JAVA COLLECTIONS LIBRARY COMP 103 JAVA COLLECTIONS LIBRARY Thomas Kuehne, Marcus Frean, Thomas Kuehne School of Engineering and Computer Science, Victoria University of Wellington 2016-T2, Lecture 2

TODAY Administrivia: Libraries and code reuse 2 Libraries and code reuse Java’s Collections Library ArrayList (our first example) Interfaces and Classes Administrivia: No cap on foreign material in submitted assignments but only original parts will be marked Register for tutorials with myAllocator by 3.00pm Friday! (email Alex Potanin in case you have problems)

Programming with Libraries 3 Modern programs (with GUIs, databases, network access, etc.) are too big to build from scratch, so we have to reuse code written by other people.... Libraries contain code designed for reuse Java has a huge number of standard libraries... organised in packages, which are groups of Classes The ecs100 library has some very useful classes (e.g. UI) There are LOTS of other libraries as well Learning to use libraries is ESSENTIAL Java API

Libraries for COMP103 4 ecs100 Special classes for text/graphical input and output, especially for GUI java.util Collection classes, and other utility classes java.io Classes for input and output javax.swing Large libraries of classes for GUI jawa.awt programs We will use libraries in almost every program

Using Libraries Read the documentation to pick useful library 5 Read the documentation to pick useful library Import the package or class into your program import java.util.*; import ecs100.*; Read the documentation to identify how to use constructors for making instances methods that you can call interfaces that you can / need to implement Use the classes as if they were part of your program Java API

“Standard” Collections 6 Common ways of organizing a collection of values: Each of these constitutes a different type of collection Collection Bag Graph Tree Stack Queue List Map Set

Java Collections Library Standard collections (eg. Set, List, Map, etc.) are available to use Collections specified as interfaces Collection, Set, List, Queue – are all interfaces Using collections implies using one of the many concrete classes that implement these interfaces. E.g. You can use: ArrayList (a class) which implements List (an interface)

You have been using ArrayList 8 Part of the Java Collections framework Stores a LIST of items a collection of items kept in a particular order Part of the java.util package ⇒ need import java.util.*; at the head of your file You can make a new ArrayList object, and put items in it Don’t have to specify its size: Like an infinitely stretchable array Need to specify the type of items Cannot use the [...] notation You have to call methods to access and assign

Using ArrayList: declaring 9 List of students (assume we have a class called Student) Array private static final int maxStudents = 1000; private Student[ ] students = new Student[maxStudents]; private int count = 0; ArrayList private ArrayList <Student> students = new ArrayList <Student>(); Type of elements in the list is between “<” and “>” after ArrayList No initial size(!), No maximum size

TIP: Read Documentation from sidebar of course homepage Using ArrayList: methods 10 ArrayList has many methods size(): returns the number of items in the list add(item): adds an item to the end of the list add(index, item): inserts an item at index (moves later items up) set(index, item): replaces the item at index with item contains(item): true if the list contains an item that equals item get(index): returns the item at position index remove(item): removes an occurrence of item remove(index): removes the item at position index (both “removes” move the later items down) You can use the “for each” loop on an ArrayList, as well as a regular for loop TIP: Read Documentation from sidebar of course homepage

Using ArrayList 11 private List <Student> students = new ArrayList <Student> (); Student s = new Student(“Davy Jones”, 300012345); students.add(s); for (Student st: students) UI.println(st.toString()); for (int i = students.size()-1; i>=0; i--) UI.println(students.get(i).toString()); if (students.contains(currentStudent)) { UI.println(currentStudent); students.remove(currentStudent); } .... ....

Collection An object that serves as a container for other objects 12 An object that serves as a container for other objects Type of Elements (a collection of ….) Structure (none/linear/hierarchical) Constraints (duplicates allowed? Access: add anywhere, remove from one end only, get from top, etc…)

Example Bag : an example of a collection 13 Bag : an example of a collection Collection type Element type Type of Elements Bag of Students Structure No structure/order Constraints Duplicates allowed, add/remove anywhere (no ordering)

Is ArrayList a Collection type then? 14 Type of Elements ArrayList of Students Structure linear order Constraints Duplicates allowed, add/remove anywhere actually no! ArrayList is not a Collection type. List is! ArrayList is just ONE WAY to implement a List

extends implements interface (a specification / promise)  what it is class (an implementation)  how it is implemented Interfaces are organised in a hierarchy; they “extend” one another

Java Collections Library 16 Interfaces… Collection most general: “a container” List ordered collection Set unordered, no duplicates Queue ordered collection with limited access (add at one end, remove from other) Map key-value pairs …specify the types Classes… List classes: ArrayList, LinkedList, Vector Set classes: HashSet, TreeSet, EnumSet, LinkedHashSet,… Map classes: EnumMap, HashMap, TreeMap, LinkedHashMap, WeakHashMap, … … …implement the interfaces abstract concrete

Java 8 adds “default” bodies, though Java Interfaces 17 At the simplest level: An interface is a bunch of method signatures with a name At a higher level: An interface is an abstract concept that defines: the operations that are supported by an object of this type (how it will behave) No concrete details! No constructors - can’t make an instance No fields - doesn’t say how to store the data No method bodies - doesn’t say how to perform the operations Details provided by Classes that implement the Interface Assertions & JML Java 8 adds “default” bodies, though

Declaring a List Earlier, we declared a list of students like this: 19 Earlier, we declared a list of students like this: private ArrayList <Student> students = new ArrayList <Student>(); BETTER / NOW: declare as an instance of the interface List: private List <Student> students = new ArrayList <Student>();

Summary Libraries Collections 20 Libraries Collections Interfaces – correspond to ADTs. No concrete details e.g., Collection, List, Set, … Classes – provide implementations for interfaces e.g., ArrayList  implements an interface (List) from the Java Collections Library