JAVA COLLECTIONS LIBRARY School of Engineering and Computer Science, Victoria University of Wellington COMP T2, Lecture 2 Thomas Kuehne

TODAY 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  ecs100 library has been updated (  /lib/userlib/, in particular Mac users)

3 Programming with Libraries 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 (eg. UI)  There are LOTS of other libraries as well  Learning to use libraries is ESSENTIAL Java API

4 Libraries for COMP103  ecs100Special classes for text/graphical input and output, especially for GUI  java.utilCollection classes, and other utility classes  java.ioClasses for input and output  javax.swingLarge libraries of classes for GUI jawa.awt programs We will use libraries in almost every program

 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 5 Using Libraries Java API

6 “Standard” Collections  Common ways of organizing a collection of values:  Each of these is a different type of collection Collection BagGraphTreeStackQueueListMapSet

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).... 7

8 8 You have been using ArrayList  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

9 9 Using ArrayList: declaring 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 students = new ArrayList ();  Type of elements in the list is between “ ” after ArrayList  No initial size (!), No maximum

10 Using ArrayList: methods 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

11 Using ArrayList private List students = new ArrayList (); Student s = new Student(“Davy Jones”, ); 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); }....

12 Collection  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…)

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

14 Is ArrayList a Collection type then?  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  what it is  an implementation (= a class)  how it is implemented Interfaces are organised in a hierarchy; they “extend” one another

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 Java Collections Library 16 abstract concrete

Java Interfaces 17  At the simplest level:  An interface is a bunch of method signatures with a name (Method signatures only, no bodies)  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

18 Abstract Data Types (ADT)  an ADT is a data type, described at an abstract level: Specifies the operations that are supported by an object of this type Outlines how it will behave  A Java Interface corresponds to an Abstract Data Type Specifies what methods can be called on objects of this type (specifies name, parameters and types, and type of return value) Behaviour of methods is given in comments only (and hence cannot be enforced)

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

 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 Summary 20

 How do we define the type of element a collection contains ?  Constructing a new object of an appropriate collection class.  What can we do with collections ?  What methods can we call on them ?  How do we iterate through all the elements of a collection ?  How do we choose the right collection interface and class ?  What if there isn’t the right class for what we want ? What’s Next ? 21