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 isn’t

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 don’t reference things, but hold things

– e.g., int size = 30;– creates and assigns without using new

– Variable that isn’t 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 stringSystem.out.println( someString );Or, equivalentlySystem.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 zeroaString.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) Stringsfor 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 don’t 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 moveSeveralBlockskarel.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 likepublic class Math

{public static int abs(int x) {…}

}

• This allows calls likeint absoluteValue = Math.abs(-12);

9.5 Other Classes

• Variables can also be static (or shared)– Static variables don’t 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 doesn’t 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