Abstract Classes

• AbstractCollection
• JComponent graphical user interface component

Example: Abstract Collections

AbstractCollection is one of the Java utility classes. Its purpose is to remember items, so you can add or delete items from the collection. One example of a subclass would be ArrayList, but there are others like HashSet.

Some of the methods it has are:

• size()
• isEmpty()

The AbstractCollection is “abstract” because it does not specify a way of remember items. However, once you know the number of items, it is easy to determine if the collection is empty - it has zero items. That means some of the code for AbstractCollection looks like this:

public abstract int size();

public boolean isEmpty() { 
  return size() == 0;
}

The major use of abstract classes is that it allows you to build lots of other functions on top of one function that you might not yet know.

Example: GUI

The Java UI tutorial has an example showing abstract classes like JComponent.

This figure shows (among other things) that everything in the user interface inherits from an abstract JComponent class, and that all menu items inherit from a JAbstractButton class.

The getAccessibleName method (line 360) is one example of a non-abstract method in JComponent. It sees if the object has a name, and if it does not, it tries to make one by looking at other components like the title of the current window. If that failes, it looks for the JLabel property and uses that (if possible). Nothing in this sequence is abstract, so this method can be written once in the JComponent class and does not need to be re-written in subclasses.