Collections and Maps

Transcription

1 Software and Programming I Collections and Maps Roman Kontchakov / Carsten Fuhs Birkbeck, University of London

2 Outline Array Lists Enhanced for Loop ArrayList and LinkedList Collection Interface Sets and Maps Section 6.8, Chapter 15 available online on companion web page slides are available at roman/sp1 SP

3 Arrays an array collects a sequence of values of the same type 1 // empty array of 5 Students (objects) 2 Student[] students = new Student[5]; 3 // list of initial values 4 String[] data = { "I", "V", "X", "L" }; students data String[ ] [0] = "I" [1] = "V" [2] = "X" [3] = "L" Student[ ] [0] = null [1] = null [2] = null [3] = null [4] = null SP

4 The Length of Arrays is Fixed come up with a guess on the maximum number of elements and keep a companion variable for the current size 1 final int LENGTH = 100; // max number of elements 2 double[] data = new double[length]; 3 int currentsize = 0; // companion variable 4 Scanner in = new Scanner(System.in); 5 while (in.hasnextdouble()) // read all doubles 6 if (currentsize < data.length) { 7 data[currentsize] = in.nextdouble(); 8 currentsize++; 9 } 10 for (int i = 0; i < currentsize; i++) // start at 0! 11 System.out.println(data[i]); // print them SP

5 Removing an Element removing the element at the position pos: data[0] data[0] data[pos-1] data[pos] data[pos+1]... data[pos-1] data[pos]... data[currentsize-2] data[currentsize-1] 1 for (int i = pos; i < currentsize - 1; i++) 2 data[i] = data[i+1]; // move elements up 3 currentsize--; // update currentsize SP

6 Inserting an Element inserting a newelement at the position pos: data[0] data[0] data[pos-1] data[pos-1] data[pos]... data[currentsize-1] newelement data[pos+1] 1 if (currentsize < data.length) { data[currentsize] 2 for (int i = currentsize; i > pos; i--) // reverse 3 data[i] = data[i-1]; // move elements down 4 data[pos] = newelement; // store the new element 5 currentsize++; // update currentsize 6 } SP

7 ArrayList (in java.util package) 1 // String is the type of elements 2 ArrayList<String> friends = new ArrayList<String>(); // append an element to the array, increasing its size 5 friends.add("allan"); 6 // accessing elements: the index i must be 7 // >= 0 and < friends.size() 8 int i = 0; // for example 9 String name = friends.get(i); 10 friends.set(i, "Harry"); 11 System.out.println(friends); // print out the contents NB: do not forget = new ArrayList<...>() (otherwise, it is null) NB: Python s lists essentially correspond to Java s ArrayLists SP

8 Enhanced For Loop with ArrayList 1 for (String name: friends) { 2 System.out.println(name); 3 } is equivalent to 1 for (int i = 0; i < friends.size(); i++) { 2 String name = friends.get(i); 3 System.out.println(name); 4 } NB: array: a.length String: a.length() array list: a.size() arrays are part of language strings and array lists are objects so, it is a method call SP

9 Inserting and Removing Elements 1 ArrayList<String> friends = new ArrayList<String>(); 2 friends.add("tom"); // adds an element to the end 3 friends.add("laura"); // now ["Tom", "Laura"] 4 // inserts an element at index 1 5 friends.add(1, "Giles"); 6 // now ["Tom", "Giles", "Laura"] 7 // removes the element at index 0 8 friends.remove(0); // now ["Giles", "Laura"] NB: inserting and removing elements is slow: on average, takes n/2 steps for an array with n elements SP

10 LinkedList: Inserting Elements LinkedList<String> head Node value = "Tom" prev = null next Node Node value = "Laura" prev next = null value = "Giles" prev SP next 9

11 LinkedList: Inserting Elements 1 public void insertafter(node current, String s) { 2 Node newnode = new Node(s); 3 newnode.next = current.next; 4 newnode.prev = current; 5 current.next.prev = newnode; 6 current.next = newnode; 7 } NB: effective insertion and deletions indexing, however, is slow: finding the nth element takes n steps SP

12 LinkedList Class 1 // a list of Strings 2 LinkedList<String> list = new LinkedList<String>(); 3 // adding elements 4 list.addlast("tom"); // to the end 5 list.addfirst("laura"); // to the beginning 6 list.addfirst("giles"); // to the beginning 7 // accessing elements 8 String fname = list.getfirst(); // the first 9 String lname = list.getlast(); // the last 10 // removing 11 String removed = list.removefirst(); // "Giles" SP

13 LinkedList: Iterators and For Loop an iterator is used to access elements inside a linked list 1 Iterator<String> iterator = list.iterator(); 2 while (iterator.hasnext()) { 3 String name = iterator.next(); 4 // do something with name 5 } is equivalent to 1 for (String name: list) { 2 // do something with name 3 } NB: This also goes on under the hood for the enhanced for-loop on ArrayList and other classes that implement the interface Iterable SP

14 LinkedList: Iterators and Removing 1 Iterator<String> iterator = list.iterator(); 2 while (iterator.hasnext()) { 3 String name = iterator.next(); 4 if (name.equals("tom")) { 5 // removes the last traversed element 6 iterator.remove(); 7 // can be called only once after next() 8 } 9 } SP

15 List Iterator a list iterator is obtained by using list.listiterator() 1 public interface ListIterator<E> 2 extends Iterator<E> { 3 boolean hasprevious(); 4 E previous(); 5 6 void add(e e); 7 void set(e e); 8 9 int nextindex(); 10 int previousindex(); 11 } SP

16 Interface Collection both ArrayList and LinkedList implement the Collection interface 1 public interface Collection<E> { 2 int size(); 3 boolean isempty(); 4 boolean contains(object element); 5 boolean add(e element); 6 boolean remove(object element); 7 Iterator<E> iterator(); } this is an example of a generic interface: E is any type name SP

17 More Collections: Sets sets cannot contain duplicate elements (lists can), three class implementations HashSet: stores the elements in a hash table best-performing implementation, but no order TreeSet: stores its elements in a tree and orders its elements based on their values uses a Comparator or natural order: the elements implement Comparable LinkedHashSet: a hash table with a linked list running through it, orders its elements based on the order in which they were inserted into the set more: docs.oracle.com/javase/tutorial/collections/toc.html SP

18 Set: An Example 1 public static Set<String> ex(set<string> set) { 2 set.add("charles"); 3 set.add("dora"); 4 set.add("alice"); 5 set.add("bob"); 6 return set; 7 } 1 // LinkedHashSet preserves insertion order 2 System.out.println(ex(new LinkedHashSet<String>())); 3 // TreeSet sorts in the natural order: see compareto() 4 System.out.println(ex(new TreeSet<String>())); 5 // HashSet has no particular order: see hashcode() 6 System.out.println(ex(new HashSet<String>())); SP

19 More Collections: Maps a map is an object that maps keys to values it models the mathematical function abstraction Q: is array a map? a map cannot contain duplicate keys: each key can map to at most one value 1 public interface Map<K,V> { 2 V put(k key, V value); 3 V get(object key); 4 V remove(object key); 5 boolean containskey(object key); 6 boolean containsvalue(object value); } SP

20 Map: An Example 1 import java.util.*; 2 public class Freq { 3 public static void main(string[] args) { 4 Map<String, Integer> m = 5 new HashMap<String, Integer>(); 6 for (String a : args) { 7 Integer freq = m.get(a); 8 m.put(a, (freq == null)? 1 : freq + 1); 9 } 10 System.out.println( 11 m.size() + " distinct words:\n" + m); 12 } 13 } SP

21 Take Home Messages a collection groups together elements and allows them to be retrieved later a list is a collection that remembers the order of its elements a set is an unordered collection of unique elements a map keeps associations between key and value objects SP