8th july

The Download Class

The Download class is the workhorse of the Download Manager. Its primary purpose is to download a file and save that file’s contents to disk. Each time a new download is added to the Download Manager, a new Download object is instantiated to handle the download. The Download Manager has the ability to download multiple files at once.

The Download Variables

Download begins by declaring several static final variables that specify the various constants used by the class. Next, four instance variables are declared. The url variable holds the Internet URL for the file being downloaded; the size variable holds the size of the download file in bytes; the downloaded variable holds the number of bytes that have been downloaded thus far; and the status variable indicates the download’s current status.

The Download Constructor

Download’s constructor is passed a reference to the URL to download in the form of a URL object, which is assigned to the url instance variable. It then sets the remaining instance variables to their initial states and calls the download( ) method. Notice that size is set to –1 to indicate there is no size yet.

The download( ) Method

The download( ) method creates a new Thread object, passing it a reference to the invoking Download instance. To use threads, the Download class simply implements the Runnable interface by overriding the run( ) method. After the download( ) method has instantiated a new Thread instance, passing its constructor the Runnable Download class, it calls the thread’s start( ) method. Invoking the start( ) method causes the Runnable instance’s (the Download class’) run( ) method to be executed.

The run( ) Method

run( ) sets up variables for the network stream that the download’s contents will

be read from and sets up the file that the download’s contents will be written to. Next, a

connection to the download’s URL is opened by calling url.openConnection( ). Since we know that the Download Manager supports only HTTP downloads, the connection is cast to the HttpURLConnection type. After the HttpURLConnection has been created, the connection request property is set by calling connection. setRequestProperty( ).

Class URLConnection

The abstract class URLConnection is the superclass of all classes that represent a communications link between the application and a URL. Instances of this class can be used both to read from and to write to the resource referenced by the URL. In general, creating a connection to a URL is a multistep process:

1. The connection object is created by invoking the openConnection method on a URL.
2. The setup parameters and general request properties are manipulated.
3. The actual connection to the remote object is made, using the connect method.
4. The remote object becomes available. The header fields and the contents of the remote object can be accessed.

Ø  setRequestProperty

               public void setRequestProperty(String key,String value)

Sets the general request property. If a property with the key already exists, overwrite its value with the new value.

Ø  getContentLength

               public int getContentLength()

Returns the value of the content-length header field.

RandomAccessFile

public RandomAccessFile(File file,String mode)

"rw"	Open for reading and writing. If the file does not already exist then an attempt will be made to create it.

The stateChanged( ) Method

Each time the Download class needs to notify its registered Observers of a change, the stateChanged( ) method is invoked. The stateChanged( ) method first calls the Observable class’ setChanged( ) method to flag the class as having been changed. Next, the stateChanged( ) method calls Observable’s notifyObservers( ) method, which broadcasts the change notification to the registered Observers.