birt barcode tool Implementing domain objects with JPA in Java

Creation ECC200 in Java Implementing domain objects with JPA

Implementing domain objects with JPA
Paint Data Matrix In Java
Using Barcode generator for Java Control to generate, create ECC200 image in Java applications.
www.OnBarcode.com
Reading Data Matrix 2d Barcode In Java
Using Barcode reader for Java Control to read, scan read, scan image in Java applications.
www.OnBarcode.com
map to 103.79 and we would have a primary key violation! Another type you should avoid choosing as identifier is TimeStamp. An important consideration to note is that using the @Id annotation on its own works only for identities with just one field or property. In reality, you ll often have to use more than one property or field (known as composite key) to uniquely identify an entity. For sake of illustration, assume that we changed our minds and decided that a Category is uniquely identified by its name and creation date. There are two ways we can accomplish this: by using either the @IdClass or @EmbeddedId annotation. The @IdClass annotation In effect, the @IdClass annotation enables you to use more than one @Id annotation in a sensible way. This is the basic problem with using more than one @Id field or property in an entity class: it is not obvious how to compare two instances in an automated fashion. This is especially true since in cases where composite keys are necessary, one or more of the fields that constitute the primary key are often relationship or association fields (or foreign keys in database terminology). For example, although this is not the case for us, the Bid domain object might have an identity consisting of the item to bid on, the bidder, as well as a bid amount:
Creating EAN-13 In Java
Using Barcode creation for Java Control to generate, create European Article Number 13 image in Java applications.
www.OnBarcode.com
Code-39 Generator In Java
Using Barcode generation for Java Control to generate, create Code 39 image in Java applications.
www.OnBarcode.com
public class Bid { private Item item; private Bidder bidder; private Double amount; ... }
Code 3/9 Maker In Java
Using Barcode maker for Java Control to generate, create Code 3/9 image in Java applications.
www.OnBarcode.com
Making QR Code ISO/IEC18004 In Java
Using Barcode creation for Java Control to generate, create QR-Code image in Java applications.
www.OnBarcode.com
In this snippet, both the item and bidder instance variables represent relationship references to other entities. You could be tempted to combine item, bidder, and amount as a composite key for Bid, but remember that it might be the case that neither of the references is simple enough to compare instances using the equals method, as it would be for a java.lang.String or java.lang.Long. This is where a designated IdClass comes in. The best way to understand how this works is through an example. For simplicity, we ll return to our Category object with the name and creation date identity. Listing 7.2 shows how the solution might look.
Barcode Maker In Java
Using Barcode maker for Java Control to generate, create Barcode image in Java applications.
www.OnBarcode.com
USD8 Maker In Java
Using Barcode generator for Java Control to generate, create Code 11 image in Java applications.
www.OnBarcode.com
Listing 7.2 Specifying category identity using IdClass
ECC200 Decoder In None
Using Barcode decoder for Software Control to read, scan read, scan image in Software applications.
www.OnBarcode.com
DataMatrix Generator In Java
Using Barcode generator for Java Control to generate, create ECC200 image in Java applications.
www.OnBarcode.com
public class CategoryPK implements Serializable { String name; Date createDate;
Decode PDF-417 2d Barcode In None
Using Barcode scanner for Software Control to read, scan read, scan image in Software applications.
www.OnBarcode.com
EAN / UCC - 13 Creator In None
Using Barcode maker for Online Control to generate, create EAN128 image in Online applications.
www.OnBarcode.com
Stored identity fields
DataMatrix Scanner In VB.NET
Using Barcode decoder for .NET framework Control to read, scan read, scan image in .NET applications.
www.OnBarcode.com
Print PDF-417 2d Barcode In None
Using Barcode maker for Font Control to generate, create PDF-417 2d barcode image in Font applications.
www.OnBarcode.com
Implementing domain models
Generating UPC-A In None
Using Barcode maker for Font Control to generate, create GS1 - 12 image in Font applications.
www.OnBarcode.com
GS1 - 12 Generator In None
Using Barcode drawer for Software Control to generate, create UPC-A image in Software applications.
www.OnBarcode.com
public CategoryPK() {}
Make GTIN - 13 In Objective-C
Using Barcode printer for iPhone Control to generate, create GS1 - 13 image in iPhone applications.
www.OnBarcode.com
DataMatrix Printer In None
Using Barcode generator for Online Control to generate, create ECC200 image in Online applications.
www.OnBarcode.com
Empty constructor
Creating PDF417 In C#.NET
Using Barcode creator for Visual Studio .NET Control to generate, create PDF-417 2d barcode image in Visual Studio .NET applications.
www.OnBarcode.com
Printing UPCA In Objective-C
Using Barcode creation for iPad Control to generate, create GS1 - 12 image in iPad applications.
www.OnBarcode.com
equals method
comparing identity public boolean equals(Object other) { if (other instanceof CategoryPK) { final CategoryPK otherCategoryPK = (CategoryPK)other; return (otherCategory.name.equals(name) && otherCategoryPK.createDate.equals(createDate)); }
return false; } public int hashCode() { return super.hashCode(); } } @Entity @IdClass(CategoryPK.class) public class Category { public Category() {} @Id protected String name;
Hashcode implementation
IdClass specification
@Id protected Date createDate; ... }
Identity fields
As shown in listing 7.2, the CategoryPK class is designated as the IdClass for Category G. The Category class has two identity fields marked by the @Id annotation: name and creationDate G. These two identity fields are mirrored in the CategoryPK class b. The constructor C is used to create an instance of the primary key object. The equals method implemented in CategoryPK compares the two mirrored identity fields to determine if two given identities are equal D. The magic here is that at runtime, the persistence provider determines if two Category objects are equal by copying the marked @Id fields into the corresponding fields of the CategoryPK object and using CategoryPK.equals. Note that any IdClass must be Serializable and must provide a valid hashCode implementation E. In effect, all that is happening here is that we are specifying exactly how to compare multiple identity fields using an external IdClass F. The disadvantage to using @IdClass is the slight redundancy and associated maintainability problems in repeating the definition of identity fields in the entity and the IdClass. In our case the name and
Implementing domain objects with JPA
createDate fields are defined in both the Category and CategoryPK classes. However, the IdClass approach keeps your domain model clutter free, especially as
opposed to the slightly awkward object model proposed by the third approach, which uses the @EmbeddedId annotation. The @EmbeddedId annotation Using the @EmbeddedId annotation is like moving the IdClass right into your entity and using the identity fields nested inside it to store entity data. Take a look at what we mean in listing 7.3, which rewrites listing 7.2 using @EmbeddedId.
Copyright © OnBarcode.com . All rights reserved.