vaadin-framework/documentation/datamodel/datamodel-properties.asciidoc

---
title: Properties
order: 2
layout: page
---

[[datamodel.properties]]
= Properties

The [interfacename]#Property# interface is the base of the Vaadin Data Model. It
provides a standardized API for a single data value object that can be read
(get) and written (set). A property is always typed, but can optionally support
data type conversions. The type of a property can be any Java class. Optionally,
properties can provide value change events for following their changes.

You can set the value of a property with [methodname]#setValue()# and read with
[methodname]#getValue()#.

In the following, we set and read the property value from a
[classname]#TextField# component, which implements the [interfacename]#Property#
interface to allow accessing the field value.

[source, java]
----
final TextField tf = new TextField("Name");
        
// Set the value
tf.setValue("The text field value");
        
// When the field value is edited by the user
tf.addValueChangeListener(
    new Property.ValueChangeListener() {
    public void valueChange(ValueChangeEvent event) {
        // Do something with the new value
        layout.addComponent(new Label(tf.getValue()));
    }
});
----
See the http://demo.vaadin.com/book-examples-vaadin7/book#datamodel.properties.basic[on-line example, window="_blank"].

Changes in the property value usually fire a [classname]#ValueChangeEvent#,
which can be handled with a [classname]#ValueChangeListener#. The event object
provides reference to the property with [methodname]#getProperty()#. Note that
its [methodname]#getValue()# method returns the value with [classname]#Object#
type, so you need to cast it to the proper type.

Properties are in themselves unnamed. They are collected in __items__, which
associate the properties with names: the __Property Identifiers__ or __PID__s.
Items can be further contained in containers and are identified with __Item
Identifiers__ or __IID__s. In the spreadsheet analogy, __Property Identifiers__
would correspond to column names and __Item Identifiers__ to row names. The
identifiers can be arbitrary objects, but must implement the
[methodname]#equals(Object)# and [methodname]#hashCode()# methods so that they
can be used in any standard Java [classname]#Collection#.

The [classname]#Property# interface can be utilized either by implementing the
interface or by using some of the built-in property implementations. Vaadin
includes a [classname]#Property# interface implementation for arbitrary function
pairs and bean properties, with the [classname]#MethodProperty# class, and for
simple object properties, with the [classname]#ObjectProperty# class, as
described later.

In addition to the simple components, selection components provide their current
selection as the property value. In single selection mode, the property is a
single item identifier, while in multiple selection mode it is a set of item
identifiers. See the documentation of the selection components for further
details.

Components that can be bound to a property have an internal default data source
object, typically a [classname]#ObjectProperty#, which is described later. As
all such components are viewers or editors, also described later, so you can
rebind a component to any data source with
[methodname]#setPropertyDataSource()#.

[[datamodel.properties.viewers]]
== Property Viewers and Editors

The most important function of the [classname]#Property# as well as of the other
data model interfaces is to connect classes implementing the interface directly
to editor and viewer classes. This means connecting a data source (model) to a
user interface component (views) to allow editing or viewing the data model.

A property can be bound to a component implementing the [classname]#Viewer#
interface with [methodname]#setPropertyDataSource()#.

[source, java]
----
// Have a data model
ObjectProperty property =
    new ObjectProperty("Hello", String.class);
        
// Have a component that implements Viewer
Label viewer = new Label();
        
// Bind it to the data
viewer.setPropertyDataSource(property);
----

You can use the same method in the [classname]#Editor# interface to bind a
component that allows editing a particular property type to a property.

[source, java]
----
// Have a data model
ObjectProperty property =
    new ObjectProperty("Hello", String.class);
        
// Have a component that implements Viewer
TextField editor = new TextField("Edit Greeting");
        
// Bind it to the data
editor.setPropertyDataSource(property);
----

As all field components implement the [classname]#Property# interface, you can
bind any component implementing the [classname]#Viewer# interface to any field,
assuming that the viewer is able the view the object type of the field.
Continuing from the above example, we can bind a [classname]#Label# to the
[classname]#TextField# value:

[source, java]
----
Label viewer = new Label();
viewer.setPropertyDataSource(editor);

// The value shown in the viewer is updated immediately
// after editing the value in the editor (once it
// loses the focus)
editor.setImmediate(true);
----

If a field has validators, as described in
<<dummy/../../../framework/components/components-fields#components.fields.validation,"Field
Validation">>, the validators are executed before writing the value to the
property data source, or by calling the [methodname]#validate()# or
[methodname]#commit()# for the field.


[[datamodel.properties.objectproperty]]
== [classname]#ObjectProperty# Implementation

The [classname]#ObjectProperty# class is a simple implementation of the
[classname]#Property# interface that allows storing an arbitrary Java object.

[source, java]
----
// Have a component that implements Viewer interface
final TextField tf = new TextField("Name");
        
// Have a data model with some data
String myObject = "Hello";
        
// Wrap it in an ObjectProperty
ObjectProperty property =
    new ObjectProperty(myObject, String.class);
        
// Bind the property to the component
tf.setPropertyDataSource(property);
----

[[datamodel.properties.converter]]
== Converting Between Property Type and Representation

Fields allow editing a certain type, such as a [classname]#String# or
[classname]#Date#. The bound property, on the other hand, could have some
entirely different type. Conversion between a representation edited by the field
and the model defined in the property is handler with a converter that
implements the [interfacename]#Converter# interface.

Most common type conversions, such as between string and integer, are handled by
the default converters. They are created in a converter factory global in the
application.

[[datamodel.properties.converter.basic]]
=== Basic Use of Converters

The [methodname]#setConverter([interfacename]#Converter#)# method sets the
converter for a field. The method is defined in [classname]#AbstractField#.

[source, java]
----
// Have an integer property
final ObjectProperty<Integer> property =
        new ObjectProperty<Integer>(42); 
        
// Create a TextField, which edits Strings
final TextField tf = new TextField("Name");

// Use a converter between String and Integer
tf.setConverter(new StringToIntegerConverter());

// And bind the field
tf.setPropertyDataSource(property);
----

The built-in converters are the following:

[[datamodel.properties.converter.basic.built-in]]
.Built-in Converters
[options="header"]
|===============
|Converter|Representation|Model
|[classname]#StringToIntegerConverter#|[classname]#String#|[classname]#Integer#
|[classname]#StringToDoubleConverter#|[classname]#String#|[classname]#Double#
|[classname]#StringToNumberConverter#|[classname]#String#|[classname]#Number#
|[classname]#StringToBooleanConverter#|[classname]#String#|[classname]#Boolean#
|[classname]#StringToDateConverter#|[classname]#String#|[classname]#Date#
|[classname]#DateToLongConverter#|[classname]#Date#|[classname]#Long#

|===============



In addition, there is a [classname]#ReverseConverter# that takes a converter as
a parameter and reverses the conversion direction.

If a converter already exists for a type, the
[methodname]#setConverter([interfacename]#Class#)# retrieves the converter for
the given type from the converter factory, and then sets it for the field. This
method is used implicitly when binding field to a property data source.


[[datamodel.properties.converter.custom]]
=== Implementing a Converter

A conversion always occurs between a __representation type__, edited by the
field component, and a __model type__, that is, the type of the property data
source. Converters implement the [interfacename]#Converter# interface defined in
the [package]#com.vaadin.data.util.converter# package.

For example, let us assume that we have a simple [classname]#Complex# type for
storing complex values.

[source, java]
----
public class ComplexConverter
       implements Converter<String, Complex> {
    @Override
    public Complex convertToModel(String value, Locale locale)
            throws ConversionException {
        String parts[] =
            value.replaceAll("[\\(\\)]", "").split(",");
        if (parts.length != 2)
            throw new ConversionException(
                    "Unable to parse String to Complex");
        return new Complex(Double.parseDouble(parts[0]),
                           Double.parseDouble(parts[1]));
    }

    @Override
    public String convertToPresentation(Complex value,
                                        Locale locale)
            throws ConversionException {
        return "("+value.getReal()+","+value.getImag()+")";
    }

    @Override
    public Class<Complex> getModelType() {
        return Complex.class;
    }

    @Override
    public Class<String> getPresentationType() {
        return String.class;
    }
}
----

The conversion methods get the locale for the conversion as a parameter.


[[datamodel.properties.converter.converterfactory]]
=== Converter Factory

If a field does not directly allow editing a property type, a default converter
is attempted to create using an application-global converter factory. If you
define your own converters that you wish to include in the converter factory,
you need to implement one yourself. While you could implement the
[interfacename]#ConverterFactory# interface, it is usually easier to just extend
[classname]#DefaultConverterFactory#.

[source, java]
----
class MyConverterFactory extends DefaultConverterFactory {
    @Override
    public <PRESENTATION, MODEL> Converter<PRESENTATION, MODEL>
            createConverter(Class<PRESENTATION> presentationType,
                            Class<MODEL> modelType) {
        // Handle one particular type conversion
        if (String.class == presentationType &&
            Complex.class == modelType)
            return (Converter<PRESENTATION, MODEL>)
                   new ComplexConverter();

        // Default to the supertype
        return super.createConverter(presentationType,
                                     modelType);
    }
}

// Use the factory globally in the application
UI.getCurrent().getSession().setConverterFactory(
        new MyConverterFactory());
----



ifdef::web[]
[[datamodel.properties.implementing]]
== Implementing the [classname]#Property# Interface

Implementation of the [classname]#Property# interface requires defining setters
and getters for the value and the __read-only__ mode. Only a getter is needed
for the property type, as the type is often fixed in property implementations.

The following example shows a simple implementation of the [classname]#Property#
interface:

[source, java]
----
class MyProperty implements Property {
    Integer data     = 0;
    boolean readOnly = false;
    
    // Return the data type of the model
    public Class<?> getType() {
        return Integer.class;
    }

    public Object getValue() {
        return data;
    }
    
    // Override the default implementation in Object
    @Override
    public String toString() {
        return Integer.toHexString(data);
    }

    public boolean isReadOnly() {
        return readOnly;
    }

    public void setReadOnly(boolean newStatus) {
        readOnly = newStatus;
    }

    public void setValue(Object newValue)
            throws ReadOnlyException, ConversionException {
        if (readOnly)
            throw new ReadOnlyException();
            
        // Already the same type as the internal representation
        if (newValue instanceof Integer)
            data = (Integer) newValue;
        
        // Conversion from a string is required
        else if (newValue instanceof String)
            try {
                data = Integer.parseInt((String) newValue, 16);
            } catch (NumberFormatException e) {
                throw new ConversionException();
            }
        else
             // Don't know how to convert any other types
            throw new ConversionException();

        // Reverse decode the hexadecimal value
    }
}
        
// Instantiate the property and set its data
MyProperty property = new MyProperty();
property.setValue(42);
        
// Bind it to a component
final TextField tf = new TextField("Name", property);
----

The components get the displayed value by the [methodname]#toString()# method,
so it is necessary to override it. To allow editing the value, value returned in
the [methodname]#toString()# must be in a format that is accepted by the
[methodname]#setValue()# method, unless the property is read-only. The
[methodname]#toString()# can perform any type conversion necessary to make the
internal type a string, and the [methodname]#setValue()# must be able to make a
reverse conversion.

The implementation example does not notify about changes in the property value
or in the read-only mode. You should normally also implement at least the
[classname]#Property.ValueChangeNotifier# and
[classname]#Property.ReadOnlyStatusChangeNotifier#. See the
[classname]#ObjectProperty# class for an example of the implementation.

endif::web[]