SETS AND MAPSSETS AND MAPSCollections of Data

Advanced Data StructuresAdvanced Data Structures

Often referred to as the Java Collections Framework….

• Set and map data types• Hash tables• Binary trees• Heap• Priority queue

We will only discuss Set and Map

SetsSets

• Unordered collection• Fundamental operations:

– Add an element– Remove an element– Containment test (is object in set?)– List elements (in arbitrary order)

• Reject duplicates

Set ImplementationSet Implementation

<<interface>>Set

HashSet TreeSet

add(E)addAll(Collection)clear()contains(Object)containsAll(Collection)equals(Object)hashCode()isEmpty()iterator()remove(Object)removeAll(Collection)retainAll(Collection)size()toArray()toArray(T[])

UML reminder: dotted line, open triangle pointing to interface

Simple SetDemoSimple SetDemo

import java.util.*;

public class SetDemo {

private Set<String> animals;

public SetDemo() {

animals = new HashSet<String>();

}

public void printAnimals(){

for (String animal : animals)

System.out.println(animal);

}

instantiate with specificimplementation.Advantage: could easilychange implementation,rest of program stays thesame.

Nodes are notnecessarily visitedin order inserted!

Add input and main methodsAdd input and main methodspublic void getAnimals() {

Scanner in = new Scanner(System.in);

String animal = "";

do {

System.out.print("Enter an animal or Q to quit: ");

animal = in.next();

if (!(animal.equalsIgnoreCase("Q")))

animals.add(animal);

} while (!(animal.equalsIgnoreCase("Q")));

}

public static void main(String[] args) {

SetDemo demo = new SetDemo();

demo.getAnimals();

demo.printAnimals();

}

}

can use methods from interface, such as add. Also have remove, contains, clear, size and more

TRY: giraffe, bear, coyote, lion

MapsMaps• A map data type keeps associations between

keys and values• Cannot contain duplicate keys• Operations include:

– put– get– containsKey/containsValue– keySet/values – return all keys/values– size

• Java has two implementations: HashMap and TreeMap.

Map DemoMap Demoimport java.util.*;

public class MapDemo {

Map<String, Integer> sightings;

public MapDemo() {

sightings = new HashMap<String, Integer>();

}

public void loadSightings() {

sightings.put("fox", 10);

sightings.put("bear", 2);

sightings.put("deer", 60);

sightings.put("elk", 30);

}

public void printSightings() {

Set<String> keySet = sightings.keySet();

for (String key : keySet)

System.out.println(key + "->" + sightings.get(key));

}

public static void main(String[] args) {

MapDemo demo = new MapDemo();

demo.loadSightings();

demo.printSightings();

}

}

TreeSet or HashSet?TreeSet or HashSet?• With a good hash function, hashing is usually faster• Balanced trees (remember those?) can guarantee

reasonable performance, HashSet depends entirely on performance of hash function

• To use TreeSet, objects being stored must implement Comparable interface

• For TreeMap, same requirement for keys• Can supply a Comparator object to TreeSet/TreeMap

constructor (takes two objects and returns comparison)• TreeSet is preferable if want to print list of items in order

TreeSet needs Comparable itemsTreeSet needs Comparable itemspublic class Student implements Comparable<Student>{

private String name;

public Student(String name) {

super();

this.name = name;

}

// Include setters and getters

public int compareTo(Student other) {

return name.compareTo(other.name);

}

}

TreeSet ExampleTreeSet Examplepublic class StudentSet {

private Set<Student> students;

public StudentSet() {

students = new

HashSet<Student>();

}

public void addStudent(Student s) {

students.add(s);

}

public void displayStudents() {

for (Student s : students) {

System.out.println(s.getName());

}

}

}

public static void main(String[] args) {

StudentSet set = new StudentSet();

set.addStudent(new Student("Cyndi"));

set.addStudent(new Student("Bill"));

set.addStudent(new Student("Jane"));

set.displayStudents();

}

}