09 Objects III: String and StringBuffer, Singly (Doubly) Linked Lists

Transcription

1 Exercises Software Development I 09 Objects III: String and StringBuffer, Singly (Doubly) Linked Lists December 10th, 2014 Software Development I Winter term 2014/2015 Priv.-Doz. Dipl.-Ing. Dr. Andreas Riener Institute for Pervasive Computing Johannes Kepler University Linz riener@pervasive.jku.at

2 I Strings: Introduction A String in Java is a complex data type (call by reference) that stores/ contains a sequence of characters (data type char) Notation Character sequence is enclosed in double quotes "..." Example: "this is a string" Basic operations on strings Comparison of strings, e.g., is greater, smaller, equal (lexicographical; as like in a telephone book) Concatenation of two or more strings ( + -operator) Searching for substrings Conversions (to lower case, to upper case characters) And many more (see class description) Software Development I // Exercises // 09 Strings & Lists // 3

3 Strings: Class String If not otherwise specified, you should use the existing class String in the Java API (java.lang.string) and its predefined and implemented methods public final class String extends Object implements Serializable {... // end String Objects of the class String are immutable Sometimes it seems that applying certain methods to a string does change the object, but actually a new object is created and the existing one remains unchanged (Use the StringBuffer or StringBuilder classes to store character sequences that are subject to change. Instances of these classes are optimized for operations that modify the contained string.) Software Development I // Exercises // 09 Strings & Lists // 4

4 I Strings: Immutability Example public class ImmutableStrings { public static void main (String[] args) { String str = new String("SWE1 2014/15"); String str2 = str; System.out.println(str2==str); str = "SWE1 2014/15"; System.out.println(str2==str); Program output? true false Software Development I // Exercises // 09 Strings & Lists // 5

5 Strings: Constructors Constructors of the class String are called whenever the compiler (javac) encounters a character sequence enclosed in double quotes. public class String extends Object implements Serializable { public String() { public String(String value) { public String(char[] value) { public String(char[] value, int offset, int count) { public String(byte[] ascii, int hibyte, int offset, int count) { public String(byte[] ascii, int hibyte) { public String(byte[] bytes, int offset, int length, String enc) throws UnsupportedEncodingException { public String(byte[] bytes, String enc) throws UnsupportedEncodingException { public String(byte[] bytes, int offset, int length) { public String(byte[] bytes) { public String(StringBuffer buffer) { Software Development I // Exercises // 09 Strings & Lists // 6

6 Strings: Class methods The following methods are implemented as class methods, i.e., can be accessed statically by qualifying the call with the class name (String). public final class String extends Object implements Serializable { static String copyvalueof(char[] data) static String copyvalueof(char[] data, int offset, int count) static String valueof(boolean b) static String valueof(char c) static String valueof(char[] data) static String valueof(char[] data, int offset, int count) static String valueof(double d) static String valueof(float f) static String valueof(int i) static String valueof(long l) static String valueof(object obj)... // end String Software Development I // Exercises // 09 Strings & Lists // 7

7 Strings: Instance methods The following methods can be invoked on every instance of class String (String object) public final class String extends Object implements Serializable { char charat(int index) int compareto(object o) int compareto(string anotherstring) int comparetoignorecase(string str) String concat(string str) boolean endswith(string suffix) boolean equals(object anobject) boolean equalsignorecase(string anotherstring) byte[] getbytes() void getbytes(int srcbegin, int srcend, byte[] dst, int dstbegin) byte[] getbytes(string enc) void getchars(int srcbegin, int srcend, char[] dst, int dstbegin) int hashcode()... Software Development I // Exercises // 09 Strings & Lists // 8

8 Strings: Instance methods The following methods can be invoked on every instance of class String (String object) public final class String extends Object implements Serializable { int indexof(int ch) int indexof(int ch, int fromindex) int indexof(string str) int indexof(string str, int fromindex) String intern() int lastindexof(int ch) int lastindexof(int ch, int fromindex) int lastindexof(string str) int lastindexof(string str, int fromindex) int length() boolean regionmatches(boolean ignorecase, int toffset, String other, int offset, int len) boolean regionmatches(int toffset, String other, int ooffset, int len)... Software Development I // Exercises // 09 Strings & Lists // 9

9 Strings: Instance methods The following methods can be invoked on every instance of class String (String object) public final class String extends Object implements Serializable { String replace(char oldchar, char newchar) boolean startswith(string prefix) boolean startswith(string prefix, int toffset) String substring(int beginindex) String substring(int beginindex, int endindex) char[] tochararray() String tolowercase() String tolowercase(locale locale) String tostring() String touppercase() String touppercase(locale locale) String trim() // end String Software Development I // Exercises // 09 Strings & Lists // 10

10 Strings: Creation What s this? Interned String object is stored at a central place (in memory) and whenever the same literal is used/created again, the JVM will not create a new String object, but use the reference of the 'cached' String. There are several different ways (why?) to create a String object String constants or literals (interned String object) String alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"; Conversion from a character (or byte) array String s1 = new String(new char[] {'A','B','C'); String s2 = new String(s1); // copy constructor; copy of s1 Conversion of other primitive data types: // int, double, long, float, boolean, short... String s3 = String.valueOf(x == 2.4); String pi = String.valueOf( ); Concatenation of multiple strings String firstname = new String("Andreas"); String lastname = "Riener"; // short form (String literal) String name = firstname + " " + lastname; // new object System.out.println("Name: " + name); Software Development I // Exercises // 09 Strings & Lists // 11

11 Strings: Method Usage Declaration (String constant) String name = "Andreas"; // Declaration of a new string String length name.length(); // output = 7 "Andreas".length(); // output = 7 Character operations name.charat(2); // output = d name.indexof("a"); // output = 5 Substrings String s1 = name.substring(0, 2); String s2 = name.substring(3); // output = An // output = reas String concatenation String s = s1 + s2; s.startswith("andr"); // output = Anreas // false Software Development I // Exercises // 09 Strings & Lists // 12

12 Strings: String Constants // String Objects Different behavior (internal representation) Example for String constants/literals (Interned String object) String a = "strange"; String b = "strange"; At time of creation of String b, the compiler checks if already an object with same signature (i.e., character sequence) exists in the memory; if true, in b a reference to the already existing String object is stored (reasons: immutability of strings; allows compiler optimizations) Real String objects String c = new String("strange"); String d = new String("strange"); System.out.println((a == b) + " " + (c == d)); Output: true false A and B reference the same String object (literal) Software Development I // Exercises // 09 Strings & Lists // 14 C and D are two different String objects

13 I Strings: Array of Strings So far we have looked at arrays of primitive types (int, float, double, etc.) private int[] numbers = new int[] {12,4,7,17,21,8,8,3,16,2; numbers Array of Strings (special case of array of objects) Declaration of the String array: private String[] countries; Creation (initialization) of the array: countries = new String[10]; // allocates memory to hold references to 10 String objects Create String objects and add them to the array: countries[0] = new String("Canada"); countries[5] = new String("Australia"); String literals/constants: countries[1] = "Austria"; countries[2] = "Austria"; countries System.out.println( countries[1]==countries[2]); Output: true Canada Australia Italy recall: jagged 2 dimensional array Software Development I // Exercises // 06 Arrays (Declaration, Initialization, Usage; Multidim. Arrays) // 15

14 I Strings: Classes StringBuffer, StringBuilder Strings are immutable and cannot be modified at runtime; StringBuffer and StringBuilder objects can change its length dynamically and are modifiable. StringBuilder vs. StringBuffer The only difference between StringBuffer and StringBuilder is, that operations on a StringBuilder object are unsynchronized and operations on instances of the class StringBuffer are synchronized. Single-threaded applications: Use of StringBuilder is more efficient... Constructors (example StringBuilder) public StringBuilder() Creates an empty string (no characters in it) with an initial length of 16 characters; buffer will be automatically enlarged if necessary public StringBuilder(int length) Creates an empty string with the given length (space to hold length chars) public StringBuilder(String str) Creates a new StringBuffer object and initializes it with the String str. The initial capacity of the string builder is str.length() Software Development I // Exercises // 09 Strings & Lists // 16

15 Strings: Class StringBuilder Example: Common usage of Stringbuilders StringBuilder a = new StringBuilder("Merry!"); // Append a String to the existing sequence a.append("!!!"); // Merry!!!! // Append an int value to the existing sequence; will be converted into corresponding characters a.append(3); // Merry!!!!3 // Append a double to the existing sequence a.append(3.14); // Merry!!!!33.14 // Delete characters in the string a.delete(6, 20); // Merry! // Insert a character sequence at a certain position a.insert(5, " Christmas"); // Merry Christmas! Software Development I // Exercises // 09 Strings & Lists // 17

16 Strings: Class StringBuilder Example: Common usage of Stringbuilders (continued) // Replace a certain character sequence a.replace(6, 12, "X"); // Merry Xmas! // Delete a character at a certain position a.deletecharat(10); // Merry Xmas // Cut the string at a certain position a.setlength(5); // Merry // Reverse the string a.reverse(); // yrrem // create an immutable String String s1 = a.tostring(); // yrrem // Create a new string with a certain substring String s2 = a.substring(0, 1); // y Software Development I // Exercises // 09 Strings & Lists // 18

17 I StringBuffer: Example Anagram We are looking for an algorithm (Java program) that reads in two words from the user and checks, whether or not the second word is an anagram of the first. The second word is an anagram of the first word if it can be created by permutation of the first word (not to be confused with palindromes!) Examples item time anagram acres caress not an anagram scare cares anagram react crater not an anagram Software Development I // Exercises // 09 Strings & Lists // 19

18 StringBuffer: Example Anagram public class Anagram { public static void main(string[] arg) { String s1, s2; System.out.println ("first word? "); s1 = Input.readString(); System.out.println ("second word? "); s2 = Input.readString(); System.out.print ("The words are "); if (!isanagram(s1, s2)) System.out.print("NOT "); System.out.print("anagrams."); // end main // end Anagram Software Development I // Exercises // 09 Strings & Lists // 20

19 StringBuffer: Example Anagram (2) static boolean isanagram (String s1, String s2) { boolean isanagram = false; if (s1.length() == s2.length()) { // create editable copy of s2 StringBuffer sb2 = new StringBuffer(s2); // strike out letters of s1 in sb2 for (int i = 0; i < s1.length(); i++) { int pos = sb2.tostring().indexof(s1.charat(i)); if (pos!= -1) sb2.setcharat(pos, (char)0); // mark with non-print char // end for // check if all letters are striked out int i = 0; while (i < sb2.length() && sb2.charat(i) == (char)0) i++; isanagram = (i == sb2.length()); // end if return isanagram; // end isanagram Software Development I // Exercises // 09 Strings & Lists // 21

20 Strings: Summary How to decide which class (String, StringBuffer, or StringBuilder) to use for String objects? 1) If the text is not likely to be modified in the future, one should select String (String objects are immutable); in addition, usage of string literals is preferred over String objects (compiler optimization) 2) If the text is likely to be modified but will be accessed by a single thread only, one should use StringBuilder (unsynchronized) 3) If the string object is likely to be modified and more than one thread have access to it, one should use StringBuffer (synchronized) Software Development I // Exercises // 09 Strings & Lists // 22

21 Lists and Vectors in Java

22 I (Linked) Lists: Introduction As objects (variables of complex data type) in Java are accessed per reference, an object can store a reference to another object. This reference can also refer to an instance of the same class This fact makes it possible to link objects with each other ( list of objects) Advantage of lists The major benefit of linked lists is that you do not have to specify a fixed size for your list (like arrays). You can have an arbitrary number of nodes and its number can be dynamically modified at runtime (adding and removing nodes) Types of lists: There is more than one type of a linked list Singly linked lists (the simplest type). The root node links one way through all the nodes. Last node links to null. Circular lists. All nodes are linked together forming a circle. Very effective for some list operations (shifting elements, etc.) Doubly linked lists. Every node stores a reference to its previous node a well as its next. Good if you need to move back by a few nodes, and don t want to run from the beginning of the list. Software Development I // Exercises // 09 Strings & Lists // 24

23 I (Linked) Lists: Introduction Linking objects with each other ( lists of objects) List of objects can be created by declaring a variable referencing the first element of the list (head). Each element in the list stores another reference pointing to the following element in the list. For this concept, it is necessary to somehow identify which element is the last one in the list (tail) null reference, i.e., the last element does not reference any other object Software Development I // Exercises // 09 Strings & Lists // 25

24 I (Linked) Lists: Introduction Arrays versus Linked Lists (1) 1) Create an empty integer array 2) Fill it partially with data. The last element is allocated but not used (wastes memory) 3) Add another element. The array is now full, no further elements can be added (elements can be deleted or replaced) 4) Arrays have a fixed size. If the array is full, no more elements can be added (linked lists are not fixed size!) 5) If you delete an element from the middle, and want no holes in the array, you will need to shift everything after the deleted element down. (same applies to inserting in the middle) Underlying source: Software Development I // Exercises // 09 Strings & Lists //

25 I (Linked) Lists: Introduction Arrays versus Linked Lists (2) head 1) This is what an empty linked list lookes like. Since we don t have any nodes, its reference/ pointer is null. head 2) Linked list with three elements/nodes. Each node points to the next node in the chain. head is a node with a reference to the first node, the next reference of the last node is null. head 3) Linked list with deleted element [2]. Element [1] (with value 1), previously pointing to [2], is now pointing to [3]. References need to be updated! If forgotten, unreferenced elements are lost and will get automatically garbage collected. head 4) The second node [2] can be added back at a later time between [1] and [3], and another 1 node added at the end of the list. The next-reference of [1] is set to [2] and the next-reference of [2] is set to the old next-reference of [1], which is [3] Software Development I // Exercises // 09 Strings & Lists // 27 null node=new Node(1); head=node; 3 [1].next=[3]; 4 [2].next=[1].next; [1].next=[2]; [3].next=[4];

26 I Lists: Structure Example: List of House objects public class House { int floors, noflifts; Person owner;...; House next; 54 house1 floors noflifts owner next = #67 67 house2 floors noflifts owner next = #80 80 house3 93 house4 floors noflifts owner next = #93 floors noflifts owner next = null Software Development I // Exercises // 09 Strings & Lists // 28

27 Lists: Inserting To access elements in the list, the reference to the first element has to be known. Given the head of the list, all the other elements can be reached by following the "next" references House first;... House house = first; while (house!= null) { // do something with the house house = house.next; Inserting a new house object at the beginning to the list requires to change the reference to the first element: house = new House(); house.next = first; first = house; // house initialization To append a new (house) object at the end of the list, the "next" reference has to be followed until reaching the last element (tail) of the list, and the new objects has to be inserted at the very end; other option: keep a reference to the last element in advance (e.g., House last;) Software Development I // Exercises // 09 Strings & Lists // 29

28 I SWE1 UE09 Lists.zip Lists: Example NumberList NumberList represents a linked list which stores an ascending list of integer numbers. Each node is an instance of the class NumberNode. class NumberNode { public int num; public NumberNode next; // next node in list public NumberNode(int num) { // constructor this.num = num; head Task: Implement two methods add(int num) adds a number to the list (ensure that the list remains sorted) getfrequency(int num) returns the number of occurrences of the given integer number num. Make sure that you do not iterate through the entire list but make use of the fact that the list is sorted. Software Development I // Exercises // 09 Strings & Lists // 30

29 SWE1 UE09 Lists.zip Lists: Example NumberList public class NumberList { NumberNode head; // adds the new number at the correct position of the sorted list public void add(int num) { NumberNode q = new NumberNode(num); NumberNode p = head, prev = null; while (p!= null && p.num < num) { // find the location to insert prev = p; // the new node p = p.next; q.next = p; Remember: Short-circuit evaluation (SCE) if (p == head) { // check if the head needs to be changed... head = q; else { prev.next = q; // continued on next slide... Software Development I // Exercises // 09 Strings & Lists // 31

30 SWE1 UE09 Lists.zip Lists: Example NumberList // continuing from previous slide... // counts the number of occurrences of the given number public int getfrequency(int num) { int count = 0; NumberNode p = head; // go through the list, increase counter if the number is found while (p!= null && p.num <= num) { if (p.num == num) { count++; p = p.next; return count; Software Development I // Exercises // 09 Strings & Lists // 32

31 Lists: Predefined List Classes (Java API) In Java there are already some classes in different libraries predefined to maintain objects in lists, e.g., Class String characters only Class Stack arbitrary objects (data types) LIFO (Last In First Out) Class Vector (better: ArrayList) arbitrary objects (data types) like arrays, but growable/shrinkable size random access to elements (via index) Class LinkedList arbitrary objects (data types) implementation of a doubly-linked list (implements List and Deque interfaces) Software Development I // Exercises // 09 Strings & Lists // 33

32 O SWE1 UE09 Lists.zip Example: ArrayList The implemented class should maintain an arbitrary number of lines, each of them containing character sequences (String). The end of the list is denoted by the predefined string quit. Input user enters lines with text stopper sign to end read-in Output (on the command line) total number of entered lines; print out the middle line all entered lines in reverse order Think of a solution with the ArrayList class, i.e. import java.util.arraylist; ArrayList lines = new ArrayList(); Software Development I // Exercises // 09 Strings & Lists // 34

33 I SWE1 UE09 Lists.zip Example: ArrayList Parametrization If not parametrized, 'lines' can contain arbitrary elements (of type Object) Warning message - ArrayList is a raw type. References to generic type ArrayList<E> should be parameterized - Type safety: The method add(object) belongs to the raw type ArrayList. References to generic type ArrayList<E> should be parameterized public class ArrayListExercise { public static void main(string[] args) { final String STOPPER = "quit"; java.util.arraylist lines = new java.util.arraylist(); java.util.arraylist<string> lines = new java.util.arraylist<string>(); System.out.println("Enter lines. Use " + STOPPER + " to end the program."); String line = Input.readString(); while (!line.equals(stopper)) { lines.add(line); line = Input.readString(); System.out.println("Number of lines: " + lines.size()); System.out.println("Middle line: " + (String) lines.get(lines.size()/2)); Software Development I // Exercises // 09 Strings & Lists // 35 // Print the lines in reverse order System.out.println("Lines in reverse order:"); for (int i = lines.size() - 1; i >= 0; i--) { System.out.println((String) lines.get(i));

34 I SWE1 UE09 Lists.zip Example: ArrayList Sample output Enter lines. Use quit to end the program: Hi, how are you today? quit Number of lines: 5 Middle line: are Lines in reverse order: today? you are how Hi, Software Development I // Exercises // 09 Strings & Lists // 36

35 I SWE1 UE09 Lists.zip Example: LinkedList<String> import java.util.*; public class LinkedListExercise { public static void main(string args[]) { // create a linked list (type of elements: String) LinkedList<String> list= new LinkedList<String>(); // add elements to the linked list list.add("roland"); list.add("thomas"); list.add("verena"); list.add("christian"); list.add("jürgen"); list.addlast("wolfgang"); list.addfirst("alexander"); list.add(1, "Andreas"); System.out.println("Contents of the list: " + list); // remove elements from the linked list list.remove("jürgen"); System.out.println("Contents of list after deletion: " + list); list.remove(2); // remove 3rd element (0...len-1) System.out.println("Contents of list after deletion: " + list); Software Development I // Exercises // 09 Strings & Lists // 37

36 I SWE1 UE09 Lists.zip Example: LinkedList<String> // continued java.lang.indexoutofboundsexception: Index: 20, Size: 6 at java.util.linkedlist.checkelementindex(linkedlist.java:553) at java.util.linkedlist.remove(linkedlist.java:523) at LinkedListExercise.main(LinkedListExercise.java:34) // what happens in case of removing inexistent object (out of bounds)? list.remove(20); // remove first and last elements list.removefirst(); list.removelast(); System.out.println("list after deleting first and last: " + list); list.remove(list.size()-1); // equivalent to list.removelast(); System.out.println("list after additional deleting of last: " + list); // get and set a value Object val = list.get(2); list.set(2, (String) val + " Changed"); System.out.println("list after change: " + list); Contents of the list: [Alexander, Andreas, Roland, Thomas, Verena, Christian, Jürgen, Wolfgang] Contents of list after deletion: [Alexander, Andreas, Roland, Thomas, Verena, Christian, Wolfgang] Contents of list after deletion: [Alexander, Andreas, Thomas, Verena, Christian, Wolfgang] list after deleting first and last: [Andreas, Thomas, Verena, Christian] list after additional deleting of last: [Andreas, Thomas, Verena] list after change: [Andreas, Thomas, Verena Changed] Software Development I // Exercises // 09 Strings & Lists // 38

37 SWE1 UE09 cinema example g1,2,4,5.zip SWE1 UE09 cinema example g3.zip Object-oriented Programming: Assignment 07 Live coding session

38 Exercises Software Development I 09 Objects III: String and StringBuffer, Singly (Doubly) Linked Lists December 10th, 2014 Software Development I Winter term 2014/2015 Priv.-Doz. Dipl.-Ing. Dr. Andreas Riener Institute for Pervasive Computing Johannes Kepler University Linz riener@pervasive.jku.at