Next we're going to consider addressing another fundamental defect in the implementations we've considered so far that those implementations are only good for strings. What if we want to have queues and stacks of other types of data? And that brings us to the topic of generics. Alright. So, we implemented stack of strings but in applications we have all different types of data that we might want to implement like stack of int say or URLs or cars or vans or whatever data that we might be processing. So how are we going to implement stacks and queues for that types of data. Well, first thing that we might that we might consider and actually we're forced to consider this one in lots of programming environment, is to implement a separate stack class for each type of data that we're using. That really seems unsatisfactory. We have our carefully crafted code that does array resizing and so forth and we're going to copy that code and change the data type string to the data type van or int to everywhere. And what if we have hundreds of different types of data that we're processing. We have hundreds of different implementations. Unfortunately that situation at the beginning of Java where we stuck with that and there are plenty of programming languages where basically we're stuck with that so what we want to look at is a modern approach to avoiding having multiple implementations for each type of data. So the a quick hack that is widely used is to use casting to implement to reuse the code for different data types. So, we make our implementation with type Object so everything in Java is a sub type of Object and then the client, when the client comes to use it, will simply cast the result to the corresponding type. I don't want to spend a lot of time with this cuz I think this is a unsatisfactory solution. So, in this example we have two types with two stacks one of apples and one of oranges. And then, it's up to the client when it pops something off the apple stacks to cast at the apple to keep the type checking system happy. The problem with this is that the client code has to do this, this casting and it's kind of an insidious bug if it doesn't quite get it. So, the third attempt that we're going to talk about uses generics. And that way the client code doesn't do casting. We can discover mistakes in typed mismatches at compile-time instead of at run-time. So, in this case, we put, with generics, we can have a type parameter on our class and that include, that's inside angle brackets in this code and then, we can [cough] if we have a stack of apples and we tried to push an orange unto a stack of apples then we're going to get a compile-time error because that's stack was declared to only consist of, of apples. And just the guiding principal in good modular programming is that we should welcome compile-time errors and avoid run-time errors because if we can detect an error at compile-time, then we can ship our product or deploy our implementation our implementation of an API and have some confident that it's going to work for any client whereas, the error is not going to get discovered until run-time it might occur with some client development. Now, years after, we have to deploy our software and be extremely difficult on everyone. Okay. So actually with a good generic implementation it's not difficult to simply [cough], take every place that we used string and replace it with a generic type name as in this code here. On the left is our implementation of a stack of strings using link list. On the right is a generic implementation. So, every place that we used string type on the left we used the word item on the right. And at the top, the class declaration we declared an angle brackets that item is the generic type that we're going to use. The implementation could hardly be more straightforward and it's an excellent way to solve the problem of handling multiple types of data with one implementation. With arrays, it doesn't quite work and again all programming languages and, you know, many programming languages nowadays have difficulties with this and Java's got a particular difficulty. So, what we would like to do is just declare a new array using our generic name Item as in the highlighted line here. Otherwise it's the same. Unfortunately, Java does not allow generic array creation. So there's various technical reasons for that and you can read, read extensive debates about this on the web that's going to go beyond our scope. For now, what we need to do is put a cast in to make this work. So, we create an array of objects and then we cast it down to an array of items. Now in my view, a good code has zero cast. So, we want to avoid cast as much as possible because it, it, it really is declaring some kind of weakness in what we're doing. But in this case we have to put in this one cast and so what we've heard about that is the ugly cast it doesn't, it doesn't make you feel good about the code. It's not something that you will come up with on your own and that's, and that's an undesirable feature, I think for codes so simple as this. But fortunately, we can get through pretty much everything that we're going to do in this course just knowing about this one of lay cast. So now, when we compile this program we get a, a warning message from Java. It says that we're using unchecked or unsafe operations and we should recompile with a minus -Xlint equals unchecked for details. So, we can go ahead and do that and it says that you have put in, in your code an unchecked cast and we're warning you about that cuz you shouldn't be putting in unchecked cast. And okay, that's fine and you're going to see that when you do compiles using code like these. I, I think maybe they might have added to this warning statement "We apologize for making you do this". It's not our fault that we had to do that, we had to do that cuz of your requirement about not allowing us to declare generic arrays. So with that note please don't think there's something wrong with your code if you follow our prescriptive and, and get this warning message. Okay then, it's one of the detail that Java takes care of and that's what about primitive types [cough] so the generic type that we're using is for objects and you know, we're casting down from array of objects. So in order to handle generic types we have to use Java's wrapper object types. So Integer with the capitalized rapid type for int and so forth and many of you were probably familiar with that. And there's a process called auto-boxing which automatically cast between primitive types and wrappers so all of that handles of the, the problem of dealing with primitive types, kind of behind the scenes. And the bottom line is that we can articulate an API for generic stacks that works for any type of data and we've got two implementations, link list and arrays that, that performed very well for [cough] any type of data using the, the resizing or link list as we've described.
