Announcements

1
Announcements

• Newsgroup cornell.class.cs100
• Assignment W1 due tomorrow at the beginning of
  class
• Assignment P1 can be picked up at the end of
  class

2
Todays Topics

• Review
• Classes
• Instance of a Class (an object)
• Fields
• Declaration of class variables
• The new operator
• Class methods

3
Review

• Whats the difference between a variable and a
  constant?
• When might you want to use an if statement?
• What is the purpose of block statements?

4
Review Example 1

• final int THRESHOLD 65
• System.out.println(Enter temperature )
• int temperature Integer.parseInt(stdin.readLine(
  ))
• if (temperature lt THRESHOLD)
• System.out.println(Its cold.\n)

5
Review Example 2

• final boolean FOURLEGS true
• final boolean BARKS true
• final boolean MEOWS false
• if (FOURLEGS false)
• System.out.println(not four-legged)
• else if (BARKS)
• System.out.println(4 legs, probably a dog)
• else if (MEOWS)
• System.out.println(4 legs, probably a cat)
• else
• System.out.println(4 legs, no idea what.)

6
Rewrite of Previous example

• final boolean FOURLEGS true
• final boolean BARKS true
• final boolean MEOWS false
• if (FOURLEGS false)
• System.out.println(not four-legged)
• if ( FOURLEGS BARKS (MEOWS false))
• System.out.println(4 legs, probably a dog)
• if (FOURLEGS MEOWS (BARKS false))
• System.out.println(4 legs, probably a cat)
• if (FOURLEGS (MEOWS false) (BARKS
  false))
• System.out.println(4 legs, no idea what.)

7
Review Formatting Output

• What does the following produce?
• \tCourse\tStudents\n\tCS100\tMany
• Course Students
• CS100 Many

8
Why Classes?

• Recall a variable is a named box into which one
  can place values
• Suppose we want boxes (variables) that contain
  more than one value?
• For exampleCoordinate c

x
-532
x -532
y 25
9
More examples

• Coordinate c1
• Coordinate c3
• Coordinate is a class. Its like a type, except
  that you (the programmer) define it.
• c1 and c3 are instances of the class we call
  them objects
• c1.x, c1.y, c3.x, c3.y are fields of objects c1
  and c3 respectively.

x 3 y 4
10
Declaring a class in Java

• public class Coordinate
• public int x
• public int y
• keywords public and class
• name of the class -- naming convention
  capitalize all words in class names, e.g.
  StringBuffer
• body of the class (within and )
• field declarations (normal type and class
  declarations)
• methods and constructors (later in the course)

11
Type declarations -- Class declarations

• int x// x
• Coordinate w// w
• null is a Java constant -- it denotes the absence
  of an object
• Just as x hasnt been assigned (so defaults to
  0), w hasnt been assigned, it has only been
  declared

0
null
12
Take a breath

• So, objects (like c, as declared in Coordinate c)
  have a type -- the type is the objects class
  (here Coordinate)
• Classes have methods and fields
• fields are data variables associated with a class
  and its objects
• methods contain the executable code of a class

13
Creating a class instance

• To create a new instance of a class, use the
  operator new
• Coordinate w// w
• w new Coordinate()// w

null
x ? y ?
14
Referencing fields
x 3 y 5
// w
6
// x
x w.x
x 3 y 5
// w
3
// x
15
Assigning to fields
// w
w.x 7
// w
16
Instance1 Instance2 ??

• Assigning one instance to another creates aliases
  -- 2 names refer to the same object.

// w
// h
h w
// w
// h
17
What if you want to copy?
// w
// h

• h.x w.x
• h.y w.y

// w
x 3 y 5
// h
18
Methods of Classes

• Classes can have methods that operate on fields
  of the class

public class Coordinate public int x public
int y // Set field x to pp public void
setX(int p) x pp // Set field y to
q public void setY(int q) y q //
Return the sum of the squares of the
fields public int sumSquares() return xx
yy
19
Return types of methods

• // Return the sum of the squares of the fields
• public int sumSquares()
• return xx yy
• sumSquares should return an integer to wherever
  it is called from

20
Returns

• return ltexpressiongt
• terminates execution of the method in which it
  appears
• returns value of ltexpressiongt to the place of the
  call to the method
• NB ltexpressiongt must match the return type in
  the method header
• If the type is not void, Java insists that a
  return statement exist
• Can also have plain return with no expression
  if void

21
Method calls

• Suppose we want to call some of the methods of
  class Coordinate
• Use object name followed by a dot . followed by
  method name
• c.setX(3) // x field of c becomes 9
• a c.sumSquares() // a becomes . . . 81?

22
Example

• c.setX(3)
• c.setY(2)
• // Store 85 in ss c.sumSquares()

x 9 y 2
// c

23
Another example

• d new Coordinate()
• d.setX(c.sumSquares())
• d.setY(c.x)

x 7225 y 9
// d