Java

This page will be a collection of resources on Java programming.

Documentation

• Thinking in Java. This links to a mirror for the third edition.
• Official Sun documentation.

Differences from C++

• Inner classes: Although nested classes exist in C++, they are rarely used, and anyway it seems that the only effect of nested classes is on namespaces. Inner classes in Java have the interesting features that they can access their enclosing class object, and all of its members, directly.

• Inner local classes: Local classes defined inside a statement block (a function block for example) can even access the final local variables of the function. Nice!

• Anonymous inner classes: When you define the body of a class directly after a new keyword, you are defining an anonymous class. This class can derive from an existing class or implement an interface. The syntax is:

MyInterface my_instance = new MyInterface()
{
    // Here you write the body of the class

    {
        // Here we could perform instance initialization
    }
};

You cannot have a constructor in an anonymous class since the class does not have a name. You can, however, pass arguments to the base constructor, and you can also perform instance initialization (see the previous code).

Note that with anonymous classes, you cannot have more than one object of the class. This limitation is not present with inner local classes, so the only point of an inner local class (vs. an anonymous class) is to create more than one instance of the class.

• Note that there is no such thing as anonymous or inner interfaces. What you do generally, however, is create a class that will implement an interface.

Libraries

Dozer

• Don't forget than you need to have a default constructor in order for Dozer to correctly instantiate the destination objects.

• If you are not using Java 1.5, and you are dealing with a collection mapping, you must provide hints regarding the collection contents. Else it will assume the target collection contains the same objects than the source collection (which often is not the case).

FileUpload

• If you use a HTTP POST request with the multipart/formdata encoding, probably the HTTP headers won't include the encoding. At least that happens when using Firefox and GWT. So on the server side, Java won't know which encoding to use for this ServletRequest. Directly setting the correct encoding on the request object does not work later with fields retrieved via the FileUpload API. You have to explicitly provide the encoding when retrieving those fields:

file_item.getString(request.getCharacterEncoding());

This would work if you previously set the request encoding (else you can just provide "UTF-8" directly).

Hints & Techniques

• To round a float to the upper integer, use Math.ceil().