save# method,
or automatically every time the value changes.

[source, java]
----
class Person {
    private String name;
    public String getName() { /* ... */ }
    public void setName(String) { /* ... */ }
}

TextField nameField = new TextField();

Binder<Person> binder = new Binder<>();

// Bind nameField to the Person.name property
// by specifying its getter and setter
binder.bind(nameField, Person::getName, Person::setName);

// Bind an actual concrete Person instance.
// After this, whenever the user changes the value
// of nameField, p.setName is automatically called.
Person p = new Person();
binder.setBean(p);
----

For more information on data binding, see <<../datamodel/datamodel-forms.asciidoc#datamodel.forms,"Binding Data to Forms">>


== Validating Field Values

User input may be syntactically or semantically invalid.
[classname]#Binder# allows adding a chain of one or more __validators__ for
automatically checking the validity of the input before storing it to the data
object. You can add validators to fields by calling the [methodname]#withValidator#
method on the [interfacename]#Binding# object returned by [methodname]#Binder.forField#.
There are several built-in validators in the Framework, such as the [classname]#StringLengthValidator# used below.

[source, java]
----
binder.forField(nameField)
    .withValidator(new StringLengthValidator(
        "Name must be between 2 and 20 characters long",
        2, 20))
    .bind(Person::getName, Person::setName);
----

Failed validation is by default indicated with the error indicator of the field, described in
<<../application/application-errors#application.errors.error-indicator,"Error
Indicator and Message">>. Hovering mouse on the field displays the error message
returned by the validator. If any value in a set of bound fields fails validation,
none of the field values are saved into the bound property until the validation
passes.


=== Implementing Custom Validators

Validators implement the [interfacename]#Validator# interface that simply
extends [interfacename]#java.util.function.Function#, returning a special type
called [interfacename]#Result#. This return type represents the validation outcome:
whether or not the given input was valid.

[source, java]
----
class MyValidator implements Validator<String> {
    @Override
    public ValidationResult apply(String value, ValueContext context) {
        if(value.length() == 6) {
            return ValidationResult.ok();
        } else {
            return ValidationResult.error(
                "Must be exactly six characters long");
        }
    }
}
----

Since [interfacename]#Validator# is a functional
interface, you can often simply write a lambda expression instead of a full class
declaration. There is also an [methodname]#withValidator# overload that creates a
validator from a boolean function and an error message. If the application requires
more sophisticated validation diagnostics (e.g. locale-specific), there is a
method [methodname]#withValidator#, which uses a boolean function and an [classname]#ErrorMessageProvider#.
The [classname]#ErrorMessageProvider# can compose diagnostic messages based on the locale of the validation
and the source component value, which are provided with the [classname]#ValueContext#.

[source, java]
----
binder.forField(nameField)
    .withValidator(name -> name.length() < 20,
        "Name must be less than 20 characters long")
     .bind(Person::getName, Person::setName);

----

== Converting Field Values

Field values are always of some particular type. For example,
[classname]#TextField# allows editing [classname]#String# values. When bound to
a data source, the type of the source property can be something different,
say an [classname]#Integer#. __Converters__ are used for converting the values
between the presentation and the model. Their usage is described in
<<../datamodel/datamodel-forms.asciidoc#datamodel.forms.conversion,"Conversion">>.


(((range="endofrange", startref="term.components.fields")))