What is a wrapper class? How are such classes useful? More on stackoverflow.com

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A wrapper is a class or function whose main purpose is to just forward calls to another class or function. The point of a wrapper is so you can use a different interface to access something. There are a couple big reasons to have a wrapper. One use is to simplify the interface. The functions that are part of the Windows API that operate on files, for example, require you to manually check an error code if something goes wrong. The File class wraps those functions and checks the error code for you, converting it into an exception. The Windows API functions also have a lot of optional parameters, but, since they were written in C, which doesn't have function overloading, you have to manually put in null or some default value for all of the optional arguments every time. Since C# supports overloading, the wrapper provides overloads so you don't have to specify the optional arguments. Another thing you'd use a wrapper for is if you want to access an object through an interface that the class doesn't implement. You can write a wrapper class that implements the interface and calls methods on the real object that do whatever operations. For example, Google Drive, Dropbox, and Microsoft OneDrive all have different APIs. If you're writing a program and want to be able to access files from any of them, you'd probably want to write wrappers that all implement the same interface so that you don't have to write all your file access functions three times. There's plenty of other reasons to use a wrapper, but the basic idea is that you're exposing an interface that you want to access something that has a different interface. More on reddit.com

For one thing, you can't put primitive types in a java.util.Container, such as a java.util.ArrayList, for example; you can only put objects in one of those. So you can't make an ArrayList—you need to make an ArrayList instead. More generally, generic types must be object types in Java, not primitive types. More on reddit.com