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Generics

Generics. Parametrized classes and methods. What are generics. Generics are classes or interfaces that can be instantiated with a variety of types. They have 1 or more formal type parameters Using a generic you specify an actual type Also known as “parametrized types” Or types with “holes”.

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Generics

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  1. Generics Parametrized classes and methods Generics

  2. What are generics • Generics are classes or interfaces that can be instantiated with a variety of types. • They have 1 or more formal type parameters • Using a generic you specify an actual type • Also known as “parametrized types” • Or types with “holes” Generics

  3. Before generics List myList = new ArrayList(); myList.add(“Anders”); … String str = (String)myList.get(0); Type casts are generally considered bad since they may fail at runtime If you add something that is not a string myList.add(new Integer(56)); After generics List<String> = new ArrayList<String> (); myList.add(“Anders”); … String str = myList.get(0); myList is now typed String in this case We don’t need type casting. We cannot add anything but Strings myList.add(56); Does not compile No more type casts Generics

  4. Benefits of generics • Making code better • Compile time: Compiler discovers wrong uses of types. • Runtime: No ClassCastException • Making code more flexible • Generic classes can be instantiated with different types. Generics

  5. Type inference- the ”diamond”Java 7 feature • Java 6 • List<String> someList = new ArrayList<String>(); • Java 7 • List<String> someList = new ArrayList<>(); • Map<T, Set<E>> multiMap = new HashMap<>(); • The compiler will automatically “calculate” there type of the right hand side object • <> is supposed to look like a diamond ♦ Generics

  6. Defining and using your own generic class • Definition public class Catalog<T> { List<T> catalog = new ArrayList<T>(); public boolean add(T element) { catalog.add(element); } } • Usage Catalog<Borrower> bc = new Catalog<Borrower>(); Catalog<Book> bookCatalog = new Catalog<Book>(); Generics

  7. Erasure • At compile-time all the types are checked and removed (erased) from the class file. • What is left is called the “raw type”. • The class file has no knowledge of the types. • Example • Catalog<Borrower> is “erased” to Catalog Generics

  8. Erasure (2) • Types are checked and removed (erased) • No generics in the JVM. • All instances of a generics class have the same run-time class List<String> list = new ArrayList<String>(); System.out.println("list type: " + list.getClass()); • Writes ArrayList, nothing said about String. if (list instanceof ArrayList<String>) … • Illegal, since <String> is not present at run-time if (list instanceof ArrayList) … • Legal Generics

  9. Relationships among generics • Object is a super type of String. • ArrayList<Object> is not a super type of ArrayList<String> • Object and String are erased at compile time. • Runtime system has no knowledge of the types. Generics

  10. Raw types • The raw types are the generic types, but without a specified actual type • ArrayList, Collection, Catalog Generics

  11. An example: Using raw types to break “security” public String loophole(Integer x) { List<String> ys = new LinkedList<String>(); List xs = ys; xs.add(x); return ys.get(0); } • We used xs as an old-fashioned (un-typed) alias to ys. Using xs we were able to insert an Integer into ys. • Generally • Avoid un-typed collections. From: Gilad Bracha: Generics in the Java Programming Language, page 12 Generics

  12. Wildcard types • Yet another feature to solve the problems with relationships among generics. • New syntax: ? • MyClass<?> • MyClass of any type • Example • Boolean java.util.Collection.containsAll(Collection<?> c) • http://download.oracle.com/javase/7/docs/api/java/util/Collection.html Generics

  13. Wild card types with upper bounds • Sometimes the types cannot be chosen completely free • We have to make sure that the type has certain features • Example • Java.util.Collection.addAll(Collection<? extends E> c) • Can use any collection of types that extends E (which is the element type of this Collection • The type is said to have an “upper bound” (in the class hierarchy) • In this case E • http://download.oracle.com/javase/7/docs/api/java/util/Collection.html Generics

  14. Generics methods • Methods (static and non-static) can be generic • The class needs not be generic. • Syntax • <T> T method(T element) { …} • Most often with static methods • Example • <T> List<T> Collections.synchronizedList(List<T> list) • http://download.oracle.com/javase/7/docs/api/java/util/Collections.html Generics

  15. Wild cards with lower bounds • Most often used with generic methods • Examples from java.util.Collections • static <T> void fill(List<? super T> list, T obj) • Replaces all of the elements of the specified list with the specified element. • T is the type of the new object • List must have elements which are super types of T • T is a lower bound in the object hierarchy • Example • List<JComponent> list; • … • Collections.fill(list, new JButton()); Generics

  16. Bounded wildcards • ? extends T upper bound • Some type that is a subtype of T • Or T itself • ? super T lower bound • Some type that is a super type of T • Or T itself • The “Get and Put Principle” • Use wildcard with extends • When you want to get values out of a structure • Use wildcard with super • When you want to put values into a structure Generics

  17. Comparing and sorting • The class java.util.Collections has static methods for sorting elements in lists • <T extends Comparable<? super T>> void sort(List<T> list) • <T> void sort(List<T> list, Comparator<? super T> c) • This is a lot of syntax! • First sort method orders elements according to the natural order • int: 1,2,3,4, etc. • String: a, b, c, etc. • Second sort method orders elements according to the specified comparator • A comparator compares 2 elements and returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second. Generics

  18. Using generics with legacy code • Legacy code means old code • In this case code written prior to Java 5.0 is considered old • How do you deal with legacy classes that don’t use generics (but ought to)? • Examples • Generics: legacycode.Cat.java • Compiler sends you warnings • … uses unchecked or unsafe operations • When you call methods on a raw type • … unchecked cast • When you cast from raw to generic type. • You may have to live with such warnings • Or change the legacy code Generics

  19. Other programming languages • C++ templates • Code is expanded • No erasures (no wildcards): Code bloat Generics

  20. References • Sun MicrosystemsThe Java Tutorial • Generics http://download.oracle.com/javase/tutorial/extra/generics/index.html • Easy introduction to generics. • GiladBracha • http://java.sun.com/docs/books/tutorial/extra/generics/index.html • GiladBrachaGenerics in the Java Programming Language, Sun Microsystems 2004 • http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf • Also known as the ”Generics Tutorial” • Thorough introduction to generics. Generics

  21. More references • Naftalin & WadlerJava Generics and Collections, O’Reilly 2007 • Page 1-150 is about generics • Niemeyer & KnudsenLearning Java, 3rd edition, O’Reilly 2005 • 8. Generics, page 214-248 Generics

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