Comp 212 Lecture 18: Java Generics

1. Comp 212: Intermediate Programming Lecture 18 Java Generics Comp 212: Intermediate Programming Lecture 18 Java Generics By: Anupam Chanda By: Anupam Chanda

2. 2 Todays Lecture Todays Lecture Java generics Java generics Parameterized classes and methods Parameterized classes and methods Compiler provides type safety Compiler provides type safety Syntax and semantics Syntax and semantics Examples Examples Generics-based implementation of the list framework next class Generics-based implementation of the list framework next class

3. 3 Outline Outline Motivation Parameterized classes Parameterized classes Parameterized methods Parameterized methods Upper bounded wildcards Upper bounded wildcards Lower bounded wildcards Lower bounded wildcards Unbounded wildcards Unbounded wildcards

4. 4 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 Cast Exceptions at Runtime Cast Exceptions at Runtime

5. 5 public class IntBox { Integer data; public IntBox(Integer data) { this .data = data; } public Integer getData() { return data; } } public class StrBox { String data; public StrBox(String data) { this .data = data; } public String 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 public class FooBox { Foo data; public FooBox(Foo data) { this .data = data; } public Foo getData() { return data; } } Infinite many classes possible Nave Solution Nave Solution

6. 6 Passing Parameters to Methods: An Analogy Passing Parameters to Methods: An Analogy public abstract class Sum { public static int sum_0_1() { return (0+1); } public static int sum_15_22() { return (15+22); } } public class Main { public static void main(String[] nu) { int j = Sum.sum_0_1(); int k = Sum.sum_15_22(); } } Bad infinite many methods public abstract class NewSum { public static int sum( int m, int n) { return (m+n); } } public class NewMain { public static void main(String[] nu) { int j = NewSum.sum(0,1); int k = NewSum.sum(15,22); } } Pass parameters to methods Methods accept parameters

7. 7 Java Generics: Key Idea Java Generics: Key Idea Parameterize type definitions Parameterize type definitions Parameterized classes and methods Parameterized classes and methods Provide type safety Provide type safety Compiler performs type checking Compiler performs type checking Prevent runtime cast errors Prevent runtime cast errors

8. 8 Parameterized Classes Parameterized Classes public class OldBox { Object data; public OldBox(Object data) { this .data = data; } public Object 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 public class Box { E data; public Box(E data) { this .data = data; } public E getData() { return data; } } 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

9. 9 How to Use Parameterized Classes How to Use Parameterized Classes public class Box { E data; public Box(E data) { this .data = data; } public E getData() { return data; } } Box intBox = new Box(42); int x = intBox.getData(); //no cast needed Box strBox = new Box(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

10. 10 When to Use Parameterized Classes When to Use Parameterized Classes Particularly useful for container classes Particularly useful for container classes Containers hold but do not process data Containers hold but do not process data All collections framework classes in Java 5.0 defined using generics All collections framework classes in Java 5.0 defined using generics See the Java 5.0 API documentation See the Java 5.0 API documentation

11. 11 Parameterized Classes: Syntax Note Parameterized Classes: Syntax Note A class can have multiple parameters, e.g: public class Stuff { } Subclassing parameterized classes allowed, e.g: /* Extending a particular type */ class IntBox extends Box { } Or /* Extending a parameterized type */ class SpecialBox extends Box { } SpecialBox is a subclass of Box. /* Following assignment is legal */ Box sb = new SpecialBox(Hi);

12. 12 Parameterized Classes in Methods 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 aMethod( int i, Box 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 data; public Box(E data) { this .data = data; } public E getData() { return data; } public void copyFrom(Box b) { this .data = b.getData(); } } //We have added an infinite number of types of Boxes //by writing a single class definition

13. 13 So Far So Far Type safety violations Type safety violations Using casts Using casts Parameterized classes solve this problem Parameterized classes solve this problem Provide type safety Provide type safety Enforced by the compiler Enforced by the compiler Particularly useful for container classes Particularly useful for container classes A parameterized class is another type A parameterized class is another type Next bounded parameterized classes Next bounded parameterized classes

14. 14 Bounded Parameterized Types Bounded Parameterized Types Sometimes we want restricted parameterization of classes. We want a box, called MathBox that holds only Number objects. We cant use Box because E could be anything. We want E to be a subclass of Number . public class MathBox extends Box { public MathBox(E data) { super (data); } public double sqrt() { return Math.sqrt(getData().doubleValue()); } }

15. 15 Bounded Parameterized Types (Contd.) Bounded Parameterized Types (Contd.) public class MathBox extends Box { public MathBox(E data) { super (data); } public double sqrt() { return Math.sqrt(getData().doubleValue()); } } The 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(5); //Legal new MathBox(32.1); //Legal new MathBox(No good!); //Illegal

16. 16 Bounded Parameterized Types (Contd.) Bounded Parameterized Types (Contd.) Inside a parameterized class, the type parameter serves as a valid type. So the following is valid. public class OuterClass { private class InnerClass { } } Syntax note: The syntax is valid even if B is an interface.

17. 17 Bounded Parameterized Types (Contd.) 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: For instance: interface A { } interface B { } class MultiBounds { }

18. 18 So Far So Far Parameterized classes Parameterized classes Bounded parameterized types Bounded parameterized types To restrict parameter types To restrict parameter types Next parameterized methods Next parameterized methods

19. 19 Parameterized Methods Parameterized Methods Consider the following class: public class Foo { //Foo is not parameterized public T aMethod(T x) { //will not compile without //to indicate that this is a //parameterized method. return x; } public static void main(String[] args) { Foo foo = new Foo(); int k = foo.aMethod(5); String s = foo.aMethod("abc"); } } Fix foo and vary parameter to aMethod() public class Bar { //Bar is parameterized public T aMethod(T x) { return x; } public static void main(String[] args) { Bar bar = new Bar(); int k = bar.aMethod(5); String s = bar.aMethod("abc"); //Compilation error here } } Once Bar object is fixed, we are locked to a specific T .

20. 20 Use of Parameterized Methods Use of Parameterized Methods Adding type safety to methods that operate on different types Adding type safety to methods that operate on different types Return type dependent on input type Return type dependent on input type

21. 21 So Far So Far Parameterized classes Parameterized classes Bounded parameterized types Bounded parameterized types Parameterized methods Parameterized methods Next wildcards Next wildcards Bounded Bounded Upper Upper Lower Lower Unbounded Unbounded

22. 22 Upper Bounded Wildcards in Parameterized Types Upper Bounded Wildcards in Parameterized Types We start to run into some new issues when we do some things that seem normal. For instance, the following seems reasonable: Box numBox = new Box(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 . The type of numBox we desire is a Box of any type which extends Number . Box numBox = new Box(31);

23. 23 Upper Bounded Wildcards in Parameterized Types (Contd.) Upper Bounded Wildcards in Parameterized Types (Contd.) public class Box { public void copyFrom(Box 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 object public class Box { public void copyFrom(Box b) { this .data = b.getData(); //b.getData() is a //subclass of this.data } } is called upper bounded wildcard because it defines a type that is bounded by the superclass E .

24. 24 Lower Bounded Wildcards in Parameterized Types 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 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 b) { b.data = this .getData(); //b.data() is a superclass of this.data() } is called a lower bounded wildcard because it defines a type that is bounded by the subclass E .

25. 25 Unbounded Wildcards 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(31); Box b2 = new Box(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 doesnt need to be specified. 2. The compiler can do additional type checks because it knows the type of b1 .

26. 26 Conclusions Conclusions Java generics Java generics Parameterized classes and methods Parameterized classes and methods Type safety Type safety Syntax and semantics through examples Syntax and semantics through examples Links to tutorials on the lecture page Links to tutorials on the lecture page Generics-based implementation of the list framework next class Generics-based implementation of the list framework next class