엄현상 (Eom, Hyeonsang) School of Computer Science and Engineering Seoul National University

Transcription

1 엄현상 (Eom, Hyeonsang) School of Computer Science and Engineering Seoul National University C O P Y R I G H T S E O M, H Y E O N S A N G A L L R I G H T S R E S E R V E D

2 - String - Overloading - Operations - Formatter Class - Scanner - Array Outline - Containers - About Containers - Container Disadvantage - Printing Containers - Container Taxonomy C O P Y R I G H T S E O M, H Y E O N S A N G A L L R I G H T S R E S E R V E D

3 Immutable Strings Objects of the String class are immutable. String class every method in the class that appears to modify a String actually creates and returns a brand new String object containing the modification. The original String is left untouched. import static net.mindview.util.print.*; public class Immutable { public static String upcase(string s) { return s.touppercase(); public static void main(string[] args) { String q = "howdy"; print(q); // howdy String qq = upcase(q); print(qq); // HOWDY print(q); // howdy >> howdy HOWDY howdy

4 Overloading + vs. StringBuilder operator + has been overloaded for String objects. Overloading an operation has been given an extra meaning when used with a particular class. (The + and += for String are the only operators that are overloaded in Java, and Java does not allow the programmer to overload any others.) The + operator allows you to concatenate Strings public class Concatenation { public static void main(string[] args) { String mango = "mango"; String s = "abc" + mango + "def" + 47; System.out.println(s); >> abcmangodef47

5 Overloading + vs. StringBuilder Cont d public class UsingStringBuilder { public static Random rand = new Random(47); public String tostring() { StringBuilder result = new StringBuilder("["); for(int i = 0; i < 25; i++) { result.append(rand.nextint(100)); result.append(", "); result.delete(result.length()-2, result.length()); result.append("]"); return result.tostring(); public static void main(string[] args) { UsingStringBuilder usb = new UsingStringBuilder(); System.out.println(usb); >> [58, 55, 93, 61, 61, 29, 68, 0, 22, 7, 88, 28, 51, 89, 9, 78, 98, 61, 20, 58, 16, 40, 11, 22, 4]

6 Overloading + vs. StringBuilder append(a + ": " + c) the compiler will jump in and start making more StringBuilder objects again. StringBuilder has a full complement of methods insert( ), replace( ), substring( ) and even reverse( ) But the ones you will generally use are append( ) and tostring( ). Note the use of delete( ) to remove the last comma and space before adding the closing square bracket. StringBuilder introduced in Java SE5 Prior : StringBuffer, which ensured thread safety

7 Operations on Strings

8 Operations on Strings Cont d

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10 System.out.format() public class SimpleFormat { public static void main(string[] args) { int x = 5; double y = ; // The old way: System.out.println("Row 1: [" + x + " " + y + "]"); // The new way: System.out.format("Row 1: [%d %f]\n", x, y); // or System.out.printf("Row 1: [%d %f]\n", x, y); >> Row 1: [ ] Row 1: [ ] Row 1: [ ]

11 The Formatter class %[argument_index$][flags][width][.precision]conversion Need to control the minimum size of a field. This can be accomplished by specifying a width. The Formatter guarantees that a field is at least a certain number of characters wide by padding it with spaces By default the data is right justified this can be overridden by including a - in the flags section.

12 The Formatter class Cont d import java.util.*; public class Receipt { private double total = 0; private Formatter f = new Formatter(System.out); public void printtitle() { f.format("%-15s %5s %10s\n", "Item", "Qty", "Price"); f.format("%-15s %5s %10s\n", "----", "---", "-----"); public void print(string name, int qty, double price) { f.format("%-15.15s %5d %10.2f\n", name, qty, price); total += price; public void printtotal() { f.format("%-15s %5s %10.2f\n", "Tax", "", total*0.06); f.format("%-15s %5s %10s\n", "", "", "-----"); f.format("%-15s %5s %10.2f\n", "Total", "", total * 1.06); public static void main(string[] args) { Receipt receipt = new Receipt(); receipt.printtitle(); receipt.print("jack s Magic Beans", 4, 4.25); receipt.print("princess Peas", 3, 5.1); receipt.print("three Bears Porridge", 1, 14.29); receipt.printtotal(); >> Item Qty Price Jack s Magic Be Princess Peas Three Bears Por Tax Total 25.06

13 Formatter conversions

14 String.format( ) String.format() a static method which takes all the same arguments as Formatter s format( ) but returns a String. It can come in handy when you only need to call format( ) once: public class DatabaseException extends Exception { public DatabaseException(int transactionid, int queryid, String message) { super(string.format("(t%d, q%d) %s", transactionid, queryid, message)); public static void main(string[] args) { try { throw new DatabaseException(3, 7, "Write failed"); catch(exception e) { System.out.println(e); >> DatabaseException: (t3, q7) Write failed

15 Creating regular expressions

16 Creating regular expressions Cont d

17 Creating regular expressions

18 Scanner The Scanner class added in Java SE5 relieves much of the burden of scanning input The Scanner constructor It can take just about any kind of input object, including a File object (which will also be covered in the I/O chapter), an InputStream, a String, or in this case a Readable With Scanner, the input, tokenizing, and parsing are all ensconced in various different kinds of "next" methods. A plain next( ) returns the next String token there are "next" methods for all the primitive types (except char) as well as for BigDecimal and Biglnteger. All of the "next" methods block they will return only after a complete data token is available for input.

19 Scanner import java.util.*; public class BetterRead { public static void main(string[] args) { Scanner stdin = new Scanner(SimpleRead.input); System.out.println("What is your name?"); String name = stdin.nextline(); System.out.println(name); System.out.println( "How old are you? What is your favorite double?"); System.out.println("(input: <age> <double>)"); int age = stdin.nextint(); double favorite = stdin.nextdouble(); System.out.println(age); System.out.println(favorite); System.out.format("Hi %s.\n", name); System.out.format("In 5 years you will be %d.\n", age + 5); System.out.format("My favorite double is %f.", favorite / 2); >> What is your name? Sir Robin of Camelot How old are you? What is your favorite double? (input: <age> <double>) Hi Sir Robin of Camelot. In 5 years you will be 27. My favorite double is

20 Scanner delimiters import java.util.*; public class ScannerDelimiter { public static void main(string[] args) { Scanner scanner = new Scanner("12, 42, 78, 99, 42"); scanner.usedelimiter("\\s*,\\s*"); while(scanner.hasnextint()) System.out.println(scanner.nextInt()); >>

21 StringTokenizer StringTokenizer Before regular expressions (in J2SE1.4) or the Scanner class (in Java SE5) the way to split a string into parts was to "tokenize" But now it s much easier and more succinct to do the same thing with regular expressions or the Scanner class.

22 StringTokenizer import java.util.*; public class ReplacingStringTokenizer { public static void main(string[] args) { String input = "But I m not dead yet! I feel happy!"; StringTokenizer stoke = new StringTokenizer(input); while(stoke.hasmoreelements()) System.out.print(stoke.nextToken() + " "); System.out.println(); System.out.println(Arrays.toString(input.split(" "))); Scanner scanner = new Scanner(input); while(scanner.hasnext()) System.out.print(scanner.next() + " "); >> But I m not dead yet! I feel happy! [But, I m, not, dead, yet!, I, feel, happy!] But I m not dead yet! I feel happy!

23 Arrays Most efficient way to hold references to objects Limitation: size of an array is fixed Benefits Array knows what type it holds, compile-time type checking Knows its size, you can ask

24 Returning an Array Returning Java array == returning a reference Reference knows the type of the array Doesn t matter where or how array is created Array is around as long as needed, GC cleans up

25 Arrays of Primitives Arrays can hold primitive types directly Containers can only hold references Can use wrapper classes to put primitives into containers, but that s read only

26 java.util.arrays Algorithms for array processing: binarysearch( ) equals( ) fill( ) The same object duplicated sort( ) Unstable Quicksort for primitives Stable merge sort for Objects Overloaded for Object and all primitives

27 Sorting No support for sorting in Java 1.0/1.1 Explain this one to me. They forgot?? Your class must implement Comparable Single method, compareto(object rv) Negative value if the argument is less than the current object Zero if the argument is equal Positive if the argument is greater

28 Imposing a Different Order If a class doesn t implement Comparable or you d like a different order Create a Comparator class Two methods, compare( ) and equals( ) Don't have to implement equals( ) except for special performance needs Just use the default Object equals( ) The compare( ) method must return a negative integer, zero, or a positive integer if the first argument is less than, equal to, or greater than the second, respectively Primitives can only sort in ascending order

29 Containers Divides the issue of "holding your objects": 1. Collection: a group of individual elements, often with some rule applied to them List holds elements in a particular sequence Set has no duplicates 2. Map: a group of key-value object pairs Can return a Set of its keys A Collection of its values Set of its pairs Maps can easily be expanded to multiple dimensions

30 Unlike arrays, built-in functionality Example introduces basic container types Printing containers >> [dog, dog, cat] [cat, dog] {cat=rags, dog=spot import java.util.*; public class PrintingContainers { static Collection fill(collection c) { c.add("dog"); c.add("dog"); c.add("cat"); return c; static Map fill(map m) { m.put("dog", "Bosco"); m.put("dog", "Spot"); m.put("cat", "Rags"); return m; public static void main(string[] args) { System.out.println(fill(new ArrayList())); System.out.println(fill(new HashSet())); System.out.println(fill(new HashMap())); ///:~

31 Filling Containers Containers.fill( ) duplicates a single object into the container Still a problem to add interesting data Will use another generator For Maps, it needs to produce an object containing two objects: Pair

32 Container Disadvantage Unknown type Containers all hold Object, so they hold anything Collection of Cat will also hold a Dog Casting back to the correct type Throwing an exception ; if you try to cast to the wrong type problem because you don t know until the program is running

33 Printing containers //: c09:cat.java public class Cat { private int catnumber; Cat(int i) { catnumber = i; void print() { System.out.println("Cat #" + catnumber); ///:~ //: c09:dog.java public class Dog { private int dognumber; Dog(int i) { dognumber = i; void print() { System.out.println("Dog #" + dognumber); ///:~ //: c09:catsanddogs.java // Simple container example. import java.util.*; public class CatsAndDogs { public static void main(string[] args) { ArrayList cats = new ArrayList(); for(int i = 0; i < 7; i++) cats.add(new Cat(i)); // Not a problem to add a dog to cats: cats.add(new Dog(7)); for(int i = 0; i < cats.size(); i++) ((Cat)cats.get(i)).print(); // Dog is detected only at run-time ///:~

34 The abstraction Iterators selecting each element in a sequence (traverse a Collection) Possibly make many iterators, and select more than one element at a time Hides underlying collection, so you can easily change from one type to another A design pattern

35 Iterators import java.util.*; class Hamster { private int hamsternumber; Hamster(int i) { hamsternumber = i; public String tostring() { return "This is Hamster #" + hamsternumber; class Printer { static void printall(iterator e) { while(e.hasnext()) System.out.println(e.next()); public class HamsterMaze { public static void main(string[] args) { ArrayList v = new ArrayList(); for(int i = 0; i < 3; i++) v.add(new Hamster(i)); Printer.printAll(v.iterator()); ///:~

36 Container Taxonomy

37 The Collection Interface boolean add(object) boolean addall(collection) void clear( ) boolean contains(object) boolean containsall(collection) boolean isempty( ) Iterator iterator( ) boolean remove(object) boolean removeall(collection) boolean retainall(collection) int size( ) Object[ ] toarray( ) Object[ ] toarray(object[ ] a)

38 Lists have additional abilities void add(index, Object) boolean addall(index, Collection) Object get(index) int indexof(object) int lastindexof(object) ListIterator listiterator() ListIterator listiterator(index) Object remove(index) Object set(index, Object) // Means: replace List sublist(fromindex, toindex)

39 Lists produce ListIterators boolean hasnext() Object next() boolean hasprevious() Object previous() int nextindex() int previousindex() add(object) void remove() void set(object) // Replace

40 ArrayList ArrayList & LinkedList Default choice for a simple sequence Optimal for rapid random access ListIterator should be used only for back-and-forth traversal of an ArrayList, but not for inserting and removing elements LinkedList Optimal sequential access with inexpensive insertions and deletions from the middle of the list possibly be used as a stack, a queue, and a deque addfirst( ), addlast( ), getfirst( ), getlast( ), removefirst( ), and removelast( ) ListIterator can be used for insertion and removal

41 Queues, Stacks and Deques Not there Use LinkedList addfirst( ), addlast( ) removefirst( ), removelast( ) getfirst( ), getlast( )

42 Sets Mathematical set abstraction No duplicates Two interfaces Set (identical to Collection) SortedSet Implementations HashSet for maximally fast lookups TreeSet to maintain sorted order Order determined by Comparable or Comparator) You can extract an ordered sequence from a TreeSet: Comparator comparator() Object first() Object last() SortedSet subset(fromelement, toelement) SortedSet headset(toelement) SortedSet tailset(fromelement)

43 Sets import java.util.*; import com.bruceeckel.util.*; public class Set1 { static Collections2.StringGenerator gen = Collections2.countries; public static void testvisual(set a) { Collections2.fill(a, gen.reset(), 10); Collections2.fill(a, gen.reset(), 10); Collections2.fill(a, gen.reset(), 10); System.out.println(a); // No duplicates! // Add another set to this one: a.addall(a); a.add("one"); a.add("one"); a.add("one"); System.out.println(a); // Look something up: System.out.println("a.contains(\"one\"): " + a.contains("one")); public static void main(string[] args) { System.out.println("HashSet"); testvisual(new HashSet()); System.out.println("TreeSet"); testvisual(new TreeSet()); ///:~

44 Maps Alternately termed a map, a dictionary or an associative array Like an ArrayList, but instead of using an integral index to look something up, another object is used as a key Store key/value pairs of Objects Duplicate Keys not allowed Returns keys as a Set view Returns values as a Collection

45 TreeMap SortedMap with elements stored in a tree Comparator comparator( ) Object firstkey( ) Object lastkey( ) SortedMap submap(fromkey, tokey) SortedMap headmap(tokey) SortedMap tailmap(fromkey) Viewing keys or pairs will be in sorted order Order determined by Comparable or Comparator

46 HashMap HashMap a Map with very fast lookup: uses a hash code to organize the objects Should typically be your first choice for a Map Program counts occurrences of random numbers:

47 HashMap import java.util.*; class Counter { int i = 1; public String tostring() { return Integer.toString(i); >> {19=526, 18=533, 17=460, 16=513, 15=521, 14=495, 13=512, 12=483, 11=488, 10=487, 9=514, 8=523, 7=497, 6=487, 5=480, 4=489, 3=509, 2=503, 1=475, 0=505 public class Statistics { public static void main(string[] args) { HashMap hm = new HashMap(); for(int i = 0; i < 10000; i++) { // Produce a number between 0 and 20: Integer r = new Integer((int)(Math.random() * 20)); if(hm.containskey(r)) ((Counter)hm.get(r)).i++; else hm.put(r, new Counter()); System.out.println(hm); ///:~

48 HashMap key classes A common pitfall if a class is to be used as a key for a HashMap, you must override hashcode( ) and equals( ) from the common root class Object

49 Old 1.0/1.1 Containers They ll appear in older standard libraries And sometimes new ones Don t use them in new code Vector -> ArrayList BitSet (maybe use) Stack -> LinkedList Hashtable -> Map

50 Enumeration: old Iterator Smaller interface than Iterator, longer method names nextelement( ) moves to and produces the next item hasmoreelements( ) indicates the end Can actually appear quite a bit, even in new library code Treat it just like an Iterator without a remove( ) method

51 Summary Array associates numerical indices to objects Holds objects of a known type Fixed size

52 Summary Sequence (List) associates numerical indices to objects Only holds Object references Automatically resizes itself Set only holds one of each object HashSet for speed, TreeSet for sorted order Map associates objects with other objects Provides rapid random access HashMap orders with hashcode( ) & equals( ) TreeMap keeps sorted order