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Hook is a programming mechanism to provide future functionality expansions. An hook is often contained in a library, whereas the expansion is a function written by the developer.

The boundary between hook and callback is very fuzzy and from the coding point of view they are essentially the same. But we express a different use and context with each term.

A callback is usually called after an operation has been completed. It's for example possible register a callback to be called back after an ajax call has been completed.

A hook is a more generic term to indicate a code expansion and it's usually more common in low level programming.

There are essentially two types of hooks:

• Delegation: this is widely used in low level programming; it's implemented with function pointers in C and via delegation pattern in Java.
• Override: this is widely used in both low and high level programming. It consist in overriding an inherited empty method with real functionality.

There is essentially one type of callback:

• Delegation: this is widely used in high level programming, especially for GUI events. It's essentially the same as the delegation for the hooks, but as mentioned to change is the context.

Let's see some examples.

Here's a callback with delegation in AWT:
 
	JButton button = new JButton("Execute");

	button.addActionListener(new ActionListener() {
 
		public void actionPerformed(ActionEvent e)
		{
			System.out.println("I'm an event expansion");
		}
	}); 

JButton is the delegator, whereas the anonymous class is the delegate, that expands the functionality for a click event.

Here's is an example of hook with override:

	public abstract class Builder
	{
		protected void preProcess() {}
		
		public void build() {
			
			preProcess();
			
			// Operations to build
		}
	}

preProcess() is the hook, which does nothing, until when a subclass expands the method.

Here's an other example of hook with override, from the AWT source code:

	
public abstract class MouseAdapter implements MouseListener {
	public void mouseClicked(MouseEvent e) {}
	public void mousePressed(MouseEvent e) {}
	public void mouseReleased(MouseEvent e) {}
	public void mouseEntered(MouseEvent e) {}
	public void mouseExited(MouseEvent e) {}
}

MouseAdapter is an implementation of the adapter pattern and contains a series of hooks that allows the developer to implement only the method that he really needs, but not all the methods of the interface MouseListener.