Moving Beyond Robots to Objects

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Chapter 9

• Moving Beyond Robots to Objects

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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

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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?

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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

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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

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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

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9.1 Objects

• What does this do?
• int size 30
• int bigger size 30System.out.println(bigger)
  
• How about this?
• int size
• int bigger size 1
• System.out.println(bigger)

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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.

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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( )

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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() )

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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

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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

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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

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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

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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()lt n

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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() lt 7

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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 ?????

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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.

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9.4 Parameters for Robots

• When we ask an object to perform a service, we
  need to send additional information.
• aString.charAt(whichCharacter)
• or

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9.4 Parameters for Robots

• if we want karel to move several blocks.
• public void moveSeveralBlocks(int howManyBlocks)
• for(int i 0 i lt howManyBlocks i)
  move()
• calls to moveSeveralBlocks
• karel.moveSeveraBlocks(4) // moves karel 4
  corners
• karel.moveSeveraBlocks( someIntValue() )

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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!

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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.

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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)

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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!

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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

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9.8 Java Arrays

• Racer racers new Racer20
• creates an array but doesnt create any Robots

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9.8 Java Arrays

• for(int i 0 i lt racers.length i)
• racersi 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.

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9.9 Important Ideas From This Chapter

• String
• array
• static method, static variable
• final class
• final method
• final