This lesson ventures into interesting uncharted territory.
Until now, our
classes have needed to have names.
But, Java doesn't actually require this!
Let's explore anonymous classes and their uses...
Please note that the next two lessons are on fairly advanced topics. You will see and need to understand code that uses these ideas, but testing on them will be limited.
We've seen how to define what is called a named class. This should be familiar to us by now:
However, Java also allows us to define so-called anonymous classes. Let's go through an example carefully:
In contrast to named classes, anonymous classes:
classor implement an
Anonymous classes can capture the value of variables that are available when they are created.
This can be extremely useful.
However, there are limitations to this approach.
Most importantly, the variables that are captured must either be
final or effectively
Let's look at an example:
We can create anonymous classes in Java. Cool! But... so what!? What problems can these classes solve?
Surprisingly, anonymous classes turn out to be common and quite powerful. Let's look at an example.
Imagine that we want to count the number of elements in an
int array that meet some condition.
The condition could be that the element was positive, or negative, or odd, or even, or divisible by three, or whatever.
One approach would be to write separate methods for each thing we would want to count:
Wow, this is getting tedious—and we've only done three! Imagine if we had a bunch of different conditions we needed to handle... But they are all very similar. There must be a better way.
Let's see how to rewrite the code above using an anonymous class in a way that makes the counting logic completely flexible.