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Whether you're building a web- or desktop-based application with Java SE or Java EE, many Java applications need to integrate data from a relational database. This course describes how to read and manage data from relational databases such as MySQL and SQL Server using the Java Database Connectivity (JDBC) API.
Author David Gassner explains how to choose a JDBC driver and connect to one or more databases. He also provides detailed instructions on reading, selecting, and updating data; calling stored procedures; managing data via JavaBean classes or with prepared statements; and working with metadata.
So far in this chapter, I have been describing how to retrieve data from ResultSet using specifically-typed Getter methods. For example, in this version of my project, GenericGetters, in my Tours class, I have calls to the methods getInt, getString, and getDouble, and I'm selecting the method to match the data type of the column. So, the tour ID is an integer, so I call getInt, tour name is a string column, so I'll call getString, and so on. In Java 7, there is a new approach, a Generic Getter method called getObject that you can use on every column of ResultSet, and you explicitly data type the return value using a different syntax.
Now again, this new Generic Getter method is only available in Java 7. So if you're using an older version of Java, you should stick with these explicitly data typed method calls. But if you are working with Java 7 and therefore JDBC 4.1, this new approach is just another option, another tool in your toolkit. I'll start this exercise by re-factoring this code a bit. I'm going to take the expression that's retrieving the tourId, and I'll extract it to a local variable of the method. I'll select the expression, then I'll right-click, scroll down and choose Refactor, and then Extract Local Variable.
I'll name my new variable the same as the column name, tourId. So that creates a local variable here, and then I'm using that value on the next line to build my string buffer. I'll do the same sort of re-factoring for the tourName. This is all just to make the code a little bit more readable. I'll select the expression, right-click, choose Refactor > Extract Local Variable, and again, I'll name the new variable to match the column name, tour name. I'll group these two variable declarations together, and then I'll take another that already exists, the declaration of the price variable, and I'll move that up.
So now I have one section of code where I'm getting all the values I need from the current row. I'm going to reformat the code a little bit, and make it even easier to read, and now I'm ready to use the new Generic Getter method. I'm going to keep a copy of this code around so I can compare the two styles of coding to each other. I'll select and copy and then paste in, and then I'll comment out the second version, and I'll make the changes to the first version. Here's how you use the Generic Getter. I'll start with the integer. Instead of calling getInt and passing in either the name of the column or the ordinal position of the column as an integer, you call getObject.
So, I'll get rid of that, and instead of getInt, I'll call getObject. You'll see that there are a few versions of this method, but I'm using this version, the one that accepts a column label as the first argument and then a class. The class will determine the data type that will be returned from the method. You'll see that we're using the diamond operator, and the T in the middle means type. What is the data type that we're looking for? So I'll call getObject, and just as before, I'll pass in the name of the column that I'm retrieving from, and then, I'll pass in a class and its class field, and it looks like this for an integer, Integer.class.
That means that I want to retrieve the tourId column, and I want to data type it as a class. Now, you might think that I would have to explicitly cast the value as I'm passing it back. For example, adding casting syntax at the beginning. But because of the way the getObject method is written, when I say that I'm passing in an integer class here, that's the class that's going to be returned, and so it will automatically be returned as my primitive integer, and I don't need this casting syntax. Now, let's do the same thing with the string.
I'll get rid of the call to the getString method, and instead I'll call getObject, once again, I'll call the version that accepts the name of the column and the type. I'll pass in the name of the column as tour name, and the type will be string.class. Finally, I'll do the same thing with the price. I'm retrieving a double value. So I'll call the getObject method, and I'll pass in the name of the column, price, and the data type which I'm going to set initially as Double.class. So, take a look at these two versions of the code. In the first one, you're calling the same method over and over again, getObject, and you're setting the data type using that second argument.
In the second version, which is the older version, you're setting the data type with the actual method names. I'll run this application, and I'll see that it works exactly the same as it has in previous versions of the project. If you see a pop-up dialog asking you whether you want to run an application or an applet, choose Java application. But here's a little problem with this version of the code. I'll go back to my Main class and show that I have started off working against MySQL, and this code works with MySQL. But now I'm going to switch to HSQLDB or HyperSQL, and I'll run the code again.
And this time I get an error. Depending on the state of your installation of Eclipse, you might not see this dialog. Instead, you might go right to the debug perspective. I'll see that I'm getting a ClassCastException which was not caught by MySQL Exception catch clause. I need to add a little bit more try catch code to see what's happening. So, I'll terminate the project and go back to my code, and I'm going to add in another catch section. And this time, instead of SQLException, I'll catch the global exception object, Exception.
And here, I'll just do a little bit of system error output, and I'll pass in the Error object. I'll run the code again, and this time I can see the explicit error, BigDecimal cannot be cast to double. So what's going on here? When you call the getObject method with a double column or a float, the specification for JDBC says that a BigDecimal will be returned, not a double or a float. Now, if you have watched my Java Essential Training course, you might remember that I described why the Big Decimal class exists.
The double and float objects can't be guaranteed to be absolutely precise because of the nature of how floating-point arithmetic happens on the computer, you'll frequently have additional decimal values that you don't want. Whereas the BigDecimal object can always return a very specific value. With some database drivers, the BigDecimal can be converted to the Double, and that was the case with MySQL. But with other database drivers, such as HyperSQLs, you have to always use the BigDecimal. So, for portability between databases, if you're working with columns that are doubles, floats, currency, or anything else that might have fractional values, I recommend always returning a BigDecimal.
So I'm going to change the data type of the value that I'm returning into, the price, and I'll change it to BigDecimal. I'll make sure to choose it so that I get an import statement for the class. Then I'll go to my getObject call, and I'll change that from Double.class to BigDecimal.class. I'll save and run the code again. And with HyperSQL, it now works, where it broke before. I'll go back to my Main class and change back to MySQL, and I'll run the code again. And I'll see that it works there as well.
If you use the BigDecimal to contain your doubles and floats, you'll pretty much guarantee that things work fine across different databases. But if you try to stuff things into a double or a float value using this new Generic Getter method, it will work on some databases but not on others. So that's a look at how you can use the new Generic Getter method. In the rest of the code throughout this course, I'm going to continue to use the older style because it is compatible with older versions of Java. But if you're working with Java 7, you might want to upgrade to this new version of the code.
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