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    <p>first of all, for code clarity, instead of writing</p> <pre><code> private DataPersistenceInterface DataDAO; </code></pre> <p>write :</p> <pre><code> private DataPersistenceInterface dataPersistenceInterface; </code></pre> <p>Always use camel case for java properties and with the same name of the class you are using. A best pratice would also be to name the implementation the same name of the interface plus a Impl:</p> <pre><code>DataPersistenceInterfaceImpl </code></pre> <p>It is a pattern problem, if you want to create a connection using the constructor of the Dao, you have three choices:</p> <p>1) You can add a closeConnection method to the DataPersistenceInterface interface by adding it to the interface signature:</p> <pre><code>public void closeConnection(); </code></pre> <p>2) But because the AppSystem class does know which implementation it uses to create the DataPersistenceInterface (DataDao), you can cast back the interface to the implementation. You will have your closeConnection() back. This would be my best guest.</p> <pre><code> public void closeConnection() { ((DataDAO)dataPersistenceInterface).closeConnection(); } </code></pre> <p>3) Depending of the usage of your AppSys, don't care about closing the connection. Put the close connection method in the finalize() method of the DataDAO implementation. The connection will be close when the AppSys application will end. </p> <pre><code>protected void finalize() throws Throwable { try{ connection.close; }catch(Exception e){ ... } } </code></pre> <p>The best pratice would be to use a ConnectionPool like c3po, or Apache ConnectionPool.</p>
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    1. CO1) totally agree, do that! 2) please don't. It's a common practice to define interfaces and use variables declared with their interface type to hide the real implementation (*polymorphism*). If you want to switch to another implementation, go ahead, if you go with the implementation type, you have to change the type in every usage. 3) DON'T DO THAT. you may use a try-catch-finally block, but the execution of the finalize-method is not guaranteed by the VM, and there are several cases reported where it behaved ways differently than expected.
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    2. COYes I agree for all your remarks... Saying "depending of the usage", I mean if you do a Hello World for your teacher, But you are right...
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