Lecture 10: Methods

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Lecture 10 Methods

• Yoni Fridman
• 7/17/01

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Outline

• Overview
• Method format
• Arguments
• The returned value
• Variable scope
• Using methods
• Commenting methods
• The method main()

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Overview

• Methods provide a way to break a program into
  smaller chunks.
• Youve already seen many examples
• System.out.println(Hello)
• Math.sqrt(25.0)
• playerWantsToHit()
• getNewCard()
• A method is a chunk of code that can be executed
  by calling its name.
• But where is the code thats executed when a
  method is called?

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Method Format
public static int triple(int n) n 3
return n
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Arguments

• A method can have any number of arguments.
• getNewCard() takes no arguments.
• System.out.println(Hello) takes 1 argument.
• Math.pow(3, 2) takes 2 arguments.
• Etc.
• When an argument of type int, double,
  or boolean is passed to a method, the
  method makes a copy.
• Strings and arrays work differently. More
  later.

public static int triple(int n) n 3
return n
int n 5 int nTripled triple(n) System.out.pr
intln(nTripled) System.out.println(n)
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The Returned Value

• As weve seen, a method can return a value that
  it computed.
• getNewCard() returns a random value between 1 and
  10.
• Math.pow(x, y) returns xy.
• The type of the returned value must be stated.
• The return statement must be included
  at the end of the method body this
  indicates the value being returned.

public static int triple(int n) n 3
return n
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The Returned Value

• A method can do stuff, but then return nothing.
• Example System.out.println(Hello)
• In this case, the return type is void, and
  there doesnt have to be a return statement.

public static void showTriple(int n) n
3 System.out.println(n)
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Variable Scope

• Variables declared inside a method have local
  scope. That is, they only exist inside the
  method.
• What will happen if you try to use
  nTimes3 outside of the method
  triple()?
• Error!
• Also, as soon as the method ends, the
  local variable is lost.

public static int triple(int n) int nTimes3
n 3 return nTimes3
int n 5 int nTripled triple(n) System.out.pr
intln(nTimes3)
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Using Methods

• Why use methods?
• So the same code doesnt have to be repeated.
• Example getNewCard()
• To make the program easier to read.

initializePlayerHands() while (playerHits)
drawNewCard() checkIfBusted() while
(dealerTotal lt 17) drawNewCard() determineWin
ner()
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Commenting Methods

• Write a comment block right before each method,
  which includes three parts
• Method description What the method does.
• Input A description of the arguments passed to
  the method.
• Output A description of the value being
  returned from the method.

// This method triples the value of an int. //
Input n any int. // Output An int that
equals three times the input. public static int
triple(int n) n 3 return n
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The Method main()

• Now we can understand (almost) this line of code
• public static void main(String args)
• Its a method called main that takes in an
  array of Strings and doesnt return anything.
• When you execute a program (by hitting the
  button), that tells Java to execute the main()
  method.

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Homework

• Read 3.1 3.5.
• HW5 out today, due Monday.
• Poker.