c# create barcode image Figure 1.3 Using inheritance for different billing strategies in Java

Printer ECC200 in Java Figure 1.3 Using inheritance for different billing strategies

Figure 1.3 Using inheritance for different billing strategies
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The paradigm mismatch
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In chapter 3, section 3.6, Mapping class inheritance, we discuss how object/ relational mapping solutions such as Hibernate solve the problem of persisting a class hierarchy to a database table or tables. This problem is now quite well understood in the community, and most solutions support approximately the same functionality. But we aren t quite finished with inheritance as soon as we introduce inheritance into the object model, we have the possibility of polymorphism. The User class has an association to the BillingDetails superclass. This is a polymorphic association. At runtime, a User object might be associated with an instance of any of the subclasses of BillingDetails. Similarly, we d like to be able to write queries that refer to the BillingDetails class and have the query return instances of its subclasses. This feature is called polymorphic queries. Since SQL databases don t provide a notion of inheritance, it s hardly surprising that they also lack an obvious way to represent a polymorphic association. A standard foreign key constraint refers to exactly one table; it isn t straightforward to define a foreign key that refers to multiple tables. We might explain this by saying that Java (and other object-oriented languages) is less strictly typed than SQL. Fortunately, two of the inheritance mapping solutions we show in chapter 3 are designed to accommodate the representation of polymorphic associations and efficient execution of polymorphic queries. So, the mismatch of subtypes is one in which the inheritance structure in your Java model must be persisted in an SQL database that doesn t offer an inheritance strategy. The next aspect of the mismatch problem is the issue of object identity. You probably noticed that we defined USERNAME as the primary key of our USER table. Was that a good choice Not really, as you ll see next.
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1.2.3 The problem of identity
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Although the problem of object identity might not be obvious at first, we ll encounter it often in our growing and expanding example e-commerce system. This problem can be seen when we consider two objects (for example, two Users) and check if they re identical. There are three ways to tackle this problem, two in the Java world and one in our SQL database. As expected, they work together only with some help. Java objects define two different notions of sameness:
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Object identity (roughly equivalent to memory location, checked with a==b) Equality as determined by the implementation of the equals() method (also called equality by value)
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Understanding object/relational persistence
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On the other hand, the identity of a database row is expressed as the primary key value. As you ll see in section 3.4, Understanding object identity, neither equals() nor == is naturally equivalent to the primary key value. It s common for several (nonidentical) objects to simultaneously represent the same row of the database. Furthermore, some subtle difficulties are involved in implementing equals() correctly for a persistent class. Let s discuss another problem related to database identity with an example. In our table definition for USER, we ve used USERNAME as a primary key. Unfortunately, this decision makes it difficult to change a username: We d need to update not only the USERNAME column in USER, but also the foreign key column in BILLING_DETAILS. So, later in the book, we ll recommend that you use surrogate keys wherever possible. A surrogate key is a primary key column with no meaning to the user. For example, we might change our table definitions to look like this:
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create table USER ( USER_ID BIGINT NOT NULL PRIMARY KEY, USERNAME VARCHAR(15) NOT NULL UNIQUE, NAME VARCHAR(50) NOT NULL, ... ) create table BILLING_DETAILS ( BILLING_DETAILS_ID BIGINT NOT NULL PRIMARY KEY, ACCOUNT_NUMBER VARCHAR(10) NOT NULL UNIQUE, ACCOUNT_NAME VARCHAR(50) NOT NULL, ACCOUNT_TYPE VARCHAR(2) NOT NULL, USER_ID BIGINT FOREIGN KEY REFERENCES USER )
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The USER_ID and BILLING_DETAILS_ID columns contain system-generated values. These columns were introduced purely for the benefit of the relational data model. How (if at all) should they be represented in the object model We ll discuss this question in section 3.4 and find a solution with object/relational mapping. In the context of persistence, identity is closely related to how the system handles caching and transactions. Different persistence solutions have chosen various strategies, and this has been an area of confusion. We cover all these interesting topics and show how they re related in chapter 5. The skeleton e-commerce application we ve designed and implemented has served our purpose well. We ve identified the mismatch problems with mapping granularity, subtypes, and object identity. We re almost ready to move on to other parts of the application. But first, we need to discuss the important concept of associations that is, how the relationships between our classes are mapped and handled. Is the foreign key in the database all we need
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