Tha Java Programming Language

Size: px

Start display at page:

Download "Tha Java Programming Language"

Karin Roberts
6 years ago
Views:

1 Tha Java Programming Language Kozsik Tamás ( ) to/

2 III. Arrays, collections and other baseclasses

3 Some of the baseclasses Object String StringBuffer Integer, Double,... System Math

4 Object The superclass of every other class. (Default, in case of no extends.) Thus every object belongs to the Object class. Defines basic procedures which every object shuld have. String tostring() boolean equals( Object o ) converts to string compares others: clone, getclass, hashcode, finalize, etc.

5 tostring Works for any objects: System.out.println( "o = " + o ); Because of the + sign, o will automatically be converted into a String, calling the tostring method. Many different classes defines a specialized tostring for converting their own objects into strings. The default function defined in Object writes the name and hashcode of the dynamic type of the object on the screen. (The latter one is a number.)

6 equals vs. == The equality of objects can be examined with the equals method. By default, that is the same as ==. But when are two dates equal? They are equal if they report the same date/time, aren't they? The == tells when two references are equal. (I.e. when two objects are the same.) For example the following will not be true: new Date(1970,4,28) == new Date(1970,4,28) Usually, classes redifine the equals method for defining the equality of two objects. The following can be true: (new Date(1970,4,28)).equals( new Date(1970,4,28) )

7 The equals method of the Date public class Date { int year, month, day; class public boolean equals( Object d ){ if( d instanceof Date ) return year == ((Date)d).year && month == ((Date)d).month && day == ((Date)d).day; else return false; } public int hashcode(){ // this is also necessary return 400*year + 31*month + day; }...

8 Example with dates Date d1 = new Date(2001,1,1); Date d2 = new Date(2001,1,1); Date d3 = new Date(1789,7,14); d1.equals(d2)? d1.equals(d3)? d1 == d2? d1 == d3? (new Object()).equals( new Object() )? new Object() == new Object()?

9 The String class A sequence of Unicode characters String s = "Hello!"; String s = new String("Hello!"); The content of a String object cannot be modified. Instead, a new string has to be created. s = "Hi?"; s = new String("Hello?"); But the language have a lot of constructing, querying and comparing procedures. length, charat, compareto, concat, endswith, replace, substring, trim, valueof, indexof, equalsignorecase, tolowercase,...

10 The StringBuffer class A sequence of Unicode characters Its content can be modified without creating a new object. Has a different representation as the String class, so different procedures can be implemented in it efficiently. append, insert, reverse, setcharat, setlength Procedures not just transform the object, but also return a reference to it. It is good for: StringBuffer b = new StringBuffer("Hello!"); b.append(" Hi!").append(" Salut!").reverse();

11 Wrapper classes There are wrapper classes defined for primitive types: Integer, Byte, Double, Boolean, etc. int i = 42; Integer j = new Integer(i); You can use them when you want to use a primitive type value as an object. For example when you need the tostring... System.out.println(42); The following is needed: new Integer(42).toString() Another important example a bit later...

12 Wrapper classes (2) boolean Boolean byte Byte char Character short Short int Integer long Long float Float double Double Procedures e.g. for the Integer class: static int parseint(string s [, int radix]) Integer( String s ) static String tostring( int i [, int radix]) final static int MIN_VALUE, MAX_VALUE

13 Some other important classes Math Math.PI, Math.E abs(), sqrt(), pow(), log(), min(), floor(), round(), rint(), random(), sin(), asin(), toradians() System in, out, err currenttimemilis(), exit(int), gc() Number Common superclass of the numeric

14 Problem Specification of the Java language does not allow the compilers to create extra objects for Strings with the same values. Test the compiler you use! And what happens in case of two Integer objects?

15 For the String problem: String s = new String("Hello!"); String z = new String("Hello!"); if( s == z )...

16 Arrays For storing several values of the same type Efficient access: indexing Inserting and deleting are time-taking procedures

17 Reviving an old example Declaration of an array type variable int t[]; int[] t;

18 Creating an array Variable declaration will not create the array. Array types are similar to classes. Actually, arrays are objects. int[] t = new int[10]; int t[] = new int[10]; An array type variable is just a reference. int[] s; int x = s[0]; int y = s.length; Wrong! It will not work until an array object is created!

19 Accessing an element of an array Indexed from 0 to length-1. Length is given at creating the array. Length cannot be changed. However, a reference to an array "does not care about" the length of the array. int[] x = new int[5]; x = new int[7]; int y[] = x; Querying length: Array index: args.length args[i] Execution system checks the index

20 Initializing an array double[] t = new double[3]; t[0] = 1.0; t[1] = 2.0; t[2] = 3.0; double[] s = new double[3]; for( int i = 0; i < s.length; i++ ) s[i] = i; double[] v = {1.0, 2.0, 3.0};

21 Array of objects Actually, an array consisting of references. Point[] t; t = new Point[3]; for (int i=0; i<3; i++) t[i] = new Point(i,2); Point[] s = { new Point(1,2), new Point(2,2),

22 Multi-dimensional arrays No such thing in Java. But an array of arrays can be created. int[][] mdt = new int[3][2]; for (int i=0; i<3; i++) for (j=0; j<2; j++) mdt[i][j] = i*j;

23 mdt[1] = new int[4]; mdt[1][0] = 2; mdt[1][1] = 4; mdt[1][2] = 8; "Matrix" of an irregular form int mdt[][]; mdt = new int[2][]; mdt[0] = new int[3]; mdt[0][0] = 7; mdt[0][1] = 2; mdt[0][2] = 9;

24 for( int i=0; i<10; i++ ) Heterogenous arrays An array can contain objects of different classes...if they have a common superclass The static types of the elements of an array are the same. The static type determines the procedures that can be used for the elements. The dynamic types of the elements of an array can be different. The dynamic type determines the implementation to be executed. Employee t[] = new Employee[10]; for( int i=0; i<10; i++ ) t[i] = new Employee(); t[42] = new Employer();

25 The array type as a class Similar in many ways... "Subtypes" Object[] t = new Employee[3]; Caution! After this, the following will raise a run-time error: t[0] = new Object();

26 Feladat Define a Matrix type. Procedures: Creating (initializing with zeros or a given element) an Accessing and setting the value of element Multiplication by a scalar Addition Multiplication

27 Other collections sequence set stack heap graph vector list queue tree hashtabl e

28 Efficiency "Trade-off" Array: constant access, linear insert List: linear access, constant insert Heap: logarithmic access and insert Choose the most sufficient for the given problem.

29 Opportunities in Java Arrays Vector, Stack and Hashtable classes from the java.util package parts of the language since JDK 1.0 Collections Framework standard part of the language since JDK 1.2 Other, not standard directories can be also accessed, e.g. JGL.

30 Vector Quite rapid access/insert/deletion first, last, in between Can store Objects boolean contains( Object element ) int indexof( Object element ) Object elementat( int index ) Object lastelement() Object removeelementat(int index) boolean removeelement( Object element ) void insertelementat( Object element, int index ) void addelement( Object element )

31 Example for using a Vector import java.util.*; class ShortWords { public static void main(string args[]){ Vector v = new Vector(); for( int i=0; i<args.length; i++ ){ if( args[i].length() < 10 ){ v.add(args[i]); } } } }

32 Elements of a Vector The type of the elements is Object. Vector lastwithoutwine( Vector v ){ for( int i=0; i<v.size(); i++ ){ String s = (String) v.getelementat(i); if( s.endswith("wine") ) v.removeelementat(i); } return v; }

33 A sequence of numbers If you want to use an array: int[] t = {2,42,37}; new int[3] Integer[] t = { new Integer(2), new Integer(42), new Integer(37) }; Data are stored quite differently in the memory. If you want to use something else, for example Vector: Vector v = new Vector(); v.add( new Integer(42) ); v.add( new Integer(37) );

34 Enumeration Iterator with which an element of a collection can be accessed. For instance a Vector... void writeselement( Enumeration e ){ while( e.hasmoreelements() ){ Object o = e.nextelement(); System.out.println(o); } } writeselement(v.elements())

35 Capacity Any number of elements can be stored in Vector objects. An "array-like" internal representation provides the efficiency of positioning procedures. Sometimes it "becomes full": then it allocates some more memory space to put data in. Increases its capacity automatically. Initial capacity can be set in the constructors. Vector() Vector( int initialcapacity ) Vector( int initialcapacity, int capacityincrement )

36 Problem Write a method which selects the first occurrances of those elements from a vector which occur several times in the vector. Do it in the following way. Querying the Enumeration object, put the elements into another vector one after the other. Before putting one into the other vector, see whether it is in that vector already or not. If yes, put that element into a third vector, too. Then get the elements from that third vector with the help of an Enumeration, and delete the first occurrances of its elements from the first vector.

37 Stack: stack data type A subclass of the Vector. Regular stack procedures: Object push( Object element ) Object pop() Object peek() boolean empty()

38 Hashtable If you do not want to index with numbers, but with other objects... key - value pairs can be stored Hashtable ht = new Hashtable(); ht.put("hat", "red"); ht.put("coat", "blue"); String colorofhat = (String) ht.get("hat"); Search and insert of a pair by its key: Object put( Object key, Object value) Object get( Object key ) Keys and values can be any objects. There can be different keys within a table...

39 More Hashtable procedures searching, deleting, querying keys and values, etc. boolean containskey ( Object key ) boolean containsvalue ( Object value ) Object remove ( Object key ) Enumeration keys() Enumeration elements() int size() void clear() Efficient storage and search by the hashcode() of keys.

40 Example for using a Hashtable Count the occurrance of command-line arguments. java Count go go far away {far=1, go=2, away=1}

41 import java.util.*; class Count { public static void main(string[] args){ Hashtable h = new Hashtable(); for( int i=0; i<args.length; i++ ){ String key = args[i]; Integer number; if( h.containskey(key) ){ number = (Integer) h.get(key); int onemore = number.intvalue() + 1; number = new Integer(oneMore); } else number = new Integer(1); h.put( key, number ); } System.out.println(h); } }

42 Problem Let command-line arguments be numbers. For every different number save its location in the sequence of command-line arguments. If a number occurs more than once, save all its locations. According to this registration, write the numbers and their locations on the screen. java Collect : : 4 1: 2 11: 0 5 {32=[1, 3, 6], 5=[4], 1=[2], 11=[0, 5]}

43 import java.util.*; class Collect { public static void main(string[] args){ Hashtable h = new Hashtable(); for( int i=0; i<args.length; i++ ) putin( h, new Integer(i), new Integer(args[i]) ); writeselement(h); // System.out.println(h); } public static void putin( Hashtable h, Integer pos, Integer number ){...}

44 public static void putin( Hashtable h, Integer pos, Integer number ){ } Vector v; if( h.containskey(number) ) v = (Vector) h.get(number); else v = new Vector(); v.add(pos); h.put( number, v );

45 public static void writeselement( Hashtable h ){ Enumeration numbers = h.keys(); while( numbers.hasmoreelements() ){ Integer number = (Integer) numbers.nextelement(); System.out.print(number + ": "); Vector v = (Vector) h.get(number); Enumeration positions = v.elements(); while( positions.hasmoreelements() ) System.out.print( positions.nextelement() + " " ); System.out.println(); } }

46 Collections Framework A system of Collections, adding abstraction level Interfaces: collection, map, set, list Map Collectio n Set List SortedMap SortedSet

47 Collection: collection boolean add(object o) boolean contains(object o) boolean remove(object o) abstraction boolean addall(collection c) boolean containsall(collection c) boolean removeall(collection c) boolean retainall(collection c) void clear() boolean isempty() int size() Iterator iterator() Object[] toarray()

48 List: a sequence data type the same element can occur several times the order of elements is important two lists are equal if they have the same elements in the same order like Vector, and what is more: Vector implements this interface short procedure names: set, get, add remove removes the first occurrance of an element, add and addall appends it to the end of the list

49 Implementations of List two different implementations linked (list) the LinkedList class efficient insert into the beginning of the list efficient deletion during enumeration vector-like the ArrayList class efficient positioning procedures the generally used one (usually more efficient) Vector is its subclass

50 Example for the List interface import java. util.*; public class insertintolist { public static void main( String[] args) { List s = new ArrayList(); s.add("route"); s.add(new Integer(66)); s.add("route"); s.add("to LA"); System.out.println(s); } } [Route, 66, Route, to LA]

51 Set: set data type each element can occur only once in a set the order of elements is indifferent two sets are equal if they contain the same elements contain: union: intersection: difference: contains, containsall addall retainall removeall Implementation: HashSet Another implementation: TreeSet, which also implements SortedSet thus you can sort the elements of a set

52 An example for the Set interface import java. util.*; public class insertintoset { public static void main( String[] args) { Set s = new HashSet(); s.add("route"); s.add(new Integer(66)); s.add("route"); s.add("to LA"); System.out.println(s); } } [to LA, Route, 66]

53 Iterator: enumerating a collection Like Enumeration interface a bit shorter procedure names you can delete during enumeration Procedures: boolean hasnext(), Object next(), void remove() ListIterator is its subclass. Lists have that iterator. More procedures... boolean hasprevious(), Object previous() void add(object), void set(object) int nextindex(), int previousindex()

54 Example for using an Iterator import java.util.*; public class Iteration { public static void main(string[] args){ Collection c = new ArrayList(); for( int i=0; i<10; i++) c.add( new Double(Math.random()) ); Iterator it = c.iterator(); while( it.hasnext() ){ Double d = (Double) it.next(); if( d.doublevalue() < 0.6 ) it.remove(); } System.out.println(c); } }

55 Map: map data type Like HashTable key - value pairs Iteration on keys: keyset().iterator() on values: values().iterator() on pairs (its name is Entry): entryset().iterator() Two different implementations: HashMap and TreeMap TreeMap implements SortedMap as well (sorted!) We usually use HashMap, because that is more efficient.

56 Sorting Arrays: with the sort method of the Arrays class Lists (List): with the sort method of the Collections class If there is a natural sort defined on the elements of the array/list. (See also the Comparable interface!) int[], Integer[] List in which Double objects will be stored You can also define a new sort. (See also the Comparator interface!)

57 By a natural sort Comparable: int compareto(object o) import java.util.*; class Sort { public static void main( String[] args ) throws ClassCastException { Arrays.sort(args); for( int i=0; i<args.length; i++ ) System.out.println(args[i]); } } Hello java Rendez Hi Hello Salut Hi Salut

58 Problem Let command-line arguments be integers. Sort them and write them on the screen!

59 Defining a new sort Comparator: int compare( Object o1, Object o2 ) class LengthComparator implements Comparator { public int compare(object o1, Object o2) { int h1 = ((String) o1).length(); int h2 = ((String) o2).length(); if( h1<h2 ) return -1; if( h2<h1 ) return 1; return 0; } }

60 import java.util.*; class Sort { public static void main( String[] args ) throws ClassCastException { Arrays.sort(args, new LengthComparator()); for( int i=0; i<args.length; i++ ) System.out.println(args[i]); } } Hi java Rendez Szia Hi Salut Szia

61 List sorting In the same way, with the Collections.sort procedure. It can be either a natural sort or a userdefined one.

62 Other procedures minimum and maximum search max() binary search binarysearch() reverse reverse() fill, copy fill(), copy() shuffle shuffle() min(), Arrays, Collections

63 Problem Give another soultion for sorting numbers: create a Comparator class which compares Strings in the following way: converts them into numbers with the parseint method and then compares those numbers. Solve the sorting of numbers in the following way: instead of putting command-line arguments into an array, put them into an ArrayList object!

CONTAİNERS COLLECTİONS

CONTAİNERS COLLECTİONS CONTAİNERS Some programs create too many objects and deal with them. In such a program, it is not feasible to declare a separate variable to hold reference to each of these objects. The proper way of keeping