This would be a practical tutorial on working with generics in Java. It would be step by step simple explanation with examples<\/p>\n
Content<\/strong><\/p>\n <\/p>\n Assuming we need a class that contains a method that takes in an integer argument and prints it to the console. We would have something like shown below:<\/p>\n <\/p>\n Then we can call this class from the main method like so <\/p>\n The problem with this is that when we want to print a double, float, string or any other type, this code would not do. We would need to create different classes for them. For instance: DoublePrinter<\/em>, FloatPrinter<\/em> etc. This lead to much code duplication<\/p>\n <\/p>\n We could simply modify the IntegerPrinter class to accept a generic type. The type would be specified\u00a0 when the class is called.<\/p>\n The new class would look like this:<\/p>\n <\/p>\n What we did here was to use a generic argument, T which could be any type. So when the Printer is called, we then have to provide the type parameter. This is shown below:<\/p>\n <\/p>\n So you can see here that we used the same class for printing Integer, Double and String.<\/p>\n Note<\/strong>: Generics don’t work with primitive types. You must use wrapper classes.<\/p>\n <\/p>\n In the previous section, considered a class with a generic type. So the class can be instantiated with any type. Assuming we want to limit the types that can be used. We could then use bounded generics. Here, we want the generic type to be only subclasses of a given class. For example,\u00a0 let’s say we have class Shape. And we have subclasses like: Circle, Triangle and Square.<\/p>\n So we would like to generic type to be only one of these three. We would modify our generic type definition like this:<\/p>\n <\/p>\n So in the main method, we can now create a Printer object, but the only type argument would be those derived from Shape. This is shown below:<\/p>\n Note<\/strong>: I already create Shape, Triangle, Circle and Square. Get all codes here.<\/p>\n <\/p>\n Note that we have called the printSides() method available in the Shape class. So we are able to use it. This is possible because the generic type extends the Shape class. You can also extend interfaces as well. This ensures that your the type would implement the interfaces. But you can have multiple bounds: you can have one class and multiple interfaces.<\/p>\n Watch the video for explanation.<\/p>\n <\/p>\n You can also create a method that takes in a generic type. Here, the type would be inferred from the arguments provided when the method is invoked. For example, the function below take two generic types: T and V. Then when we call it we provide a string and an integer and it prints them out<\/p>\n\n
1. Why Do We Need Generics<\/strong><\/h4>\n
public<\/span> class<\/span> IntegerPrinter<\/span> {<\/span>\r\n\t\r\n private<\/span> Integer toPrint;<\/span>\r\n\t\r\n public<\/span> IntegerPrinter<\/span>(<\/span>Integer toPrint)<\/span> {<\/span>\r\n this<\/span>.<\/span>toPrint<\/span> =<\/span> toPrint;<\/span>\r\n }<\/span>\t\r\n public<\/span> void<\/span> print<\/span>()<\/span> {<\/span>\r\n\t\t \r\n System.<\/span>out<\/span>.<\/span>println<\/span>(<\/span>toPrint);<\/span>\r\n }<\/span>\t\r\n}<\/span>\r\n<\/pre>\n
\n<\/p>\npublic<\/span> static<\/span> void<\/span> main<\/span>(<\/span>String[]<\/span> args)<\/span> {<\/span>\r\n\t\r\n IntegerPrinter printer =<\/span> new<\/span> IntegerPrinter(<\/span>34<\/span>);<\/span>\r\n printer.<\/span>print<\/span>();<\/span>\r\n}<\/span>\r\n<\/pre>\n2. How Generics Solve the Problem<\/strong><\/h4>\n
public<\/span> class<\/span> Printer<\/span><<\/span>T><\/span> { \/\/ You can have more multiple generics as well<\/span>\r\n\t\r\n\tprivate<\/span> T toPrint;<\/span>\r\n\t\r\n\tpublic<\/span> Printer<\/span>(<\/span>T toPrint)<\/span> {<\/span>\r\n\t\tthis<\/span>.<\/span>toPrint<\/span> =<\/span> toPrint;<\/span>\r\n\t}<\/span>\r\n\t\r\n\tpublic<\/span> void<\/span> print<\/span>()<\/span> {<\/span>\r\n\t\tSystem.<\/span>out<\/span>.<\/span>println<\/span>(<\/span>toPrint);<\/span>\r\n\t}<\/span>\t\r\n}<\/span>\r\n<\/pre>\npublic<\/span> static<\/span> void<\/span> main<\/span>(<\/span>String[]<\/span> args)<\/span> {<\/span>\r\n\t\r\n\tPrinter<<\/span>Double><\/span> printer1 =<\/span> new<\/span> Printer<>(<\/span>34.5<\/span>);<\/span>\r\n\tprinter1.<\/span>print<\/span>();<\/span>\r\n\t\r\n\tPrinter<<\/span>Integer><\/span> printer2 =<\/span> new<\/span> Printer<>(<\/span>34<\/span>);<\/span>\r\n\tprinter2.<\/span>print<\/span>();<\/span>\r\n\t\r\n\tPrinter<<\/span>String><\/span> printer3 =<\/span> new<\/span> Printer<>(<\/span>\"Hello World\"<\/span>);<\/span>\r\n\tprinter3.<\/span>print<\/span>();<\/span>\r\n}<\/span>\r\n<\/pre>\n3. Bounded Generics<\/strong><\/h4>\n
public<\/span> class<\/span> Printer<\/span><<\/span>T extends<\/span> Shape><\/span> {<\/span>\r\n<\/pre>\npublic<\/span> static<\/span> void<\/span> main<\/span>(<\/span>String[]<\/span> args)<\/span> {<\/span>\r\n\t\r\n\tPrinter<<\/span>Circle><\/span> printer1 =<\/span> new<\/span> Printer<>(<\/span>new<\/span> Circle(<\/span>1<\/span>));<\/span>\r\n\tPrinter<<\/span>Square><\/span> printer2 =<\/span> new<\/span> Printer<>(<\/span>new<\/span> Square(<\/span>4<\/span>));<\/span>\r\n\t\r\n\tPrinter<<\/span>Integer><\/span> printer3 =<\/span> new<\/span> Printer<>(<\/span>5<\/span>)<\/span> \/\/ Throws an exception<\/span>\r\n\t\r\n\tCircle c1 =<\/span> new<\/span> Circle(<\/span>1<\/span>);<\/span>\r\n\tSquare s2 =<\/span> new<\/span> Square(<\/span>4<\/span>);<\/span>\r\n\t\r\n\t\t\t\r\n\tprinter1.<\/span>print<\/span>();<\/span>\r\n\tprinter2.<\/span>print<\/span>();<\/span>\r\n\tc1.<\/span>print<\/span>();<\/span>\r\n\ts2.<\/span>printSides<\/span>();<\/span>\r\n\t\r\n}<\/span>\r\n<\/pre>\n4. Generic Methods<\/strong><\/h4>\n
public<\/span> class<\/span> Generics<\/span> {<\/span>\r\n\t\t\r\n\tpublic<\/span> static<\/span> void<\/span> main<\/span>(<\/span>String[]<\/span> args)<\/span> {<\/span>\r\n\t\t\r\n\t\tshow(<\/span>\"Student\"<\/span>,<\/span> 19<\/span>);<\/span>\r\n\t\tshow(<\/span>\"Employee\"<\/span>,<\/span> 45<\/span>);<\/span>\r\n\t\t\r\n\t}<\/span>\r\n\t\r\n\tprivate<\/span> static<\/span> <<\/span>T,<\/span> V><\/span> void<\/span> show(<\/span>T data1,<\/span> V data2)<\/span> {<\/span>\r\n\t\tSystem.<\/span>out<\/span>.<\/span>println<\/span>(<\/span>\"First argument: \"<\/span>