Methods

1
Methods

• CSC 171 FALL 2001
• LECTURE 3

2
History

• The abacus

3
HistoryThe Father of Computing

• 1822 - Charles Babbage
• The Difference Engine
• First Government grant
• Babbages ideas were not widley accepted because
  of poor documentation

4
Methods

• Variables describe static aspects
• nouns
• Methods describe dynamic aspects
• verbs, behaviors
• Methods do things

5
Methods

• We have seen some assignment statements where we
  do the computation in the program
• int x a 5
• String b Hello World\n

6
Methods

• We have also had some occasion to use methods
  that do computation elsewhere
• double d1 Math.sqrt(50.9)

7
Methods

• Think of a method as a black box
• We put something in
• We get something out
• Putting something in
• parameters
• Getting something out
• Return values

8
Black Box
Math
9.0
sqrt()
3.0
double x Math.sqrt(9.0)
9
A Computer Program

• public class myFirstProg
• public static void main(String args)
• System.out.println(Hello, CSC171)

10
Black Box
String args
MyFirstProg
main()
void
side effects
11
A Computer Program

• public class mySecondProg
• public static void main(String args)
• int x 5, y 8
• int product mymult(x,y)
• System.out.println( x y is
  product)
• public static int mymult(int n1, int n2)
• return n1 n2

12
Black Box
String args
MyFirstProg
main()
void
mymult()
13
Sequence of events
main()
mymult()
5,8
System.out.println()
40
5 8 is 40
void
14
Proceedural Abstraction

• We define the input and output behavior
• Contract
• We embed some complex behavior in a method.
• We refer to the method by a name
• We can write debug the method once then use
  it whenever we want.

15

• public class myThirdProg
• public static void main(String args)
• int x 5, y 8
• int product mymult(x,y)
• System.out.println( x 3 y is
• product)
• public static int mymult(int n1, int n2)
• return mycube(n1) n2
• public static int mycube(int x) return x x
  x

16
Sequence of events
mycube()
main()
5
mymult()
5,8
125
1000
System.out.println()
53 8 is 1000
void
17
Interfaces needed for project

• Interfaces are ways of writing contracts
• Client communicates the input output
  requirements
• Method names
• Parameters for each method
• Return types for each method

18
Interface definition

• public interface myMinMax
• public int myMin(int x, int y) // no body
• public int myMax(int x, int y) // no body

19
Interface use

• public class myProject
• implements myMinMax
• public int myMin(int x, int y)
• if (x lt y) return x else return y
• public int myMax(int x, int y)
• if (xgty) return x else return y