CS100A Lecture 8

1
CS100A Lecture 8

• Previous Lecture
• Programming Concepts
• Classes, Objects, and Methods --- reiterated
• Accessors
• Encapsulation
• Java Constructs
• return statement
• Visibility modifiers
• public
• private
• This Lecture
• Programming concepts
• defining your own constructor
• chaining objects together
• searching
• Java Constructs
• Field and method modifier
• static

2
Problem Setting

• We wish to represent rooms
• Class Room is a collection of rooms
• Each room has a unique ID
• The ID should be automatically assigned, unique,
  and immutable, i.e., not changeable.
• Method findRoom(int targetId) returns the room
  with ID equal to targetId, or null if there is
  no such room.

3
Class Definition

• / Collection of rooms. /
• import java.io.
• public class Room
• // Each room r has a unique r.id gt 0.
• // nextId is the ID of the next room
• // to be created.
• private static int nextId 1
• private int id
• // Create a new room with a unique ID.
• public Room()
• id nextId
• nextId nextId 1

4
Constructors

• A constructor is used to create an objects.
• A class can define its own constructor.
• Class definition
• class class-name
• constructors
• declarations
• methods
• Constructor definition
• constructor-modifier
• class-name( parameter-list )
• statement-list

5
Constructor Invocation

• The value of expression
• new class-name()
• is a reference to a new object of the given
  class-name
• If there is a user-defined constructor with no
  parameters, then it is invoked on the new object
  created by new.

6
Field Declarations

• A field declarations can have field-modifiers
• Declarations
• field-modifiers type name
• Possible field-modifiers are
• public
• private
• static
• others later
• A private field is not visible from outside the
  class definition.
• A static field is also called a class variable.
• A class variable is not an instance variable of
  each object rather, there is precisely one
  instance of a class variable no matter how many
  objects of the class are eventually created

7
Object Instances

• An object is a collection of instance variables
  (known as fields) and methods
• There is one class variable per static field
  declaration

8
Problem Setting, revisited

• We wish to represent rooms
• Class Room is a collection of rooms
• Each room has a unique ID
• Method findRoom(int targetId) returns the room
  with ID equal to targetId, or null if there is
  no such room.

9
Method Definitions, revisited

• Method definitions
• method-modifier
• return-type method-name( parameter-list )
• statement-list
• Possible method-modifiers are
• public
• private
• static
• others later
• A private method is not visible from outside the
  class definition.
• A static method is also called a class method.
• A class method is invoked on the class, not on an
  object of the class, e.g.,

10
Method findRoom

• // Return the room with id targetId,
• // or null if there is no such room.
• public static Room findRoom(int targetId)
• . . .
• But how are we going to find the room?
• Idea keep all rooms chained together.
• Objects can have fields that refer to other
  objects
• The expression
• this
• evaluated in an instance method of object o
  is a reference to o.

11
Object Instances, revisited
12
Class Definition, revisited

• / Collection of rooms. /
• import java.io.
• public class Room
• // Each room r has a unique r.id gt 0.
• // nextId is the ID of the next room
• // to be created.
• private static int nextId 1
• private int id
• // For each room r, r.previous is the
• // room created immediately before
• // r was created, or null if r was the
• // first room created. The most recent
• // room to have been created is last.
• private static Room last
• private Room previous
• . . .

13
Constructor Room, revisited

• / Collection of rooms. /
• import java.io.
• public class Room
• . . .
• // For each room r, r.previous is the
• // room created immediately before
• // r was created, or null if r was the
• // first room created. The most recent
• // room to have been created is last.
• private static Room last
• private Room previous
• // Create a new room with a unique ID linked
• // to the previous room created.
• public Room()
• id nextId
• nextId nextId 1

14
Object Instances, revisited

15
Pattern for Searching

• // Search for something.
• // Start at the first place to look.
• r the first place look
• while ( r is not what we are looking for
• and
• there are still more places to look
• )
• r the next place to look
• // Now r is either what we were looking for,
• // or is an indication that there were no
• // more places to look.

16
Method findRoom, revisited

• // Return the room with id targetId,
• // or null if there is no such room.
• public static Room findRoom(int targetId)
• Room r last
• while (r ! null r.id ! targetId)
• r r.previous
• return r
• expression expression is called a conjunction.
  It is true only when both operands are true. If
  the left operand is false, the right operand is
  not evaluated.
• Note The order of the conjunction is important!
  In particular, it would be wrong to use the
  conjunction
• r.id ! targetId r ! null
• Why?