Programming Languages and Techniques (CIS120)

Transcription

1 Programming Languages and Techniques (CIS120) Lecture 29 April 3, 2014 Overriding, Enums, Iterators

2 public class Ref<T> { private T contents; public void add(t e) { contents = e; Quiz (on paper) public boolean contains(object o) { // what should we put here?? public static void main(string[] args) { Ref<Point> r = new Ref<Point>(); r.add(new Point(1,2)); System.out.println(r.contains(null)); // should print false System.out.println(r.contains(new Point(1,2))); // should print true

3 Method Overriding

4 A Subclass can Override its Parent public class C { public void printname() { System.out.println( I m a C ); public class D extends C { public void printname() { System.out.println( I m a D ); // somewhere in main C c = new D(); c.printname(); What gets printed to the console? 1. I m a C 2. I m a D 3. NullPointerExcepQon 4. NoSuchMethodExcepQon

5 A Subclass can Override its Parent public class C { public void printname() { System.out.println( I m a C ); public class D extends C { public void printname() { System.out.println( I m a D ); // somewhere in main C c = new D(); c.printname(); Our ASM model for dynamic dispatch already explains what will happen when we run this code. Useful for changing the default behavior of classes. But can be confusing and difficult to reason about if not used carefully.

6 Overriding Example Workspace Stack Heap C c = new D(); c.printname(); Class Table Object String tostring(){ boolean equals C extends C() { void printname(){ D extends D() { void printname(){

7 Overriding Example Workspace Stack Heap Class Table c.printname(); c D Object String tostring(){ boolean equals C extends C() { void printname(){ D extends D() { void printname(){

8 Overriding Example Workspace Stack Heap Class Table.printName(); c D Object String tostring(){ boolean equals C extends C() { void printname(){ D extends D() { void printname(){

9 Overriding Example Workspace Stack Heap Class Table System.out. println( I m a D ); c D Object String tostring(){ this boolean equals C extends C() { void printname(){ D extends D() { void printname(){

10 Difficulty with Overriding class C { public void printname() { System.out.println("I'm a " + getname()); public String getname() { return "C"; class E extends C { public String getname() { return "E"; What gets printed to the console? 1. I m a C 2. I m a E 3. I m an E 4. NullPointerExcepQon // in main C c = new E(); c.printname();

11 class C { Difficulty with Overriding public void printname() { System.out.println("I'm a " + getname()); public String getname() { return "C"; class E extends C { public String getname() { return "E"; // in main C c = new E(); c.printname(); The C class might be in another package, or a library... Whoever wrote E might not be aware of the implicaqons of changing getname. Overriding What gets the printed method to the causes console? the behavior of printname to change! 1. I m a C 2. I m a E Overriding can break invariants/ 3. I m an E abstracqons relied upon by the 4. NullPointerExcepQon superclass.

12 Digression: EnumeraQons See: ChatDemo.java

13 Poll Have you started HW 08? 1. yes 2. no

14 EnumeraQons (a.k.a. Enum Types) Java supports enumerated type constructors. These are a bit like OCaml s datatypes. They can be nested inside of other classes: e.g. Command.Type public static enum Type { CONNECT, NICK, JOIN, NAMES, MESG, LEAVE, MOD, UNMOD, KICK

15 Using Enums: Switch Cases // Use of the 'enum' from Command Command.Type t = switch (t) { case CONNECT : System.out.println("Got CONNECT!"); break; case MESG : System.out.println("Got MESG!"); break; default: System.out.println("default"); MulQ- way branch, similar to OCaml s match Works for: primiqve data int, byte, char, etc., Enum types, String Not pacern matching! (Cannot bind subcomponents of an Enum) The default keyword specifies the catch all case.

16 What will be printed by the following program? Command.Type t = Command.Type.CONNECT; switch (t) { case CONNECT : System.out.println("Got CONNECT!"); case MESG : System.out.println("Got MESG!"); case NICK : System.out.println("Got NICK!"); default: System.out.println("default"); 1. Got CONNECT! 2. Got MESG! 3. Got NICK! 4. default 5. something else

17 break GOTCHA: Must use explicit break to avoid fallthrough without break, the code of the next branch will be run too! The program in the quiz prints all of the strings: Got CONNECT! Got MESG! Got NICK! default

18 EnumeraQons Enum types are just a convenient way of defining a class along with methods of a class. Enum types (implicitly) extend java.lang.enum They can contain constant data properqes As classes, they can have methods: e.g. to access a field Intended to represent constant data values. AutomaQcally generated staqc methods: valueof : converts a String to an Enum Command.Type c = Command.Type.valueOf ("CONNECT"); values: returns an Array of all the enumerated constants Command.Type[] varr = Command.Type.values();

19 Accessing elements in a collecqon

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21 Interfaces* of the CollecQons Library Iterable<E> Map<K,V> Collection<E> SortedMap<K,V> List<E> Deque<E> Set<E> SortedSet<E> *not all of them!

22 Iterator and Iterable interface Iterator<E> { public boolean hasnext(); public E next(); public void delete(); // optional interface Iterable<E> { public Iterator<E> iterator(); Challenge: given a List<Book> how would you add each book s data to a catalogue using an iterator?

23 While Loops interface Iterator<E> { public boolean hasnext(); public E next(); public void delete(); // optional example with while loops: List<Book> shelf = // create a list of Books //iterate through the elements on the shelf Iterator<Book> iter = shelf.iterator(); while (iter.hasnext()) { Book book = iter.next(); catalogue.addinfo(book); numbooks = numbooks+1;

24 For Loops interface Iterator<E> { public boolean hasnext(); public E next(); public void delete(); // optional example with for loops: List<Book> shelf = // create a list of Books //iterate through the elements on the shelf for (Iterator<Book> iter = shelf.iterator(); iter.hasnext();) { Book book = iter.next(); catalogue.addinfo(book); numbooks = numbooks+1;

25 syntax: New: For- each Loops example: //repeat body for each element in collection for (type var : coll) { body ; element type array or instance of Iterable<E> List<Book> shelf = // create a list of books //iterate through the elements on a shelf for (Book book : shelf) { catalogue.addinfo(book); numbooks = numbooks+1;

26 Another example: For- each Loops (Cont d) int[] arr = // create an array of ints //count the non-zero elements of an array for (int elt : arr) { if (elt!= 0) cnt = cnt+1; For- each can be used to iterate over arrays or any class that implements the Iterable<E> interface (notably Collection<E> and its subinterfaces).

27 Iterator example public static void iteratorexample() { List<Integer> nums = new LinkedList<Integer>(); nums.add(1); nums.add(2); nums.add(7); int numelts = 0; int sumelts = 0; Iterator<Integer> iter = nums.iterator(); while (iter.hasnext()) { Integer v = iter.next(); sumelts = sumelts + v; numelts = numelts + 1; What is printed by iteratorexample()? 1. sumelts = 0 numelts = 0 2. sumelts = 3 numelts = 2 3. sumelts = 10 numelts = 3 4. NullPointerExcepQon 5. Something else System.out.println("sumElts = " + sumelts); System.out.println("numElts = " + numelts);

28 For- each version public static void foreachexample() { List<Integer> nums = new LinkedList<Integer>(); nums.add(1); nums.add(2); nums.add(7); int numelts = 0; int sumelts = 0; for (Integer v : nums) { sumelts = sumelts + v; numelts = numelts + 1; System.out.println("sumElts = " + sumelts); System.out.println("numElts = " + numelts);