05 Method Calling

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05 Method Calling
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Outline

• What is a Method?
• Declaring a Method
• Method Calling
• Method Call Stack
• Parameter Passing
• Pass by Value

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Objectives

• Define a method
• Demonstrate how to properly declare a method.
• Demonstrate how methods call each other
• Demonstrate how methods are executed in call
  stack
• Demonstrate parameter passing
• Understand that Java passes arguments by value.

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What is a Method?

• A method refers to a piece of code referring to
  behaviors associated either with an object or its
  class
• A code found in a class for responding to a
  message
• The executable code that implements the logic of
  a particular message for a class
• An operation or function that is associated with
  an object and is allowed to manipulate the
  object's data

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Creating a Method

• Steps in declaring a method
• Set the return type
• Provide method name
• Declare formal parameters

• class Number
• int multiply(int i, int j)
• return ij
• int divide(int i, int j)
• return i/j
• double getPi()
• return 3.14159265358979
• void printSum(int i, int j)
• System.out.println(ij)

• method signature
• consists of the method name and its parameters
• must be unique for each method in a class

• return statement
• allows the method to return a value to its caller
• also means to stop the execution of the current
  method and return to its caller
• implicit return at the end of the method

A method with empty parameters A method that
does not return a value must specify void as its
return type
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Method Calling

• How to call a method
• Method name should match
• Number of parameters should match
• Type of parameters should match
• Ways of calling a method
• Calling a method through its object name
• Calling a method within the same class
• Calling a static method through its class name

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Method Calling - Example

• public class JavaMain
• public static void main(String args)
• // create a Person object
• Person you new Person()
• you.talk()
• you.jump(3)
• System.out.println(you.tellAge())
• JavaMain.talkOnly(you)
• // create object of main program
• JavaMain me new JavaMain()
• me.jumpOnly(you)
• static void talkOnly(Person p)
• p.talk()
• void jumpOnly(Person p)

• class Person
• void talk()
• System.out.println("blah, blah...")
• void jump(int times)
• for (int i0 ilttimes i)
• System.out.println("whoop!")
• String tellAge()
• return "I'm " getAge()
• int getAge()
• return 10

blah, blah... whoop! whoop! whoop! I'm 10 blah,
blah... whoop! whoop!
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Method Call Stack

• The Method Call Stack refers to all the methods
  currently active and being processed by a Java
  application

print()
compute()
check()
main()
invoke print() invoke compute() invoke
check() main() begins
execute print() execute compute() execute
check() main() ends
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Passing Parameters

• Passing parameters in Java is always Pass by
  Value!
• When passing a parameter of primitive type
• A copy of the value of the variable is passed
• The passed variable cannot be changed in the
  called method as the method only possesses a copy
  of that variable.
• When passing a parameter of reference type
• A copy of the reference (address) of the object
  is passed
• The object reference cannot be changed in the
  called method (i.e., the object cannot be
  reassigned to another object)
• The object state can be changed in the called
  method (i.e., attributes can be modified)

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Passing Parameters Strings

• String literals in Java are implemented as
  instances of String class (java.lang.String).
• Strings in java are immutable (i.e., their value
  cannot be changed).
• When passing a parameter of String type
• A new instance of String class is created and the
  value of the parameter is copied to the new
  instance.
• Like parameters of primitive type, the passed
  variable cannot be changed in the called method
  because the method only possesses a copy of that
  variable.

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Passing Parameters - Example

• public class TestJavaParameterPassing
• public static void main(String args)
• int count5
• int numbers 10,12,15
• String name Tom"
• TestJavaParameterPassing test new
  TestJavaParameterPassing()
• System.out.println(Passing a parameter of
  primitive type (count).")
• System.out.println( Initial Value of count in
  main is count)
• test.changeCount (count)
• System.out.println( Value of count in main
  remains unchanged after calling changeCount "
  count)
• System.out.println(Passing a parameter of
  reference type (numbers).")
• System.out.println( Initial Value of numbers in
  main is test.get.Numbers(numbers))
• test.changeNumbers (numbers)
• System.out.println( Values of numbers in main
  changes after calling changeNumbers "
  test.getNumbers (numbers))
• System.out.println(Passing a parameter of
  String type (name).")

• void changeNumbers(int numbers)
• numbers 020 numbers 111 numbers 218
• System.out.println( Value of numbers in
  changeNumbers method after the change is "
  getNumbers(numbers))
• String getNumbers(int numbers)
• String s""
• for (int i0 iltnumbers.length i) s
  numbersi " "
• return s
• void changeName(String name)
• String newName Tony"
• name newName
• System.out.println( Value of name in changeName
  method after the change is " name)

Passing a parameter of primitive type (count).
Initial Value of count in main is 5 Value of
count parameter in changeCount method after the
change is 10 Value of count in main remains
unchanged after calling changeCount 5 Passing a
parameter of reference type (numbers). Initial
Value of numbers in main is 10 12 15 Value of
numbers in changeNumbers method after the change
is 20 11 18 Values of numbers in main changes
after calling changeNumbers 20 11 18 Passing a
parameter of String type (name). Initial Value
of name in main is Tom Value of name in
changeName method after the change is Tony
Values of name in main remains unchanged after
calling changeName Tom
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Key Points

• A method refers to what an object or class can do
• A method must have a return type, a name, and
  optional parameters
• The method signature refers to a method name and
  its parameters
• Return statement returns a value to its caller or
  returns control to its caller
• A method that does not return a value must
  specify void as a return type
• Calling a method should match its method
  signature
• When calling a method in the same class, use only
  the method name
• When calling a method in a different class, use
  the object reference
• When calling a static method, use the class name
• Methods are invoked sequentially in the call
  stack and executed in reverse order
• Passing parameters in Java is always pass by value