Practical Session 3 Java Collections

Outline Working with a Collection The Collection interface The Collection hierarchy Case Study: Undoable StackMaps Maps The Collections class Wrapper classes

Collection A group of elements. The group size can be changed during run-time. A collection object has many useful methods for manipulating the collection: Inserting elements Deleting elements Copying the collection Iterating over the elements Computing subsets of elements … Various types of collections are defined in the standard library: Vector, ArrayList, Stack, ArrayDequeue, PriorityQueue, TreeSet, HashMap…

Collection Why shouldn’t we just use arrays? Arrays are not objects! You cannot invoke methods on arrays. Arrays have fixed size. Arrays are less convenient for representing certain types of collections: Sets Double-ended queues Dictionaries …

Working with a collection Definition syntax: Collection <type> colName = new Collection <type>() Example: import java.util.Vector; … Vector <Car> carsVec = new Vector <Car>(); Car Volvo = new Car(2.0); Car BMW = new Car(2.2); carsVec.add(Volvo); carsVec.add(BMW); for (Car c: carsVec) System.out.println(c);

Working with a collection Example - continued: Car temp = carsVec.elementAt(0); carsVec.set(0,carsVec.elementAt(1)); carsVec.set(1, temp); for (int i=0; i < carsVec.size(); i++) System.out.println(carsVec.elementAt(i));

The Collection Interface Collection is an interface in the standard java library. The Collection interface is generic. It accepts a type as a parameter. public interface Collection<E> extends Iterable<E> { int size(); boolean isEmpty(); boolean contains(Object element); boolean add(E element); //optional boolean remove(Object element); //optional Iterator<E> iterator(); Object[] toArray(); <T> T[] toArray(T[] a); …

The Collection Hierarchy (Partial illustration) Collection List Set Queue

The Collection Hierarchy (Partial illustration) Collection List Set Queue Vector

The Collection Hierarchy (Partial illustration) Collection List Set Queue Vector ArrayList PriorityQueue Stack

Case Study: Undoable Stack Various programs allow the user to undo his operations. The undo operations are performed in reverse order. In such a program, we need to add each operation with its parameters onto the stack.

Program Stack

Program Stack Resize 36

Program Stack Resize 36 Recolor 4 Resize 36

Program Stack Resize 24 Recolor 4 Resize 36 Undo: Resize(36)

Program Stack Undo: Resize(36) Resize 24 Recolor 4 Resize 36 Recolor 4

Undoable Stack Hierarchy

Undoable Stack Hierarchy TextArea -text: String -size: int -color: int + getters… + setters…

Undoable Stack Hierarchy TextArea -text: String -size: int -color: int + getters… + setters… Operation + perform(arg: int) + getArg() : int

Undoable Stack Hierarchy TextArea -text: String -size: int -color: int + getters… + setters… Operation + perform(arg: int) + getArg() : int Recolor - color: int + perform(arg: int) + getArg() : int Resize - size: int + perform(arg: int) + getArg() : int

Undoable Stack Hierarchy TextArea -text: String -size: int -color: int + getters… + setters… Operation + perform(arg: int) + getArg() : int 0..n Recolor - color: int + perform(arg: int) + getArg() : int Resize - size: int + perform(arg: int) + getArg() : int UndoStack + add (op :Operation) + undo()

Maps An object that maps keys to values. A map cannot contain duplicate keys. each key can map to at most one value.

Maps 1 2 3 4 5 6 9 מערכים: 1 7 10 13 15 30 55 6 8 4 46 11 מפות:

Words count Generates a frequency table of the words found in a sentence. Example: how much wood could a woodchuck chuck if a woodchuck could chuck wood. wood=2, could=2, how=1, if=1, chuck=2, a=2, woodchuck=2, much=1

Words count String sentence = "how much wood could a woodchuck chuck if a woodchuck could chuck wood"; Map<String, Integer> map = new HashMap<String, Integer>(); for (String word : sentence.split(" ")) { Integer freq = map.get(word); map.put(word, (freq == null) ? 1 : freq + 1); }

HashMap vs. TreeMap vs. LinkedHashMap Iteration order String sentence = "how much wood could a woodchuck chuck if a woodchuck could chuck wood"; HashMap makes absolutely no guarantees about the iteration order. It can (and will) even change completely when new elements are added. TreeMap will iterate according to the "natural ordering" of the keys according to their compareTo() method (or an externally supplied Comparator). {a=2, chuck=2, could=2, how=1, if=1, much=1, wood=2, woodchuck=2} LinkedHashMap will iterate in the order in which the entries were put into the map. {how=1, much=1, wood=2, could=2, a=2, woodchuck=2, chuck=2, if=1}

HashMap vs. TreeMap vs. LinkedHashMap  Implement of the Map interface HashMap is a map based on hashing of the keys. Keys must have consistent implementations of hashCode() and equals() for this to work. TreeMap is a tree based mapping. Its put/get operations take O(log n) time. It requires items to have some comparison mechanism, either with Comparable or Comparator. The iteration order is determined by this mechanism. LinkedHashMap is very similar to HashMap, but it adds awareness to the order at which items are added (insertion order). It’s like a hashMap that maintains a doubly-linked list.

HashMap example (1) Output: white dog - 5 black dog - 15 red dog - 10

HashMap example (2) Output: 3 red dog - 10 white dog - 20 black dog - 15

TreeMap example (1) java_string_compareto (“Read & Focus on the return value”)  Iterate elements in increasing order (smallest to largest ) Output: white dog - 20 black dog - 15 red dog - 10 If "Dog d4 = new Dog("white", 10);" is replaced with "Dog d4 = new Dog("white", 40);“ the output would be: white dog - 5 black dog - 15 red dog - 10 white dog - 20

The Collections class The Collections class contains static methods that operate on collections: min max reverse sort … Example: import java.util.Collections; import java.util.Vector; … Vector <Car> carsVec = new Vector <Car>(); Collections.reverse(carsVec);

A wrapper class is an object representing a primitive type. Wrapper Classes What happens if we want to create a collection of a primitive type? Collections can be made only of objects. Primitive types are not objects. We use wrapper classes instead. A wrapper class is an object representing a primitive type.

Wrapper Classes Integer Float Double Character Boolean int float Example: Integer intObj = new Integer(10); int number = intObj.intValue();

A collection of wrapper objects Import java.util.Vector; … Vector <Integer> numVec = new Vector <Integer>(); int size = 10; for (int i = size; i >0; i--) numVec.add(new Integer( i )); Collections.sort(numVec); for (Integer i: numVec) System.out.println( i);