Chapter 9 Moving Beyond Robots to Objects

9.1 Objects Objects are electronic things Objects can do stuff –can remember stuff –can talk to other objects Act more like people & cats than rocks & freshman

9.1 Objects Most objects are passive –only active when asked to do something –when asked to do something that isnt appropriate, it is an error

9.1 Objects Objects in the programming world –primarily robots –corners are almost like objects –we also used a random number generator object flipping a coin in RandomWalker

9.1 Objects Some objects are things –They have names –They can also have aliases –names are also called variables because the object they refer to can change –In general, a variable of an object type/class can refer to any object of its type or any subtype

9.1 Objects In Java –variable can refer to other things besides objects –some variables dont reference things, but hold things –e.g., int size = 30; –creates and assigns without using new –Variable that isnt a reference to an object –can only hold a value of its own precise type

9.1 Objects What does this do? int size = 30; int bigger = size + 30; System.out.println(bigger); How about this? int size; int bigger = size + 1; System.out.println(bigger);

9.2 Classes class is short for classificaton A class describes a set of objects of the same kind. A class describes what an object can do and/or remember A good way to think about a class is that it is a factory for creating objects of that type.

9.2 Classes How do you make a robot remember its name? class BrainyRobot extends Robot{ public BrainyRobot(String name, …) { super (…); myName = name; } public String name() { return myName; } private String myName; } –A String is a sequence of characters, usually in double Quotes( )

9.2 Classes String is a Java class myName is an instance variable –Usually, one initializes an instance variable in the constructor(s) –It is called an instance variable because each instance of the class that was instantiated has its own instance of the variable(memory) In Java we can print out a string System.out.println( someString ); Or, equivalently System.out.println( someString.toString() );

9.3 String A String is a sequence of (any) characters A String object performs services for us –e.g., remembers the sequence of characters –tell us its length: someString.length(); –return a new String: String more = someWord.concat(a word); Or, equivalently String more = someWord + a word;

9.3 String Examples: String word1 = one Fish; word1.length() returns 8 String word2 = Fox in Socks; word2.length() returns 12 String word3 = word1.concat( two Fish); word3.length() returns 17 because word3 contains the String one Fish two Fish word1.length() still returns 8 because word1 has not been altered

9.3 String String class has 30+ services but none change the characters – because there are no modifier methods Strings are immutable a string variable can refer to different Strings

9.3 String String word1 = The cat; String word2 = word1; word1 = word1 + in the Hat; word1.length() returns 18 because word1 contains the String The cat in the Hat However word2.length() returns 7 because word2 contains the String The cat

9.3 String We can look at the individual char(acters) –someString.charAt(someIntValue) Each char is stored in a numbered slot –start with zero aString.charAt(0) represents/returns the first char aString.charAt(4) represents/returns the 5th char aString.charAt(n) is an error if aString.length()<= n

9.3 String String word1 = The cat; word1.charAt(0) returns the char T word1.charAt(4) returns the char c word1.charAt(5) returns the char a word1.charAt(7) is an error because word1.length() <= 7

9.3 String We can also add (concatenate) Strings for example: String more = help me; String stillMore = more + Please; stillMore.length() returns ???? stillMore contains what sequence ????? more = someone + more; more.length() returns ???? more contains what sequence ?????

9.3 String In Java, Strings dont behave polymorphically –because the class is declared as final –prohibits you from building a subclass of String i.e., you can not extend String –methods can also be final can not be overridden –A final class means all its methods are final.

9.4 Parameters for Robots When we ask an object to perform a service, we need to send additional information. aString.charAt (whichCharacter); or

9.4 Parameters for Robots if we want karel to move several blocks. public void moveSeveralBlocks(int howManyBlocks){ for(int i = 0; i < howManyBlocks; i++) move(); } calls to moveSeveralBlocks karel.moveSeveraBlocks(4); // moves karel 4 corners karel.moveSeveraBlocks( someIntValue() );

9.4 Parameters for Robots It is possible to define several methods in the same class with the same name, provided: –they DIFFER in number and type –Compare these 4 examples – which could be in the same class??? public void move(int distance) {…} public void move(double distance) {…} public boolean move(int dist) {…} public void move(int distance, char ch) {…} Different return type only is NOT sufficient!

9.5 Other Classes Math is a special type of class Certain messages can be sent to classes/methods. –These are called static methods A static member of a class is shared by all objects in the class and, if public, shared everywhere.

9.5 Other Classes Math class has something like public class Math { public static int abs(int x) {…} } This allows calls like int absoluteValue = Math.abs(-12);

9.5 Other Classes Variables can also be static (or shared) –Static variables dont have separate copies in each object of the class –Instead, the one variable is shared by ALL objects of the class!

9.6 Still more Java System.out.println(The robot has + karel.numBeepers() + beepers); –notice the mixed use of the + operator with Strings and ints

9.8 Java Arrays Racer[] racers = new Racer[20]; –creates an array but doesnt create any Robots

9.8 Java Arrays for(int i = 0; i < racers.length; i++) racers[i] = new Racer(i, 1, East, 0); –creates the Robots and stores their reference in the racers array. –Slots are numbered from 0 just like Strings –length is used without the () –accessing an index out of range causes an ArrayIndexOutOfBoundsException run-time exception.

9.9 Important Ideas From This Chapter String array static method, static variable final class final method final