Inf1-OOP. Encapsulation. Object Oriented Programming. Encapsulation Accessing Data Immutability. Encapsulation and Collections.

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1 Inf1-OOP Encapsulation and Collections Ewan Klein, Perdita Stevens School of Informatics January 12, 2013 Encapsulation Accessing Data Immutability Enhanced for loop Collections ArrayList Maps Summary/Admin Object Oriented Programming Encapsulation Procedural programming [verb oriented] tell the computer to do this, then tell the computer to do that. Alan Kay s philosophy: software is a simulation of the real world. We know (approximately) how the real world works; design software to model the real world. Object Oriented programming (OOP) [noun oriented] Programming paradigm based on data types. Identify things that are part of the problem domain or solution. Things in the world know things: instance variables. Things in the world do things: methods. Data type: set of values and operations on those values. E.g., int, String, PrinterQueue, HotelRoom,... Encapsulated data type: hide internal representation of the data type. Separate implementation from design specification: Class provides data representation and code for operations. Client uses data type as black box. API specifies contract between client and class. Bottom line: You don t need to know how a data type is implemented in order to use it.

2 Intuition Intuition Client API adjust volume switch channel switch to standby Implementation cathode ray tube 20 screen, 22 kg Sony Trinitron KV20M10 Client API adjust volume switch channel switch to standby Implementation HD LED display 37 screen, 10 kg Samsung UE37C5800 client needs to know how to use API implementation needs to know what API to implement client needs to know how to use API implementation needs to know what API to implement Implementation and client need to agree on API ahead of time. TV Example Can substitute better implementation without changing the client. Dalek Encapsulation Unprotected Dalek Using the TV API TV mytv = new TV(); mytv.setvolume(up); mytv.setchannel(film4); mytv.standby(); public class Dalek { public double batterycharge = 5; public void batteryrecharge(double c) {... public void move(int distance) {... Disabling the Dalek Dalek d = new Dalek(); // start off with a // well-charged battery d.batterycharge = Double.NEGATIVE_INFINITY; d.batteryrecharge(1000); // battery charge still -Infinity!

3 Dalek Encapsulation Changing Internal Representation Protected Dalek public class Dalek { private double batterycharge = 5; public void batteryrecharge(double c) {... public void move(int distance) {... Disabling the Dalek Dalek d = new Dalek(); // start off with a // well-charged battery d.batterycharge = Double.NEGATIVE_INFINITY; //Triggers compile-time Error Encapsulation: Keep data representation hidden with private access modifier. Expose API to clients using public access modifier. Advantage: can switch internal representations without changing client. Encapsulated data types: Don t touch data to do whatever you want. Instead, ask object to manipulate its data. Exception...: Unresolved compilation problem: The field Dalek.batteryCharge is not visible Immutability Immutability: Advantages and Disadvantages Immutable data type: object s value cannot change once constructed. mutable Picture Dalek Java arrays immutable String primitive types Immutable data type: object s value cannot change once constructed. Advantages: Makes programs easier to debug Limits scope of code that can change values Pass objects around without worrying about modification. Disadvantages: New object must be created for every value.

4 The final Modifier Final: declaring a variable to by final means that you can assign it a value only once, in initializer or constructor. E.g., Daleks come in three versions, Mark I, Mark II and Mark III. public class Dalek { private final int mark; private double batterycharge;... Advantages: this value can be change by invoking the instance method batteryrecharge() Helps enforce immutability. Prevents accidental changes. Makes program easier to debug. this value doesn't change once the object is constructed Documents the fact that value cannot change. Enhanced for loop Enhanced for loop Enhanced for loop: more concise syntax for looping. int[] numbers = {2, 5, 6, 1, 0, 5; Ordinary for loop for (int i = 0; i < numbers.length; i++) { System.out.println(numbers[i]); Enhanced for loop for (int num : numbers) { // num is the loop variable System.out.println(num); Enhanced for loop Enhanced for loop: more concise syntax for looping. Also called for-each loop Easier to use with arrays and other iterables. General form: for ( variable declaration : iterable ){... NB the variable must have same type as elements in iterable. for (int num : numbers) { System.out.println(num); The colon (:) is often pronounced in. On each iteration, an element of the iterable gets assigned to the loop variable. Loop gets executed once for each element in the iterable. Don t need to initialize loop variable or set termination condition. But you can t keep track (via the loop counter) of where you are in the iteration (e.g., first, last, etc)

5 Enhanced for loop, 3 Squeezing into an array Another Example: Right String[] words = {"hello", "world", "we", "can", "do", "it"; for (String w : words) { System.out.println(w); Another Example: Wrong String[] words = {"hello", "world", "we", "can", "do", "it" ; for ( int w : words) { System.out.println(w); Squeezing into an array ArrayList Length of array is fixed at creation time. Can t be expanded. Can t be shrunk. Arrays are part of Java language uses special syntax. E.g., myarray[i] for accessing the ith element. ArrayList: Can grow and shrink as needed; provides methods for inserting and removing elements. Declaration ArrayList<String> cheers = new ArrayList<String>(); This is an array list of strings; counterpart to String[]. Angle brackets indicate that String is a type parameter. Can replace String with e.g. HotelRoom to get different array list type. In general: use ArrayList<E> to collect objects of type E; but E cannot be a primitive type.

6 ArrayList: Methods ArrayList: Adding A newly constructed ArrayList has size 0. ArrayList has various methods, which allow us to: keep on adding new elements; remove elements. The size changes after each addition / removal. Adding Elements ArrayList<String> cheers = new ArrayList<String>(); cheers.add("hip"); cheers.add("hip"); cheers.add("hooray"); int n = cheers.size(); // n gets value 3 add() appends each element to the end of the list. Printing an ArrayList System.out.println(cheers); Output ArrayList: More methods [hip, hip, hooray] ArrayList: More methods contains() Index of first occurrence int ind = cheers.indexof("hip"); // ind gets value 0 boolean iship = cheers.contains("hip"); remove() // iship is true Adding element at an index cheers.remove("hip"); // removes first occurrence of "hip" cheers.add(1, "hop"); // 2nd "hip" gets shunted along Elements of cheers: ["hip", "hop", "hip", "hooray"] Elements of cheers: "hop", "hip", "hooray" get(int index) cheers.get(0); // get the first element // returns "hop"

7 ArrayList and Loops ArrayList and Loops Looping over ArrayList: Standard for loop for (int i = 0; i < cheers.size(); i++) { System.out.println(cheers.get(i)); Enhanced for again for (String s : cheers) { System.out.print(s); Enhanced for again for (String s : cheers) { System.out.print(s + "\thas index: "); System.out.println(cheers.indexOf(s)); Output hop has index: 0 hip has index: 1 hooray has index: 2 Wrapper Classes Wrapper Classes: The type variable E in a generic type like ArrayList<E> must resolve to a reference type. So ArrayList<int> will not compile. All the primitive types can be turned into objects by using wrapper classes: Primitive Type boolean char double int long Wrapper Class Boolean Character Double Integer Long NB Wrapper class names are always capitalized, always complete words. Auto-boxing Conversion between primitive types and corresponding wrapper classes is automatic. Process of conversion is called auto-boxing Auto-box example Double batterycharge = 2.75; double x = batterycharge; Auto-box example ArrayList<Double> data = new ArrayList<Double>(); data.add(29.95); double x = data.get(0);

8 Import Java API Importing: To get full access to Java API, we need to import classes (strictly speaking, packages). Not necessary if class is in same folder, or part of java.lang (e.g., Math library). To use ArrayList, add the appropriate import statement at top of your file: Import example Look at sample Javadoc web page. import java.util.arraylist; Import example Wrong! import java.util.arraylist<string>; // Don t use parameter Java API: Top Node Java API: Entry for ArrayList

9 Maps Associative Arrays Associative array: I Associates a collection of unique keys with values. I Ordinary arrays: keys can only be integers. I Associative arrays allow keys of many types, most notably strings. Examples: I I Given Given Given Given a person s name, look up a telephone number. an internet domain, look up its IP address. a geo-location, look up its GPS coordinates. a word, look up its frequency in a text. Relationship between key and value: mapping. Java: associative arrays are implemented by type HashMap. Map People to their Matric Nos. Map Words to Length Keys Values Peter s "this" 4 Michael s "is" 2 Helen s "the" 3 Mary s "time" John s "and" Keys Values

10 Map People to their Matric Nos: Wrong! Map People to their Telephone Nos: Wrong! Keys Values Keys Values Peter Michael Peter Mary John s s s s s Peter Michael Helen Mary John NB Keys must be unique. A given key can only be mapped to one value. However, type of value can be array, or some other object. HashMap Mapping Words to their Lengths Import HashMap import java.util.hashmap; Declare HashMap HashMap<String, Integer> map = new HashMap<String, Integer>(); Goal: Given a string of words, derive an associative array that maps each word to its length. 1. Split the string on whitespace, to yield words. 2. For each word w, add it as a key, and associate it with value w.length(). 3. When we add the same key again, we overwrite the previous association wasteful but harmless in this case. split() method of String HashMap takes two type parameters. Here, String is type of key, Integer is type of value. String sent = "this is the time and this is the record of the time"; String[] words = sent.split(" "); // split on whitespace

11 HashMap: Add and retrieve mappings put(key, Value): put Value as the value of Key in wordlengths. HashMap<String, Integer> wordlengths = new HashMap<String, Integer>(); for (String word : words) { wordlengths.put(word, word.length()); add a key-value pair to the mapping get(key): get the value of Key in wordlengths. HashMap: Add and retrieve mappings wordlengths.keyset(): the set of keys in wordlengths. [of, record, time, is, the, this, and] Q How do we list all key-value pairs in a map? A Loop over the set of keys. for (String key : wordlengths.keyset()) { System.out.printf("%s => %s\n", key, wordlengths.get(key)); Output int wl = wordlengths.get("record"); // value is 6 HashMap: Printing Output System.out.println(wordLengths); Output {of=2, record=6, time=4, is=2, the=3, this=4, and=3 Format is {Key1 =Value1, Key2 =Value2,... of => 2 record => 6 time => 4 is => 2 the => 3 this => 4 and => 3 Access Control Summary Encapsulation and visibility: All the instance variables and methods (i.e., members) of a class are visible within the body of the class. Access modifiers: control the visibility of your code to other programs. public: member is accessible whenever the class is accessible. private: member is only accessible within the class. default: amounts to public for current purposes. Benefits of encapsulation: Loose coupling Protected variation Exporting an API: the classes, members etc, by which some program is accessed any client program can use the API the author is committed to supporting the API

12 For-each, ArrayList & HashMap Reading Use enhanced for loops whenever you don t need access to the loop variable; fewer opportunities for typos. Use ArrayList when you want your arrays to be able to grow, or you want to easily insert and remove items in the middle of an array. Use HashMap when you want to use keys other than a predetermined list of integers. For more on ArrayList and HashMap, look at the Java API: DICE: file:///usr/share/doc/java sun-manual-1.6.0/api/ index.html Oracle s website: Java collections not discussed in Sedgewick & Wayne text. Instead, see Eck or Cay Horstmann (2008) Java Concepts, Sections 6.6, 10.5