Methods and Parameters

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Methods and Parameters

• B.Ramamurthy
• SUNY at Buffalo

2
Introduction

• Methods represent the functionality of objects.
• You can customize the functions carried out by a
  method using a powerful mechanism called
  parameter passing.
• This concept was briefly introduced in the last
  set of lectures on Classes and Objects. We will
  learn more details about parameter passing in
  this discussion.

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Topics for Discussion

• 1. Why parameters?
• 2. Types of parameters formal and actual
• 3.How to specify the parameters?
• 4. Semantics of parameter passage
• 5. Return statement and return value
• 6.Example
• 7. Summary

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Why Parameters?

• Parameters are needed to specialize a method
• For example Consider drawOval
• Without any parameters it will drawing any one
  fixed size oval.
• By specifying parameters drawOval(40,30,20,100)
  you are able to set the size as well the position
  of your oval.
• This is the power that parameters offer a method
  invocation.

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Formal Parameters

• Parameters defined in the method header are
  called formal parameter.
• Example Method header for drawOval from p.741
  (Blue book)
• public abstract void drawOval ( int xsrc, int
  ysrc, int width, int height)
• Meaning Formal parameter list gives the type and
  name of characteristics that can be changed in
  the method invocation.
• Formal parameter are dummy and do not get any
  value until a method is activated/invoked.

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Actual Parameters

• During the call /invocation/ activation of a
  method actual values representing the parameters
  are specified.
• Example
• drawOval ( 20, 30, 50. 50)
• X 4 Y 60
• Z Math.pow(X , 2) A X Y
• drawOval(X, Y, Z , A)
• drawOval( X, Y, Math.pow(X,2), XY) //
  Complex expressions as parameters

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Semantics of Parameter Passage
Definition
Call / Invocation
int Sum ( int X, int Y)
Total Sum (67, 60)
return (X Y)
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Semantics (contd.)

• Number of formal parameters should be same as
  actual parameters.
• When a method is invoked the actual parameters
  are positionally matched to the formal
  parameters.
• Type of actual parameters should be same as
  corresponding formal parameters.
• Value of the actual parameters are copied into
  the formal parameters.
• Control is transferred to method.
• Execution resumes in the method.
• When a return statement is encountered the value
  specified is returned and control is transferred
  back to the caller.

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Void and non-void Methods

• Void methods do not return a value, and so can be
  called as a stand-alone
• Example drawOval(5, 4 ,6 ,6)
• non-void methods return a value, so are usually
  used in an expression or assignment statement, or
  in general, in place of a variable.
• Example
• X Math.abs(X)
• Card RandGen.nextInt()

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Example