COMP20608S General Programming 2

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COMP206-08SGeneral Programming 2
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Lectures

• Today
• Random Numbers
• Timing
• Assignment 1 Review

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Random Numbers

• import java.util.Random
• Implements a random number generator that appears
  to generate random numbers
• Construct object of Random class
• Use methods
• nextInt(n) random number 0 -gt n-1
• nextDouble() random number 0 -gt 1 (exclusive)
• Constructors use system clock no params or you
  can seed the generator

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Example

• Random generator new Random( )
• int oneToTen 1 generator.nextInt(10)

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Useful utility

• import java.util.Random
• public class ArrayUtil
• private static Random generator new Random()
• public static int randomIntArray(int length,
  int n)
• int a new intlength
• for(int i0 ilta.length i)
• ai generator.nextInt(n)
• return a
• public static void print(int a)
• for (int x a)
• System.out.print(x " ")
• System.out.println()

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Test for Array Util

• public class TestArrayUtil
• public static void main(String args)
• // 20 numbers in range 0 -gt 99
• int a ArrayUtil.randomIntArray(20, 100)
• ArrayUtil.print(a)

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Timing program runs

• Want to be able to assess how long something
  takes to run
• Can be complicated due to threads
• We will make use of a system call
• System.currentTimeMillis()
• This provides the current time in milliseconds so
  we need to do some work to convert to the elapsed
  time of the thing we are assessing.
• Can get elapsed time while running

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Stop Watch Class

• public class StopWatch
• private Boolean isRunning
• private long startTime, elapsedTime
• public StopWatch()
• reset()
• public void start()
• if (isRunning) return
• isRunning true
• startTime System.currentTimeMillis()

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• public void stop()
• if (!isRunning) return
• isRunning false
• long endTime System.currentTimeMillis()
• elapsedTime elapsedTime endTime - startTime
• public long getElapsedTime()
• if (isRunning)
• long endTime System.currentTimeMillis()
• return elapsedTime endTime - startTime
• else
• return elapsedTime
• public void reset()

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Test program for StopWatch

• public class TestStopWatch
• public static void main(String args)
• StopWatch timer new StopWatch()
• timer.start()
• try
• Thread.sleep(6000)
• catch (InterruptedException exception)
• timer.stop()
• System.out.println("Time to sleep
  "timer.getElapsedTime())

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Assignment 1 Review

• Java naming conventions
• All variables start with a lowercase letter and
  new words start with an uppercase letter
• Eg int num, int numFound
• Class names uppercase letter first (then as
  above)
• Eg Circle, Student, BinarySearchTree
• Method names as per variables
• Eg processFile, main, findOutliers

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Question 1 check chars

• public class palindrome
• public static void main (String args)
• String s args0
• for (int i0 ilts.length()/2 i)
• if (!(s.charAt(i) (s.charAt(s.length()
  -1-i))))
• System.out.println(s" is not a palindrome")
• return
• System.out.println(s" is a palindrome")

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Question 1 use String methods

• public static void main (String args)
• StringBuffer s new StringBuffer (args0)
• StringBuffer t new StringBuffer (args0)
• if (s.toString().equals(t.reverse().toString()))
  
• System.out.println(s" is a palindrome")
• else
• System.out.println(s" is not a
  palindrome")

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Complex Numbers

• public class Complex
• private float real
• private float imag
• public Complex(float r, float i)
• real r
• imag i
• public Complex()
• real 0
• imag 0

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• public Complex add(Complex a)
• Complex result new Complex()
• result.real a.real this.real
• result.imag a.imag this.imag
• return result
• public String toString()
• String result new String()
• result "Real part " this.real
• result " Imag part " this.imag
• return result
• public boolean equals (Complex y)

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equals

• The equals method can be either overloaded or
  overridden
• Here it is overloaded so there are two equals
  methods
• boolean equals(Complex other) defined in
  Complex
• boolean equals(Object other) defined in Object
• boolean equals(Complex x, Complex y) is a
  different method entirely

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Equals formal defn

• public boolean equals(Object otherObject)
• if (otherObject null) return false
• if (getClass() ! otherObject.getClass())
• return false
• Complex other (Complex) otherObject
• return (this.real other.real this.imag
  other.imag)

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Question 3 - Scanner

• import java.util.Scanner
• public class TestComplex
• public static void main (String args)
• Scanner in new Scanner(System.in)
• int xr in.nextInt()
• int xi in.nextInt()
• Complex x new Complex(xr,xi)
• String op in.next()
• int yr in.nextInt()
• int yi in.nextInt()
• Complex y new Complex(yr, yi)
• Complex result
• if (op.equals(""))
• result x.add(y)
• System.out.println("result is
  "result.toString())
• else System.out.println("Something went wrong")

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Scanner methods

• Scanner(InputStream in)
• Scanner(Reader in)
• boolean hasNext()
• String next()
• int nextInt()
• String nextLine()

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One line input

• import java.util.Scanner
• import java.util.StringTokenizer
• public class TestComplex
• public static void main (String args)
• Scanner in new Scanner(System.in)
• String s in.nextLine()
• StringTokenizer tokens new StringTokenizer(s)
• String openBracket tokens.nextToken()
• int xr Integer.parseInt(tokens.nextToken())
• int xi Integer.parseInt(tokens.nextToken())
• Complex x new Complex(xr, xi)
• String op tokens.nextToken()
• int yr Integer.parseInt(tokens.nextToken())
• int yi Integer.parseInt(tokens.nextToken())
• Complex y new Complex(yr, yi)
• String closeBracket tokens.nextToken()

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Question 4

• import java.util.ArrayList
• public class TestShapes
• public static void main(String args)
• ArrayListltShapegt shapes new ArrayListltShapegt()
  
• shapes.add(new Circle(4))
• shapes.add(new Triangle(5,10))
• shapes.add(new Rectangle(4,5))
• double totalArea 0
• for (Shape x shapes)
• totalArea x.area()
• System.out.println("Total area is "totalArea)

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Implement Shape as an Interface

• public interface Shape
• double area()

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And each shape implements area

• public class Circle implements Shape
• private double radius
• public Circle()
• radius 0
• public Circle(double r)
• radius r
• public double area()
• return Math.PIradiusradius

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Similarly for others

• public class Rectangle implements Shape
• private double width, length
• public Rectangle()
• width 0
• length 0
• public Rectangle(double w, double l)
• width w
• length l
• public double area()
• return widthlength

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And Triangle

• public class Triangle implements Shape
• private double base, height
• public Triangle()
• base 0
• height 0
• public Triangle(double b, double h)
• base b
• height h
• public double area()
• return 0.5baseheight