CS3240 - Generics

1. CS3240 - Generics L. Grewe

2. What is Generics? • Data Structures that contain data (such as lists) are not defined to operate over a specific type of data; instead, they operate over a homogeneous set, where the set type is defined at declaration. • Helps with Reusability (no longer need multiple data structures to hold different types of data) • Helps control run-time errors if you instead could have data structures containing different (heterogeneous) kinds of data.

3. Concept in other languages • e.g. C++ In C++, would write generic stack class using templates template <type t> class Stack { private: t data; Stack<t> * next; public: void push (t* x) { … } t* pop ( ) { … } };

4. Java Generic Programming Java has class Object Supertype of all object types This allows “subtype polymorphism” Can apply operation on class T to any subclass S <: T Java 1.0 – 1.4 did not have generics No parametric polymorphism Many considered this the biggest deficiency of Java Java type system does not let you “cheat” Can cast from supertype to subtype Cast is checked at run time

5. Run-Time Error without Generics • Example from sun.java.com: In the first example (lines 13 to 20), you might believe you're working with a list of Integer objects, when in reality it's a list of Strings. In the second example (lines 11 and 22 to 27), you might think you're working with a homogenous set of String, but this is a heterogeneous set of both String and Integer elements. So unless you create a new list subclass for every element type (which would undermine the advantages of OO reuse), there's no way to statically constrain the list to a set of homogeneous elements. And in this simple example, the errors are fairly easy to catch. In a bigger program, you'd have even bigger problems. 1.  2.  List stringList  = new LinkedList();  3.  List integerList = new LinkedList();  4.   5.  integerList.add(new Integer(1));  6.  integerList.add(new Integer(2));  7.   8.  stringList.add(new String("I am a String"));  9.   10. // Nothing constrains the elements to a homogeneous set.  11. stringList.add(new Integer(1));  12.   13. Iterator listIterator = integerList.iterator();  14.   15. // Compiler unaware of the list's return type and the illegal cast.  16. // Developer meant to iterate through the string list.  17. while(listIterator.hasNext()) {  18.   19.   // Illegal cast caught at runtime.  20.   String item = (String)listIterator.next();  21. }  22.   23. listIterator = stringList.iterator();  24. // No guarantee of homogeneous containers.  25. while (listIterator.hasNext()) {  26.   // fail at runtime due to heterogeneous set  27.   String item = (String)listIterator.next();  28. }  29.

6. Can generics help this problem • With generics, you achieve polymorphic behavior similar to the previous code, but with strong static type-checking; the compiler knows that the two lists are different because they contain different elements, and these lists are guaranteed to contain only a homogeneous set of elements.

7. Previous Example with Generics • As you can see from comments in the code, all the errors are caught at compile time. Don't worry about the syntax for now -- we'll cover that shortly. • In comparing the two examples, you should notice that additional type information is included in the generics code, which directs the compiler as to what type each container should contain. 1.  2.  import java.util.LinkedList;  3.  import java.util.Collections;  4.  import java.util.Iterator;  5.   6.  public class genericsExample2{  7.   8.  static public void main(String[] args) {  9.      LinkedList<String>  stringList  = new LinkedList<String>();  10.    LinkedList<Integer> integerList = new LinkedList<Integer>();  11.   12.    integerList.add(new Integer(1));  13.    integerList.add(new Integer(2));  14.   15.    stringList.add(new String("I am a String"));  16.    stringList.add(new Integer(1)); // causes a compilation error  17.   18.  

8.   /* genericsExample2.java:16: cannot resolve symbol  19.    ** symbol : method add (java.lang.Integer)  20.    */  21.   22.    Iterator<Integer> listIterator = integerList.iterator();  23.    String item;  24.    while(listIterator.hasNext()) {  25.       item = listIterator.next(); // causes a compilation error  26.         27.       /* genericsExample2.java:25: incompatible types  28.       ** found : java.lang.Integer  29.       ** required: java.lang.String  30.       */  31.    }  32.  33.    listIterator = stringList.iterator(); // causes a compilation error  34.   35.    /* genericsExample2.java:33: incompatible types  36.    ** found : java.util.Iterator<java.lang.String>  37.    ** required: java.util.Iterator<java.lang.Integer>  38.    */  39.    // the iterator is guaranteed to be homogeneous  40.    while (listIterator.hasNext()) {  41.      item = listIterator.next();  42.  43.      /* genericsEx2.java:41: incompatible types  44.      ** found : java.lang.Integer  45.      ** required: java.lang.String  46.      */  47.    }  48. } // main  49. } // class genericsExample2

9. ANOTHER Example of Problem without Generics: Cast Exceptions at Runtime public class OldBox { Object data; public OldBox(Object data) { this.data = data; } public Object getData() { return data; } } OldBox intBox = new OldBox(42); int x = (Integer) intBox.getData(); OldBox strBox = new OldBox(“Hi”); String s = (String) strBox.getData(); int y = (Integer) strBox.getData(); intBox = strBox; ClassCastException! Compiles but fails at runtime

10. Without Generics - NOT A GOOD SOLUTION public class FooBox { Foo data; public FooBox(Foo data) { this.data = data; } public Foo getData() { return data; } } public class StrBox { String data; public StrBox(String data) { this.data = data; } public String getData() { return data; } } public class IntBox { Integer data; public IntBox(Integer data) { this.data = data; } public Integer getData() { return data; } } IntBox intBox = new IntBox(42); int x = intBox.getData(); StrBox strBox = new StrBox(“Hi”); String s = strBox.getData(); int y = (Integer) strBox.getData(); intBox = strBox; Errors caught by compiler Infinite many classes possible

11. Java Generics: Key Idea • Parameterize type definitions • Parameterized classes and methods • Provide type safety • Compiler performs type checking • Prevent runtime cast errors

12. Generics: Parameterized Classes USING Generics public class OldBox { Object data; public OldBox(Object data) { this.data = data; } public Object getData() { return data; } } public class Box<E> { E data; public Box(E data) { this.data = data; } public E getData() { return data; } } • We want the box to hold a “specific” class – abstractly represented • Object does not work as we have seen earlier • Solution – parameterize the class definition • E refers to a particular type • The constructor takes an object of type E, not any object • To use this class, E must be replaced with a specific class

13. How to Use Parameterized Classes public class Box<E> { E data; public Box(E data) { this.data = data; } public E getData() { return data; } } Box<Integer> intBox = new Box<Integer>(42); int x = intBox.getData();//no cast needed Box<String> strBox = new Box<String>(“Hi”); String s = strBox.getData();//no cast needed Following lines will not compile anymore: String s = (String) intBox.getData(); int y = (Integer) strBox.getData(); intBox = strBox; Runtime errors now converted to compile time errors

14. When to Use Parameterized Classes • Particularly useful for “container” classes • Containers hold but do not process data • All collections framework classes in Java 5.0 and on defined using generics • See the Java API documentation

15. Collections now use Generic (see java.util) interface Collection<A> { public void add (A x); public Iterator<A> iterator (); } interface Iterator<E> { E next(); boolean hasNext(); } • Generic interface • Generic class implementing Collection interface class LinkedList<A> implements Collection<A> { protected class Node { A elt; Node next = null; Node (A elt) { this.elt = elt; } } ... }

16. More Collections Code from java.util • public interface List<E> { void add(E x); Iterator <E> iterator(); }public interface Iterator<E> { E next(); boolean hasNext(); } LinkedList is in java.util and implements List interface as a Generic Class. HOW TO USE THIS CODE List<Integer> myIntList = new LinkedList<Integer>(); myIntList.add(new Integer(0)); Integer x = myIntList.iterator().next();

17. Parameterized Classes: Syntax Note A class can have multiple parameters, e.g: public class Stuff<A,B,C> { … } Subclassing parameterized classes allowed, e.g: /* Extending a particular type */ class IntBox extends Box<Integer> { … } Or /* Extending a parameterized type */ class SpecialBox<E> extends Box<E> { … } SpecialBox<String> is a subclass of Box<String>. /* Following assignment is legal */ Box<String> sb = new SpecialBox<String>(“Hi”);

18. Parameterized Classes in Methods A parameterized class is a type just like any other class. It can be used in method input types and return types, e.g: Box<String> aMethod(int i, Box<Integer> b) { … } If a class is parameterized, that type parameter can be used for any type declaration in that class, e.g: public class Box<E> { E data; public Box(E data) { this.data = data; } publicE getData() { return data; } public void copyFrom(Box<E> b) { this.data = b.getData(); } }//We have added an infinite number of types of Boxes //by writing a single class definition

19. So Far… • Type safety violations • Using casts • Parameterized classes solve this problem • Provide type safety • Enforced by the compiler • Particularly useful for container classes • A parameterized class is another type • Next – bounded parameterized classes

20. Bounded Parameterized Types Sometimes we want restricted parameterization of classes. We want a box, called MathBox that holds only Number objects. We can’t use Box<E> because E could be anything. We want E to be a subclass of Number. public class MathBox<E extends Number> extends Box<Number> { public MathBox(E data) { super(data); } public double sqrt() { return Math.sqrt(getData().doubleValue()); } }

21. Bounded Parameterized Types (Contd.) public class MathBox<E extends Number> extends Box<Number> { public MathBox(E data) { super(data); } public double sqrt() { return Math.sqrt(getData().doubleValue()); } } The <E extends Number> syntax means that the type parameter of MathBox must be a subclass of the Number class. We say that the type parameter is bounded. new MathBox<Integer>(5);//Legal new MathBox<Double>(32.1);//Legal new MathBox<String>(“No good!”);//Illegal

22. Bounded Parameterized Types (Contd.) Inside a parameterized class, the type parameter serves as a valid type. So the following is valid. public class OuterClass<T> { private class InnerClass<E extends T> { … } … } Syntax note: The <A extends B> syntax is valid even if B is an interface.

23. Bounded Parameterized Types (Contd.) Java allows multiple inheritance in the form of implementing multiple interfaces. So multiple bounds may be necessary to specify a type parameter. The following syntax is used then: <T extends A & B & C & …> For instance: interface A { … } interface B { … } class MultiBounds<T extends A & B> { … }

24. Parameterized Methods public class Bar<T> { //Bar is parameterized public T aMethod(T x) { return x; } public static void main(String[] args) { Bar<Integer> bar = new Bar<Integer>(); int k = bar.aMethod(5); String s = bar.aMethod("abc"); //Compilation error here } } Once Bar<T> object is fixed, we are locked to a specific T.

25. Use of Parameterized Methods • Adding type safety to methods that operate on different types • Return type dependent on input type

26. Upper Bounded Wildcards in Parameterized Types The following is a PROBLME: Box<Number> numBox = new Box<Integer>(31); Compiler comes back with an “Incompatible Type” error message. This is because numBox can hold only a Number object and nothing else, not even an object of type Integer which is a subclass of Number. SOLUTION The type of numBox we desire is “a Box of any type which extends Number”. Box<? extends Number> numBox = new Box<Integer>(31);

27. Upper Bounded Wildcards in Parameterized Types (Contd.) public class Box<E> { public void copyFrom(Box<E> b) { this.data = b.getData(); } } //We have seen this earlier //We can rewrite copyFrom() so that it can take a box //that contains data that is a subclass of E and //store it to a Box<E> object public class Box<E> { public void copyFrom(Box<? extends E> b) { this.data = b.getData();//b.getData() is a //subclass of this.data } } <? extends E> is called “upper bounded wildcard” because it defines a type that is bounded by the superclassE.

28. Lower Bounded Wildcards in Parameterized Types Suppose we want to write copyTo() that copies data in the opposite direction of copyFrom(). copyTo() copies data from the host object to the given object. This can be done as: public void copyTo(Box<E> b) { b.data = this.getData(); } Above code is fine as long as b and the host are boxes of exactly same type. But b could be a box of an object that is a superclass of E. This can be expressed as: public void copyTo(Box<? super E> b) { b.data = this.getData(); //b.data() is a superclass of this.data() } <? super E> is called a “lower bounded wildcard” because it defines a type that is bounded by the subclass E.

29. Unbounded Wildcards Use unbounded wildcards when any type parameter works. <?> is used to specify unbounded wildcards. The following are legal statements. Box<?> b1 = new Box<Integer>(31); Box<?> b2 = new Box<String>(“Hi”); b1 = b2; Wildcard capture: The compiler can figure out exactly what type b1 is above from the right hand side of the assignments. This “capturing” of type information means: 1. The type on the left hand doesn’t need to be specified. 2. The compiler can do additional type checks because it knows the type of b1.

30. Conclusion • Java generics • Parameterized classes and methods • Type safety • Syntax and semantics through examples