static methods

1
static methods

• COSC 101
• Code available at
• http//cs.colgate.edu/faculty/jspacco/101/examples
  /bankAccountRevisited.zip

2
There are three kinds of methods

• instance methods
• static methods
• constructors
• Constructors are special instance methods

3
Using constructors

• Its like a method call
• but no instance yet
• because were creating a new instance
• no dot
• must use the keyword new
• BankAccount b1new BankAccount()

4
Defining constructors

• If you dont define any constructors
• Java will create a default or void constructor
  for you
• Takes no parameters
• initializes all fields to default value for that
  type
• 0 for primitive types
• null for reference types
• If you create at least one constructor that takes
  at least one parameter
• Java will not create the default constructor for
  you
• so if you want a default constructor
• provide one yourself

5
Defining constructors, cont

• Method declaration for constructor
• no return type
• method name must match the name of the class
  exactly
• Can take parameters

6
Classes can have as many constructors as you want

• Constructors are differentiated from one another
  by their parameters
• This process is called method overloading

7
Defining multiple constructors

• public BankAccount(double balance)
• this.balancebalance
• public BankAccount(String accountNum)
• this.accountNumberaccountNum
• public BankAccount(double balance,
• String accountNum)
• this.balancebalance
• this.accountNumberaccountNum

8
Java differentiates between constructors by their
parameters

• BankAccount b1new BankAccount()
• BankAccount b2new BankAccount(1000.0,
• 123abc)
• BankAccount b3new BankAccount(2000.0)

9
static VS instance

• instance methods
• can manipulate instance variables
• must be invoked on an instance
• b1.withdraw(100.0)
• static methods
• cannot see instance variables
• have no state
• COSC101Math.power(1.05, 8)

10
Instance methods

• Inputs Parameters passed into method
• methods can have zero or more parameters
• Output return value
• methods can only return one value
• Side-effects
• instance methods can manipulate the instance
  variables of the class
• which might have the side-effect of changing the
  state of the method

11
static methods

• state-less methods
• cannot manipulate or even see instance variables
• if a method doesnt look at any instance
  variables at all
• it should probably be static
• Common to put lots of static utility methods into
  one class
• java.lang.Math
• http//java.sun.com/j2se/1.5.0/docs/api/java/lang/
  Math.html

12
All primitives in Java are passed by value

• This just means that we actually pass a copy of
  each parameter of primitive type
• primitives are small
• 1 to 8 bytes each
• very efficient just to copy them
• BankAccount b1new BankAccount(1000.0)
• double money100.0
• b1.deposit(money)

13
All reference types are passed by reference

• This means that just a reference (or handle) to
  the object gets passed into the method
• The object still only exists in one place in
  memory
• 500.0 is passed by value
• b2 is passed by reference
• BankAccount b1new BankAccount(1000.0)
• BankAccount b2new BankAccount(2000.0)
• b1.transfer(500.0, b2)

14
Parameters passed by value cannot change the
original value

• You dont really have the original value
• you just have a copy of it!
• Thus even if you change the value of a primitive
  passed into a method
• it DOES NOT change the value of the original
  thing passed in

15
Parameters passed by reference can be changed
inside the method

• And these changes are visible outside the method
• This is because you have a reference to the
  actual object
• For example, the transfer method may change the
  balance of b2
• BankAccount b1new BankAccount(1000.0)
• BankAccount b2new BankAccount(2000.0)
• b1.transfer(500.0, b2)

16
private methods

• private methods are also called helper methods
• These methods are only usable inside the class
• In the BankAccount class, we have to remove money
  from the balance
• and return true if it was OK to take that money
  out
• false if there wasnt enough money

17
Lets add a deductMoney method

• private instance method
• because its private, it can only be used inside
  the BankAccount class
• private boolean deductMoney(double m)
• if (balance gt m)
• balance-m
• return true
• return false

18
deductMoney method can simplify other methods

• transfer method can be simplified using
  deductMoney money methods
• that means that one method will call another
  method

19
Before

• public boolean transfer(double m, BankAccount
  other)
• if (this.balance gt m)
• other.balancem
• this.balance-m
• return true
• return false

20
After

• public boolean transfer(double m, BankAccount
  other)
• if (deduct(m))
• other.deposit(m)
• return true
• return false

21
Calling instance methods from other instance
methods

• You can use the this keyword
• this.deductMoney(m)
• Or not
• deductMoney(m)
• this is implicitly the reference to the instance
  upon which a method is being invoked
• can be used to access fields
• or to invoke methods

22
Lets play definition or use

• Know the difference between defining a method and
  using a method