Classes in Java

1
Classes in Java

• class Employee
• public Employee(String n, double s, Day d)
• name n
• salary s
• hireDay d
• public void print()
• System.out.println(name " " salary "
  "
• hireYear())
• public void raiseSalary(double byPercent)
• salary 1 byPercent / 100
• public int hireYear()
• return hireDay.getYear()

2
Methods in the Employee Class

• Constructors
• public Employee(String n, double s, Day d)
• If no constructor is defined, the system default
  constructor will be called automatically.
• Primitive instance variables and object instance
  variables will be initialized numeric type to
  zeros, boolean to false, and object variables to
  null.

3
Methods in Employee Class
Accessors/Interrogators/Queries public void
print() public int hireYear() Access but no
change to the state of an object. Manipulators/Mut
ators/Actions public void raiseSalary(double
byPercent) Change the state of an object. Private
methods can also be defined for implementing
internal methods.
4
Instance Variables

• Instance variables are not initialized until a
  class is instantiated to an object.
• Private instance variables are visible with the
  class in which they are declared vs. public
  instance variable are visible to the outside
  world.
• Default constructor initializes instance
  variables to their default values.
• We probably want to keep as many instance
  variables private as possible. However, this
  implies a public accessor and a public mutator
  are needed.

5
Driver

• public class EmployeeTest
• public static void main(String args)
• Employee staff new Employee3
• staff0 new Employee("Harry Hacker",
  35000,
• new Day(1989,10,1))
• staff1 new Employee("Carl Cracker",
  75000,
• new Day(1987,12,15))
• staff2 new Employee("Tony Tester",
  38000,
• new Day(1990,3,15))
• int i
• for (i 0 i lt staff.length i)
  staffi.raiseSalary(5)
• for (i 0 i lt staff.length i)
  staffi.print()

6
User Defined Default Constructor

• Once a user defined constructor is declared. The
  systems default constructor ceased to exist.
• How about adding the following default
  constructor to Employee?
• public Employee()
• What values will be stored in the instance
  variables during instantiation?
• So, whats the problem?

7

• public class EmployeeTest
• public static void main(String args)
• //...
• Employee e new Employee()
• e.print()
• class Employee
• public Employee()
• public void print()
• System.out.println(name " " salary "
  "
• hireYear())
• public int hireYear()
• return hireDay.getYear()

8
UML Class Diagrams(for Static Modeling)
9
UML Object Diagram(for Static Modeling)
10
Mutable Objects Accessors

• Consider the getHireDay() methods that returns a
  Day object to the caller.
• public Day getHireDay()return hireDay
• Vs. another accessor method that returns a
  String, which is immutable.
• pubic String getName()return name
• Similar to parameter passing and assignment,
  primitive types are returned by value and objects
  are returned by "reference".

11
Aliasing

• The following piece of code generates aliases for
  the same object
• Employee harry new Employee()
• Day d harry.getHireDay()
• d.advance(-3650) //go back 10 years

12
Cloning

• How about making hireDay final?
• A solution to this is cloning.
• public Day getHireDay()
• return (Day) hireDay.clone()
• clone() belongs to java.lang and it returns
  Object.

13
Day's cloning method

• class Day implements Cloneable
• //...
• public Object clone()
• try
• //a lazy way to do shallow copy on
  variables
• return super.clone()
• catch (CloneNotSupportedException e)
• // this shouldn't happen, since we are
  Cloneable
• System.out.println("Bombed")
• return null
• private int day
• private int month
• private int year

14
More on cloning

• .clone() is defined for the Object class.
• To make a class cloneable, that class has to
  implement the cloneable interface and redefine
  its only .clone() method.
• class Employee implements Cloneable
• //...
• public Object clone()
• try
• return super.clone()
• catch (CloneNotSupportedException e)
• // this shouldn't happen, since we are
  Cloneable
• System.out.println("Bombed")
• return null
• //...

15
Shallow Copy

• public class EmployeeTest2
• public static void main(String args)
• Employee a new Employee("Harry Hacker",
  35000,
• new Day(1989,10,1))
• Employee b (Employee) a.clone()
• Day d1 a.getHireDay()
• Day d2 b.getHireDay()
• if (d1d2)
• System.out.println("Oops...messed up again!" )

16
Deep Copy with Copy Constructor?

• public Employee(Employee rhs)
• name new String(rhs.name) //String not
  cloneable
• salary rhs.salary
• hireDay (Day) rhs.hireDay.clone()
• //Day has no constructor
• If an instance variable is an object, its copy
  constructor should also perform deep copy.

17
Driver

• public class EmployeeTest3
• public static void main(String args)
• Employee a new Employee("Harry Hacker",
  35000,
• new
  Day(1989,10,1))
• Employee b new Employee(a)
• Day d1 a.getHireDay()
• Day d2 b.getHireDay()
• if (d1d2) System.out.println("Oops...messed
  up again!")
• When you need to return an object by-value with
  deep copy in a method, say Employee, try
• return new Employee(this) //this or some
  Employee variable.

18
Do we need to perform deep copy on Strings?
public Employee(Employee rhs) //name new
String(rhs.name) name rhs.name salary
rhs.salary hireDay (Day) rhs.hireDay.clone()

19
A better way Cloneable Employee
class Employee implements Cloneable //. . .
public Object clone() try //using
super.clone() to do shallow copy // a lazy way
to do it Employee tmp (Employee)
super.clone() tmp.hireDay (Day)
hireDay.clone() return tmp catch
(CloneNotSupportedException e) // this
shouldn't happen, since we are Cloneable
System.out.println("Bombed")
return null
20
Class Variables

• Variables declared with static.
• Similar to class methods (static methods), such
  variables belongs to a class rather than an
  object.
• They can be initialized using an initialization
  block.
• Non-static variables can also be initialized
  here.

public class Numeric //... public static int
multi(int a, int b) return
(ab) private static double buffer new
doubleMAXSIZE private static int
size static //initialization block size
100 for (int i0 iltbuffer.length
i) bufferi Math.random()
21
Good Programming Practices

• There are three ways to initialize instance
  variables
• In constructors.
• Using initialization block (similar to statics).
  
• Instance field initialization.
• public class MyClass
• //
• private final int i0 //OK
• private int j
• private static double x
• private static final double y0
• //initialization block
• j 0 //better do it in constructor
• static //static initialization block
• x0

22
Order of Execution

• When an object is being created
• Class variables are created and the static
  initializers are executed. (First time only)
• The static initialization block is executed.
• Instance variables are created and the
  initializers are executed.
• The initialization block is executed.
• The body of the constructor is executed.

23
Test Program

• class MyClass
• public MyClass()System.out.println("MyClass
  Constructor")
• private static int f()System.out.println("Initia
  lizer")return 0
• private static double g()System.out.println("Sta
  tic Initializer") return 0
• private final int if()
• private int j
• private static double x
• private static final double yg() //OK
• //initialization block
• j 0 //better do it in constructor
• System.out.println("Initialization Block")
• static
• x0 //OK
• System.out.println("Static Initialization
  Block")

24
Driver Outputs

• public class MyClassTest1
• public static void main(String args)
• MyClass a //nothing happens
• MyClass b new MyClass()
• MyClass c new MyClass()
• Static Initializer
• Static Initialization Block
• Initializer
• Initialization Block
• MyClass Constructor
• Initializer
• Initialization Block
• MyClass Constructor

25
Instance Variables that are Objects

• class MyClass
• public MyClass(int i)
• System.out.println("MyClass Constructor")
• part new Part(i)
• //...
• private Part part
• //...
• class Part
• public Part(int i)System.out.println("Part
  Constructor")
• private int k

26
Driver Outputs

• public class MyClassTest2
• public static void main(String args)
• MyClass a new MyClass(1)
• C creates instance variables implicitly first
  and then execute the constructors body.

Static Initializer Static Initialization
Block Initializer Initialization Block MyClass
Constructor Part Constructor