Attribute API

Attributes are the properties that qualify a business class. For example, an object of type alarm can be qualified by a severity and a description. An attribute is identified by its name and its value and is typed by a Java™ class.

The following figure illustrates the attribute API:

The attribute API

The interface IlpAttribute defines attributes. It provides the following methods:

String getName() returns the attribute name used as an identifier.
Object getValueClass() returns the class of the attribute value.
Object getDefaultValue() returns the default value of the given attribute. The default value is used to initialize the attribute in a newly created business object (see newInstance).
getAttributeGroup getAttributeIlpAttribute.getAttributeGroup() returns the attribute group that contains the attribute instance.
void setAttributeGroup(IlpAttributeGroup group) sets the attribute group that contains the instance.
boolean isTransient() returns whether the attribute should persist or not when the business object is stored.

JViews TGO provides a set of convenience implementations for IlpAttribute that you can use directly or subclass in order to obtain application-specific behavior:

IlpDefaultAttribute—Defines a simple attribute with a name and a value class. The attribute value is not stored in the attribute, but in the object to which this attribute belongs.
IlpStaticAttribute—Defines an attribute with a static value; in other words, this attribute is the same for all objects that contain it.
IlpReferenceAttribute—Defines an attribute with a value derived from the value of another attribute in the same object instance.
IlpObjectReferenceAttribute—Defines an attribute with a value derived from the value of another attribute in another object instance.
IlpComputedAttribute—Defines an attribute with a value that is calculated from a given formula defined by the user. For more information, see Computed attributes.
IlpBeansAttribute—Provides a wrapper for an existing JavaBean™ class property; this wrapper makes it possible to access the property (through introspection) as if it were a dynamic attribute.

Attribute group

Attributes are logically gathered in attribute groups. An attribute group can be static or dynamic. Static attribute groups are defined by the interface IlpAttributeGroup and dynamic attribute groups are defined by IlpMutableAttributeGroup.

These interfaces provide methods that allow you to perform the following operations:

Iterate over the list of attributes
Search for a certain attribute given its name
Verify whether a given attribute is present in the attribute group
Insert and remove attributes
Notify interested objects about the modifications described above

JViews TGO provides the following convenience implementations of IlpAttributeGroup :

IlpDefaultAttributeGroup—Defines a simple dynamic attribute group.
IlpExtendedAttributeGroup—Defines a dynamic attribute group that can be extended with other attribute groups. The attribute groups used are not copied, but simply referred to.

Attribute value holder

In JViews TGO, an attribute value is not carried by the attribute itself but by an attribute value holder because this value is specific to the object with which the attribute is associated. An attribute value holder is defined by the IlpAttributeValueHolder interface. Business objects and representation objects carry attribute values, and as such they implement this interface. The IlpAttributeValueHolder interface includes methods to retrieve and set the value of an attribute, and notify interested objects about changes in attribute values.

Attribute values may be set using the following method:

public void setAttributeValue (IlpAttribute attribute, Object value)

where value may be null .

In cases where attributes have not been set or initialized, the value is IlpAttributeValueHolder.VALUE_NOT_SET .

Attribute values may be retrieved using the following method:

public Object getAttributeValue (IlpAttribute attribute)

Computed attributes

A computed attribute is an attribute which value is derived from the value of one or more other attributes. To implement a computed attribute, you need to extend the abstract class IlpComputedAttribute and implement its abstract part and a constructor. The abstract part of this class is also known as the IlpAttributeValueProvider interface.

The IlpAttributeValueProvider interface contains the following methods:

public Object getValue (IlpAttributeValueHolder h);
public boolean isDependentOn (IlpAttribute a) ;

The getValue method returns a value that is computed from its attribute value holder parameter. The isDependentOn method specifies the attributes used to calculate the value of the computed attribute. Whenever the value of one of these attributes changes, the object carrying this attribute notifies its listeners that the computed attribute value has been modified. Note that the computed value is cached. Therefore, calling the getAttributeValue() method of IlpAttributeValueHolder twice calls the method IlpComputedAttribute. IlpComputedAttribute only once. This cached value is erased whenever the value of an attribute on which the computed attribute depends is modified.

The example below shows how to define a computed attribute that returns a sorted array of integers calculated from another array of integers.

How to define a computed attribute

class  SortIntAttribute extends IlpComputedAttribute {
    IlpAttribute arrayAttribute; 
    public SortIntAttribute(String name,
      IlpAttributeGroup model,
      IlpAttribute arrayAttribute) {
      super(name, model, arrayAttribute.getValueClass());
      this.arrayAttribute = arrayAttribute;
    }

    public Object getValue(IlpAttributeValueHolder h) { 
      Object value = h.getAttributeValue(arrayAttribute);
      if (value != IlpAttributeValueHolder.VALUE_NOT_SET) {
        int[] array = (int [])h.getAttributeValue(arrayAttribute);
        array = (int [])array.clone();
        Arrays. sort(array);
        return array;
      }
      return IlpAttributeValueHolder.VALUE_NOT_SET;
    }

    public boolean isDependentOn (IlpAttribute a) {
      return a == arrayAttribute;
    }
}

The constructor of SortIntAttributes takes three arguments:

name is the name of the attribute,
model specifies the attribute group (usually, an IlpClass ),
arrayAttribute specifies the attribute that the computed attribute depends on.

The constructor of the abstract class IlpComputedAttribute takes three arguments: the name, the model, and the Java class of the value it returns. If the last argument is set to null , the computed attribute returns java.lang.Object . In this example, this argument is specified and corresponds to the value class of the attribute on which the computed attribute depends.

The getValue method computes the attribute value. Computation is protected. As a consequence, if the value of the attribute on which the computed attribute depends is not initialized, or in other words if its value is IlpAttributeValueHolder, the method does not perform the computation and returns IlpAttributeValueHolder.VALUE_NOT_SET . In this example, when the value is set, the returned array is duplicated, sorted, and returned.

The implementation of the isDependentOn method is quite straightforward, since in this example the computed attribute depends only on one other attribute, which is specified in the constructor.

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Business model API