5. Java collections and Generics. 3. Generics презентация

5. Collections and Generics 3. Generics

Generics Basics JDK 5.0 introduces generics Generics allow you to abstract over types The most common examples are container types, such as those in the Collections hierarchy

Generic Classes public class ClassName<Т>{ class body } Parametric type T can be used in the class body as usual type: private T a; public void set(T a) { this.a = a; } public T get() { return a; }

Generic Objects You need to set a type in <> when creating an object of generic class: public class GenClass<Т>{ . . . . . } GenClass<Integer> cInt = new GenClass<Integer>();

How Generics Work In the invocation all occurrences of the formal type parameter are replaced by the actual type argument The compiler can check the type correctness of the program at compile-time Primitive types cannot use as actual types

Exercise. Print List Create a class with list of objects of an arbitrary given class with two methods: add for accumulation data in the list printList with a boolean parameter to print odd or even elements of the list accordingly to parameter’s value

Exercise. Print List See 531FirstGeneric project for the full text

Generics Inheritance In general, if Sub is a subtype (subclass or subinterface) of Base, and G is some generic type declaration, it is not the case that G<Sub> is a subtype of G<Base>

Generic Interfaces Generic interfaces are similar to generic classes: public interface List<E>{ void add(E x); Iterator<E> iterator(); }  public interface Iterator<E>{ E next(); boolean hasNext(); }

Generic Methods Type parameters can also be declared within method and constructor signatures to create generic methods and generic constructors: public <U> void inspect(U u){ . . . } Type inference feature allows you to invoke a generic method as you would an ordinary method, without specifying a type between angle brackets

Generic Method Example class ArrayAlg { public static <T> T getMiddle(T[] a) { return a[a.length / 2]; } } You can define generic methods both inside ordinary classes and inside generic classes

Generic Method Call When you call a generic method, you can place the actual types, enclosed in angle brackets, before the method name: String[] names = { "John", "Q.", "Public" }; String middle = ArrayAlg.<String>getMiddle(names);

Wildcards What is the supertype of all kinds of collections? Collection<Object> is not such supertype due to generics inheritance rule Collection<?> (pronounced "collection of unknown"), that is, a collection whose element type matches anything

Bounded Wildcards ? extends class_name ? stands for an unknown type that this unknown type is a subtype of class_name example: List<? extends Shape> Code <? super class_name > would be read as "an unknown type that is a supertype of class_name, possibly class_name itself

Bounded Wildcards Example public static double sumOfList(List<? extends Number> list) { double s = 0.0; for (Number n : list) s += n.doubleValue(); return s; }

Home Exercise 5.3.2 ( 1 of 2) Create TBill class that saves deal for buying treasury bills (nominal, price, amount of bills, maturity date) and calculating deal income as follows: income = (nominal – price) * amount

Home Exercise 5.3.2 (2 of 2) Create DealAnalisys class that saves deals of any type (depo – single, barrier, month capitalization, TBill) Create compareIncome method that compares yield of the deal that saved in the class object and deal given as method’s parameter

Manuals http://docs.oracle.com/javase/tutorial/extra/generics/index.html